{"count":5068,"next":"http://www.astro4edu.org/oae-api/glossary-terms/?page=4","previous":"http://www.astro4edu.org/oae-api/glossary-terms/?page=2","results":[{"term_name":"Newton's Laws of Motion","term_definition":"Newton’s Laws of Motion are a set of models which explain how objects with mass change their motion due to interactions with other objects. These interactions are described as forces. Newton's Laws explain how forces affect the motion of objects. Newton developed three laws of motion and a law of gravitation. These laws can explain the motion of most objects in the Universe, with motions involving high velocities and/or very strong gravity requiring Einstein's theories of special and general relativity respectively.","term_approval_level":"A","language_code":"en","term_number":216,"term_in_english":"Newton's Laws Of Motion","based_on_current_english_version":null,"linked_terms":[135],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/216/"},{"term_name":"North Celestial Pole (NCP)","term_definition":"","term_approval_level":"N","language_code":"en","term_number":217,"term_in_english":"North Celestial Pole (NCP)","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":[],"override_term_number":52,"categories":[],"category_ids":[],"override_url":"https://astro4edu.org/resources/glossary/term/52/","url":"https://astro4edu.org/resources/glossary/term/217/"},{"term_name":"Nova","term_definition":"A nova is a star that suddenly brightens, becoming many times brighter than before. The name derives from the Latin nova stella or new star, which was used in early-Modern Europe to describe bright stars that suddenly appeared in the sky. Novae have been observed as \"guest stars\" by many different cultures.\r\n\r\nNovae are caused by white dwarfs which accrete gas from a close binary star companion. This gas builds up in the white dwarf's atmosphere until it is hot enough to ignite through nuclear fusion. This nuclear fireball causes the white dwarf to brighten by several orders of magnitude. Unlike a Type Ia supernova, the white dwarf remains intact after this explosion. This means that the whole process can start again and the nova can recur.","term_approval_level":"A","language_code":"en","term_number":219,"term_in_english":"Nova","based_on_current_english_version":null,"linked_terms":[221,349,386,476],"alternate_terms":[],"categories":["Stars"],"category_ids":[2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/219/"},{"term_name":"Nuclear Fission","term_definition":"Nuclear fission is a process where the nucleus of a heavy element splits into two lighter nuclei. The mass of the remaining material is less than that of the original nucleus, while the deficit in mass is released as energy. \r\n\r\nThe two lighter nuclei produced by nuclear fission are often radioactive themselves and can release further energy through radioactive decay.","term_approval_level":"A","language_code":"en","term_number":220,"term_in_english":"Nuclear Fission","based_on_current_english_version":null,"linked_terms":[221,224],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/220/"},{"term_name":"Nuclear Fusion","term_definition":"Nuclear fusion is the process where the atomic nuclei of lighter elements join together to form the nucleus of a heavier element. \r\n\r\nIn the Universe, nuclear fusion plays two important roles. For one, it provides the energy supply for the radiation emitted by stars like our Sun. When sufficiently light nuclei fuse, the total rest mass of the resulting nucleus is slightly smaller than the combined rest masses of the initial nuclei. This \"mass deficit\" corresponds to the energy released by the fusion reaction, via Einstein's famous formula E=mc², which links mass m, energy E, and the speed of light c. For example, in the core of the Sun, hydrogen nuclei fuse together to form helium and release energy in the form of radiation as well as neutrino particles. \r\n\r\nThe second role of nuclear fusion is that it is responsible for producing elements in the Universe that are more complex than hydrogen and helium. After the Big Bang, only hydrogen, helium, and trace amounts of lithium nuclei existed in the Universe. Fusion reactions in the cores of stars, in the course of supernova explosions, and from explosions caused by colliding neutron stars, are the source of (essentially) all the remaining heavier chemical elements in the Universe. The chemical elements that make up the greatest part of the human body by mass, notably oxygen and carbon, were formed by nuclear fusion in the core of stars or during supernova explosions, prompting the expression \"we are star dust\".","term_approval_level":"A","language_code":"en","term_number":221,"term_in_english":"Nuclear Fusion","based_on_current_english_version":null,"linked_terms":[150,220,224],"alternate_terms":["Thermonuclear fusion","fusion"],"categories":["Stars","The Sun"],"category_ids":[2,5],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/221/"},{"term_name":"Nucleon","term_definition":"All the matter that we see around us is made up of atoms, and all atoms have the same basic structure: a tiny, dense nucleus that carries almost all of the atom's mass, surrounded by electrons. The main constituents of atomic nuclei are protons and neutrons, which are collectively called nucleons.","term_approval_level":"A","language_code":"en","term_number":223,"term_in_english":"Nucleon","based_on_current_english_version":null,"linked_terms":[224,441],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/223/"},{"term_name":"Nucleus","term_definition":"All the matter that we see around us consists of atoms, and each atom in turn consists of electrons surrounding a small, central nucleus. Atomic nuclei consist of protons, which are positively charged, and neutrons, which carry no electric charge. Although the protons repel each other due to their electric charge, there is an even stronger force, called the strong nuclear force or just the strong force, that holds the nucleus together.  Nuclei with the same number of protons form atoms that belong to the same chemical element. \r\n\r\nNuclei are tiny, only about 1/100,000th of the size of an atom – so in a sense, most of the atom is empty space! The nucleus typically accounts for more than 99.9% of an atom's total mass. That mass at such tiny size makes nuclei very dense, with typical densities of a hundred million billion kilograms per cubic meter. \r\n\r\nNuclei are important in different areas of astrophysics. In the interior of stars, energy is set free as lighter nuclei (starting with hydrogen, whose nucleus is a single proton) fuse to form successively heavier nuclei – this is what makes stars shine. Nuclear fusion in stars can form heavy nuclei up to those of iron, with supernova explosions and the interior of certain cool stars able to form even heavier nuclei. Shortly after the Big Bang, a brief phase of \"Big Bang nucleosynthesis\" had hydrogen nuclei fusing to helium and traces of other elements. Neutron stars, as the remnants of the supernova explosions of massive stars, consist of mostly neutrons stacked to similar densities as those of nuclei. Certain types of atomic nuclei, stripped of their electrons, are emitted by stars as part of stellar winds, or travel the depths of space as cosmic rays.","term_approval_level":"A","language_code":"en","term_number":224,"term_in_english":"Nucleus","based_on_current_english_version":null,"linked_terms":[31,70,149,223,430,435,441],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/224/"},{"term_name":"Observable Universe","term_definition":"The observable universe refers to the patch of the Universe we can see, which is a sphere with us at the center. The radius of the observable universe is determined by how far light has been able to travel towards us since the beginning of the Universe. Regions at the boundary of the observable universe are so far away that their light has just had enough time to reach us over the past 14 billion years; in other words: over the age of the Universe. \r\n\r\nThe most distant regions of the Universe that we can see are now over 40 billion light years away. This is because the Universe has expanded a lot since the light reaching us from those regions was emitted. Light from objects outside the observable universe has not yet had enough time to reach us. \r\n\r\nThe longer we wait, the more time light has to reach us, and the larger the observable universe grows. Other observers in the cosmos have their own observable universes: a sphere with them at the center, its radius the greatest distance over which light from other regions has had time to reach them.","term_approval_level":"A","language_code":"en","term_number":225,"term_in_english":"Observable Universe","based_on_current_english_version":null,"linked_terms":[4,96,175,178,322,374],"alternate_terms":[],"categories":["Cosmology"],"category_ids":[9],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/225/"},{"term_name":"Observation","term_definition":"Astronomical observations involve collecting and/or measuring electromagnetic radiation, particles, or gravitational waves reaching us from an astronomical object. In the past, humans observed with their eyes, and from the early 1600s through telescopes. Now a variety of cameras, spectrometers, and other instruments can also be used. The information that is collected, such as a raw image we retrieve from a camera, is known as (observational) data.\r\n\r\nThis data contains information about the object and the intervening medium (e.g. the interstellar or intergalactic medium), but still depends on the specifics of the instrument, for instance if one part of the camera is more sensitive than another. The data also depends on contaminants; for example, when collecting light from an astronomical object, we will typically also collect foreground light scattered in Earth's atmosphere. Removing instrument-specific and contaminant portions as completely as possible is called data reduction. Typical end products of observations are images, spectra, and time series (repeated observations of the same object or objects, for instance, data from pulsars or variable stars). These can be used to measure various quantities such as the angle between two objects, the time an event was observed, or the apparent magnitude of an object.\r\n\r\nObservations differ from the experiments carried out in many scientific laboratories in that the observer cannot interact with the astronomical objects themselves in the same way as a chemist mixing two chemicals can. In some contexts, observations can sometimes be supplemented by experiments on the objects themselves such as the study of meteorites or by sending space probes to objects in the Solar System.","term_approval_level":"A","language_code":"en","term_number":226,"term_in_english":"Observation","based_on_current_english_version":null,"linked_terms":[15,96,227,241,328,447,485],"alternate_terms":[],"categories":["Naked Eye Astronomy","Telescopes, Instruments and Observatories"],"category_ids":[4,3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/226/"},{"term_name":"Observatory","term_definition":"An observatory is a site or building dedicated to astronomical observations. Optical and infrared observations are usually performed with telescopes placed in domes to protect them during bad weather. Radio telescopes, and some submillimeter telescopes are often placed outside. Modern, ground-based, optical, infrared, or submillimeter astronomical observatories are erected on mountaintops above as much of Earth's atmosphere as possible. These observatories have specialized instruments such as telescopes, cameras, and spectrographs. A high location is less necessary for radio and cosmic ray astronomical observatories. Other parts of the electromagnetic spectrum (gamma, X-ray, ultraviolet radiation, and longer wavelength infrared radiation) can be studied with specific space telescopes that are sometimes also referred to as observatories. Observations for radio telescopes at different sites can be combined together. These multiple sites are often referred to collectively as one observatory.","term_approval_level":"A","language_code":"en","term_number":227,"term_in_english":"Observatory","based_on_current_english_version":null,"linked_terms":[70,96,123,155,198,274,352,371,378,435,482],"alternate_terms":["Astronomical Observatory"],"categories":["Telescopes, Instruments and Observatories"],"category_ids":[3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/227/"},{"term_name":"Opposition","term_definition":"When two astronomical objects appear to be lined up or nearly lined up with an observer, in opposite directions in the sky, they are said to be in opposition. It is not necessary for both objects to be actually visible for the observer. For instance, at a full moon, the Sun, observer on Earth, and Moon are lined up, so the visible part of the Moon's surface is fully lit up by the Sun – unless the alignment is perfect, in which case there is a lunar eclipse. When a planet, comet, or asteroid is said to be \"in opposition\", this commonly refers to the Sun and observers on Earth. When a planet is in opposition, it looks particularly bright, appears to move in a direction opposite than usual (\"retrograde motion\" as Earth moves faster on its inside track), and is particularly close to Earth.","term_approval_level":"A","language_code":"en","term_number":228,"term_in_english":"Opposition","based_on_current_english_version":null,"linked_terms":[65,182,367],"alternate_terms":[],"categories":["Naked Eye Astronomy"],"category_ids":[4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/228/"},{"term_name":"Optical Astronomy","term_definition":"Optical astronomy is the practice of studying and observing the Universe (stars, planets, dwarf planets, asteroids, etc.) in the near-infrared, in visible light, and in ultraviolet light. The reason for lumping those three kinds of electromagnetic radiation together is that the optical telescopes, with glass lenses and/or metallic mirrors, that astronomers had originally constructed for observing visible light from celestial objects, are equally well-suited for observing near-infrared or ultraviolet light. In addition, the Earth's atmosphere is transparent not only for visible light, but for the directly adjacent infrared and ultraviolet regions, allowing all three kinds of observations from the ground. Last but not least, the camera chips astronomers use for visible-light observations can also detect near-infrared and ultraviolet light. Put all of this together, and the telescopes and instruments astronomers use to observe visible light work just as well for near-infrared and ultraviolet observations. In consequence, it makes sense for astronomers to collectively describe observations in that range of the electromagnetic spectrum with a single term, namely optical astronomy. The adjective \"optical\" is also used to describe the spectral range, as in \"the optical part of the spectrum.\" Observations in that range are \"optical observations\".","term_approval_level":"A","language_code":"en","term_number":229,"term_in_english":"Optical Astronomy","based_on_current_english_version":null,"linked_terms":[378,451,459],"alternate_terms":[],"categories":["Telescopes, Instruments and Observatories"],"category_ids":[3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/229/"},{"term_name":"Optical Telescope","term_definition":"An optical telescope is an instrument that is used to observe and study astronomical objects in visible (optical) light. Optical telescopes use mirrors (a reflecting telescope) and/or lenses (a refracting telescope) to collect and focus light. Telescopes were originally developed to observe distant objects on Earth but were soon turned to astronomical use. Telescopes can range in size from a few centimeters across to diameters of over ten meters.","term_approval_level":"A","language_code":"en","term_number":230,"term_in_english":"Optical Telescope","based_on_current_english_version":null,"linked_terms":[96,279,281,352,378,451,459],"alternate_terms":[],"categories":["Telescopes, Instruments and Observatories"],"category_ids":[3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/230/"},{"term_name":"Optics","term_definition":"Optics is the science of light and its interactions with matter, and the art of constructing instruments that make use of the general principles of light–matter interactions. In astronomy, the \"optics\" of a telescope or instrument are those parts that guide light towards a detector, notably mirrors, lenses, masks, slits, or waveguides where visible light is concerned, as well as dispersive elements such as prisms and gratings that produce spectra. Active optics is the term for a mirror kept in the correct shape by active mechanical elements (\"actuators\"), while adaptive optics is a system whereby a mirror is quickly deformed in just the right way to counteract atmospheric disturbances (the \"twinkling\" of stars). The adjective \"optical\" is also used to refer to astronomy using visible light.","term_approval_level":"A","language_code":"en","term_number":231,"term_in_english":"Optics","based_on_current_english_version":null,"linked_terms":[279,281,352,427,451,459],"alternate_terms":[],"categories":["Telescopes, Instruments and Observatories"],"category_ids":[3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/231/"},{"term_name":"Orbit","term_definition":"An orbit is the path of a moving object in a system around the center of mass of that system, caused by the mutual gravitational force between the objects in the system. For systems such as the Solar System, where the central body is much more massive than the other bodies, this center of mass lies inside or close to the most massive object (in the case of the Solar System, the Sun). In a binary star system the center of mass the stars orbit often lies between the two stars.\r\n\r\nOrbits are typically elliptical in shape with the center of mass of the system lying at one focus of the ellipse. The size and shape of the orbit is defined by the semi-major axis and the eccentricity of the ellipse. More eccentric orbits have higher ellipticities. Most planets in the Solar System have orbital eccentricities very close to zero, for example, Venus (0.007) and Earth (0.017). The exceptions are Mercury (0.206) and the dwarf planet Pluto (0.244).","term_approval_level":"A","language_code":"en","term_number":232,"term_in_english":"Orbit","based_on_current_english_version":null,"linked_terms":[40,98,135,169,233,314],"alternate_terms":[],"categories":["Exoplanets & Astrobiology","Naked Eye Astronomy","Solar System"],"category_ids":[6,4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/232/"},{"term_name":"Orbital Period","term_definition":"The orbital period is the time it takes for an object to make one complete orbit of another object. Another way of stating this is that it is the time it takes for an object to arrive back to the same point in its orbit. This could be a planet going around a central star (for example, Earth orbiting the Sun); it could be a moon orbiting around a planet; it could be a star, group of stars, or nebulae going around the center of a galaxy; it could be two (binary) stars orbiting their common center of mass.","term_approval_level":"A","language_code":"en","term_number":233,"term_in_english":"Orbital Period","based_on_current_english_version":null,"linked_terms":[40,135,169,232,315],"alternate_terms":[],"categories":["Naked Eye Astronomy","Solar System"],"category_ids":[4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/233/"},{"term_name":"Outer Planets","term_definition":"In our Solar System, the outer planets are Jupiter, Saturn, Uranus, and Neptune. Their orbits are outside the asteroid belt, and all of those planets are so-called giant planets, with an extremely thick atmosphere made up mostly of hydrogen. This makes them physically distinct from the inner planets, each of which is a comparatively small rocky body with a comparatively thin atmosphere. \r\n\r\nPlanets around stars other than our Sun do not necessarily fall into inner and outer groups with similar characteristics – we know a number of stars with at least one gas giant, a \"hot Jupiter\", in close orbit.","term_approval_level":"A","language_code":"en","term_number":234,"term_in_english":"Outer Planets","based_on_current_english_version":null,"linked_terms":[129,167,212,294,314,375,449],"alternate_terms":[],"categories":["Naked Eye Astronomy","Solar System"],"category_ids":[4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/234/"},{"term_name":"Outer Space","term_definition":"Outer space, often shortened to \"space\", is the term for all regions within our Universe that lie beyond Earth's atmosphere. Spaceflight is the endeavor to fly suitable conveyances into and through outer space, and such conveyances are called spacecraft. Space sciences is an umbrella term for all branches of science that are concerned with space exploration, spaceflight, or astronomical bodies in outer space that may be reached by spacecraft, which includes astronomy and planetology.","term_approval_level":"A","language_code":"en","term_number":235,"term_in_english":"Outer Space","based_on_current_english_version":null,"linked_terms":[319,324],"alternate_terms":[],"categories":["Space Exploration"],"category_ids":[10],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/235/"},{"term_name":"Ozone","term_definition":"Ozone is a colorless or pale blue-colored gas in which each molecule consists of three oxygen atoms. It has the chemical formula O₃ and it is poisonous to humans. Its proportion in the atmosphere is small, and in some areas it may not exceed one part in a million. Depending on where the ozone is, it can protect or harm life on Earth. \r\n\r\nMost of the ozone is located in the stratosphere, where it acts as a shield that protects Earth's surface from ultraviolet radiation from the Sun. This armor has weakened due to human introduction of chlorofluorocarbons into the atmosphere, thus increasing the risk of skin cancer, cataracts, and weakened immune systems. In 1987, the Montreal Protocol banned use of these gases as refrigerants and aerosol spray propellants with the result that the stratospheric ozone layer has begun to recover.\r\n\r\nNear Earth's surface and within the troposphere, ozone is a very harmful pollutant and can cause damage to lung tissue and plants.","term_approval_level":"A","language_code":"en","term_number":236,"term_in_english":"Ozone","based_on_current_english_version":null,"linked_terms":[29,371],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/236/"},{"term_name":"Parallax","term_definition":"Parallax is the apparent change in position of a celestial object on the sky due to a change in the observer’s point of view. The position change in the sky, expressed as an angle, is determined relative to the most distant objects we know – historically, distant stars; in modern astronomy, extremely distant objects called quasars, which determine what is called the International Celestial Reference Frame. The angle of apparent position change is inversely proportional to the object's distance from us, making parallax measurements a powerful tool for determining distances in our cosmic neighborhood (\"parallax method\"). For Solar System objects, simultaneous observations from different locations on Earth can yield useful parallax values. For stars, parallax angles are given for a standard shift of observer position by one astronomical unit (the average Earth–Sun distance) at right angles to the line of sight. Observer position shifts of that magnitude can be achieved by making observations several months apart, with Earth moving along its orbit around the Sun in between. By definition, an object whose parallax angle under those conditions is 1 arcsecond is at a distance of 1 parsec (3.26 light years) from Earth. Over one year, the apparent position of a star in the sky traces out an ellipse, whose semi-major axis is the parallax angle. The most accurate stellar parallaxes to date are supplied by ESA's Gaia mission, a space telescope specifically designed for that task.","term_approval_level":"A","language_code":"en","term_number":237,"term_in_english":"Parallax","based_on_current_english_version":null,"linked_terms":[10],"alternate_terms":[],"categories":["Naked Eye Astronomy","Stars"],"category_ids":[4,2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/237/"},{"term_name":"Parsec","term_definition":"The parsec (pc) is a standard unit of distance measure in astronomy, defined as follows: Imagine a circle with a radius of one astronomical unit (which is the average Earth–Sun distance), which we are viewing face-on. From a distance of exactly one parsec, we will see the circle's radius subtend an angle of one arcsecond. This makes 1 pc equal to approximately 3.26 light years, and to as many astronomical units as there are arcseconds in one radian: 206,264.8 astronomical units. The practical importance of this definition is its relation to the parallax method of distance determination. For this method, one measures tiny position shifts of astronomical objects in the sky as the observer position changes by a given length. For an astronomical object at a distance of one parsec, a change in observer position by one astronomical unit (usually because the Earth has moved along its orbit between the two observations) corresponds to an apparent position shift, called the object's parallax (angle), of one arcsecond. This results in a simple relationship: The distance in parsecs is one divided by the parallax angle in arcseconds. Its direct geometrical meaning makes the parsec the professional astronomer's preferred measure of distance, more common in the literature than distances given in light years. Sirius is 2.7 pc distant from us, corresponding to a parallax of 1/2.7=0.37 arcseconds. Even Proxima Centauri, the closest star apart from our Sun, has a parallax of less than 1 arcsecond.","term_approval_level":"A","language_code":"en","term_number":238,"term_in_english":"Parsec","based_on_current_english_version":null,"linked_terms":[10,178,237,304],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/238/"},{"term_name":"Partial Lunar Eclipse","term_definition":"","term_approval_level":"A","language_code":"en","term_number":239,"term_in_english":"Partial Lunar Eclipse","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":[],"override_term_number":181,"categories":[],"category_ids":[],"override_url":"https://astro4edu.org/resources/glossary/term/181/","url":"https://astro4edu.org/resources/glossary/term/239/"},{"term_name":"Partial Solar Eclipse","term_definition":"","term_approval_level":"A","language_code":"en","term_number":240,"term_in_english":"Partial Solar Eclipse","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":[],"override_term_number":310,"categories":[],"category_ids":[],"override_url":"https://astro4edu.org/resources/glossary/term/310/","url":"https://astro4edu.org/resources/glossary/term/240/"},{"term_name":"Particle","term_definition":"A small constituent of matter may be referred to as a particle. In classical physics, particles move through space, and at each given moment a particle has a definite location. In quantum physics, particle properties are different. Particles do not always have a definite location, and in some ways behave similar to waves.\r\n\r\nParticle physics is the study of the smallest constituents of matter, the so-called elementary particles. For astronomy, the most important matter particles are the electron as well as the quarks, up quark and down quark, that make up the composite particles proton and neutron. Atomic matter consists of protons and neutrons forming the atomic nucleus, surrounded by electrons.\r\n\r\nElectromagnetic radiation, as the main astronomical messenger, consists of quantum particles called photons. At lower photon energies, the wave properties of these quantum particles are most important. Radio astronomers, for instance, describe the electromagnetic radiation they receive not in terms of separate particles, but as electromagnetic waves, characterized by wavelength or frequency. On the other end of the electromagnetic spectrum, for the highest-energy photons, the particle properties are the most important. High-energy astronomers who observe using X-rays or gamma rays typically make their observations using particle detectors, and describe the properties of the radiation they receive in terms of particle energies.","term_approval_level":"A","language_code":"en","term_number":241,"term_in_english":"Particle","based_on_current_english_version":null,"linked_terms":[191,242],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/241/"},{"term_name":"Particle Physics","term_definition":"Particle physics is the branch of physics that studies the smallest constituents of matter, including the particles that atoms are made of (electrons, neutrons, and protons), as well as the particles that neutrons and protons are made of (so-called quarks). These and more exotic, short-lived particles can be produced in particle accelerators. There particles are collided at high energies, resulting in the production of various types of other particles. All known elementary particles and their interactions are described by the so-called standard model of particle physics. Some astrophysical objects, notably the jets of stellar-mass or supermassive black holes, or supernova explosions, are natural particle accelerators. Cosmic particles accelerated in that way reach us as cosmic rays.","term_approval_level":"A","language_code":"en","term_number":242,"term_in_english":"Particle Physics","based_on_current_english_version":null,"linked_terms":[63,70,213,241,348,349,435,441],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/242/"},{"term_name":"Penumbra","term_definition":"The penumbra (Latin for \"almost shadow\") has two meanings. \r\n\r\nIn the first, penumbra refers to the outer, less dark, part of the shadow cast onto a body during an eclipse, in which the light is only partially blocked. For example, in a solar eclipse, observers in the penumbra region will see the Moon only partially cover the Sun and experience a partial eclipse only. \r\n\r\nIn the second, penumbra refers to the distinct outer and brighter part of a sunspot.","term_approval_level":"A","language_code":"en","term_number":243,"term_in_english":"Penumbra","based_on_current_english_version":null,"linked_terms":[310,345,372,475],"alternate_terms":[],"categories":["Naked Eye Astronomy","The Sun"],"category_ids":[4,5],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/243/"},{"term_name":"Perihelion","term_definition":"Most objects in the Solar System have elliptical orbits, with the Sun at one of the foci. Perihelion is the point along the orbit where the orbiting body is closest to the Sun. Mathematically, this point marks one end of the major axis of the ellipse. In this word \"peri\" denotes closest point and \"helion\" denotes the Sun. Accordingly, this word may only be used when the central body is the Sun. When the central body is a star that is not the Sun the term is \"periastron\"; when the central body being orbited is the Earth the term is \"perigee\". The general term regardless of the central body is \"periapsis\".","term_approval_level":"A","language_code":"en","term_number":244,"term_in_english":"Perihelion","based_on_current_english_version":null,"linked_terms":[13,98,232,314],"alternate_terms":[],"categories":["Solar System"],"category_ids":[1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/244/"},{"term_name":"Period","term_definition":"Numerous processes in astronomy are cyclic: They repeat regularly. An example is the orbit of a planet around the Sun, with the planet orbiting the Sun along the same trajectory again and again. The time it takes between one repetition of such a cycle and the next is called the cycle's period. The orbital period of a planet, for instance, is the time it takes the planet to go once around the Sun. The repetition does not need to be perfect, and orbital periods can change slowly over time. For instance, the slight systematic decrease in the period of the first binary neutron star provided the first indirect evidence for the emission of gravitational waves.","term_approval_level":"A","language_code":"en","term_number":245,"term_in_english":"Period","based_on_current_english_version":null,"linked_terms":[112,233,447],"alternate_terms":[],"categories":["Exoplanets & Astrobiology","Naked Eye Astronomy"],"category_ids":[6,4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/245/"},{"term_name":"Phase","term_definition":"In astronomy, phase refers to the state of partial illumination of a moon or planet with respect to a specific observer. A distant light source typically illuminates only half of a spherical body's surface; the rest remains dark. Similarly, only about half of a spherical body's surface is within a distant observer's sight. The phase specifies which fraction of the surface within the observer's sight is illuminated. It changes as the relative positions of the object, the observer, and the light source change.\r\n\r\nThe best-known examples are the phases of the Moon. The relative positions of the Moon, of the Earth as the location of the observer, and of the Sun as the light source, change as the Moon orbits the Earth over the course of about one month. Hence, over that time, an observer on Earth will see different phases of the Moon. The phase where all of the Moon's surface that is in the observer's sight is illuminated is called \"full Moon\"; when no illuminated surface regions are visible, we have \"new Moon\". The two half-illuminated phases are called \"first\" or \"second quarter\", respectively. Less-than-half illumination makes for a \"crescent (Moon)\".\r\n\r\nPhases are also seen in planets in the Solar System (particularly prominent for Mercury and Venus), and have been inferred for exoplanet systems. For the Moon, even non-illuminated regions are not completely dark: they reflect light reaching them from Earth, in a phenomenon known as Earthshine, first documented by Leonardo da Vinci.","term_approval_level":"A","language_code":"en","term_number":247,"term_in_english":"Phase","based_on_current_english_version":null,"linked_terms":[182,475],"alternate_terms":[],"categories":["Naked Eye Astronomy","Solar System"],"category_ids":[4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/247/"},{"term_name":"Photoelectric Effect","term_definition":"When light or other electromagnetic radiation strikes a material, electrons can, depending on the material and frequency of the light, be emitted: This is the photoelectric effect. It can be explained by considering light as particles or packets of energy called photons. For materials, mostly metals, the frequency of light should be greater than a threshold frequency (characteristic of the material) for the emission of these photoelectrons to occur. Their maximum energy is determined by the frequency; increasing the frequency of the light brings about an increase in the maximum kinetic energy of the electrons. On increasing the intensity of monochromatic light, more electrons are released, but there is no change in their maximum kinetic energy. This is because the intensity of the light is directly proportional to the number of photons.","term_approval_level":"A","language_code":"en","term_number":248,"term_in_english":"Photoelectric Effect","based_on_current_english_version":null,"linked_terms":[441],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/248/"},{"term_name":"Photometry","term_definition":"The term photometry is derived from the Greek phōs meaning light, and metro meaning to measure. Although there are various types of photometry, the basic method involves measuring the intensity of the light (photons) radiated by various astronomical objects. The light from objects passes through specialized filters (known as passbands), and that light is captured on a digital device such as a camera CCD. Separate passbands cover ranges of wavelengths, that may include infrared, visible, and ultraviolet. Astronomical telescopes often have filter groups, which are called photometric systems. Some common systems include UBVRI, JHK, and ugriz. Photometry allows different physical characteristics of astronomical objects to be measured, for example, temperature, color, and change in brightness.","term_approval_level":"A","language_code":"en","term_number":249,"term_in_english":"Photometry","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":[],"categories":["Telescopes, Instruments and Observatories"],"category_ids":[3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/249/"},{"term_name":"Photon","term_definition":"The term photon comes from the Greek phōs, which means light, and is therefore used synonymously and interchangeably with light. When astronomers refer to light, they are referring to all types of electromagnetic radiation from radio waves all the way to gamma rays. \r\n\r\nA photon is a fundamental particle, carrier for the electromagnetic force, and considered to be the smallest packet (quanta) of electromagnetic energy. The quanta of energy associated with a photon of given frequency is proportional to this frequency and inversely proportional to its wavelength. \r\n\r\nModern views consider a photon as being not only a particle but also a wave. In astronomy, photons are foundational to our ability to observe and measure various aspects of the Universe and the objects it contains.","term_approval_level":"A","language_code":"en","term_number":250,"term_in_english":"Photon","based_on_current_english_version":null,"linked_terms":[96],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/250/"},{"term_name":"Photosphere","term_definition":"The photosphere (\"light sphere\") is the layer of a star from which the light that we observe emerges. Light emitted from deeper, denser layers will be absorbed before it can escape from the star. Higher layers are less dense, and do not emit significant light.","term_approval_level":"A","language_code":"en","term_number":251,"term_in_english":"Photosphere","based_on_current_english_version":null,"linked_terms":[331],"alternate_terms":[],"categories":["Stars","The Sun"],"category_ids":[2,5],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/251/"},{"term_name":"Pisces","term_definition":"Pisces is a constellation in the Zodiac, i.e. the stars that make up this constellation are in the part of the sky that intersects with the ecliptic – with the plane defined by the Earth's orbit around the Sun. Hence, from our point of view here on Earth, we can regularly find the Sun, and also the other planets in the Solar System, in this constellation. In the case of the Sun this is from around mid-March to mid-April, which includes the time of the March equinox. (Of course if the Sun is there, we cannot see the constellation's stars.) Pisces is one of the 88 modern constellations defined by the International Astronomical Union, but it goes back much further – it was already one of the 48 constellations named by the 2nd century astronomer Claudius Ptolemy.","term_approval_level":"A","language_code":"en","term_number":252,"term_in_english":"Pisces","based_on_current_english_version":null,"linked_terms":[66,92,158,391],"alternate_terms":[],"categories":["Naked Eye Astronomy"],"category_ids":[4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/252/"},{"term_name":"Planet","term_definition":"A planet is defined by the International Astronomical Union as a celestial body in orbit around a star or the remnant of a star, that is large enough to be nearly round in shape by its own gravitational force, but not massive enough for thermonuclear fusion to ever occur in its core. It must also be large enough for its gravity to remove other objects that pass close to its orbit around the star. Therefore, they are cold bodies (as compared to stars) that shine in the visible band only by the light reflected from their stars but they do emit light at infrared wavelengths. In our Solar System, eight planets orbit around the Sun. Planets may be basically rocky objects, such as the inner planets – Mercury, Venus, Earth, and Mars – or primarily liquid and gas with a small solid core like the outer planets – Jupiter, Saturn, Uranus, and Neptune. Planets outside the Solar System are called extrasolar planets or exoplanets for short.","term_approval_level":"A","language_code":"en","term_number":253,"term_in_english":"Planet","based_on_current_english_version":null,"linked_terms":[89,106,125,129,153,158,167,189,192,212,221,234,294,314,354,375,377],"alternate_terms":["major planet"],"categories":["Chemistry","Exoplanets & Astrobiology","Naked Eye Astronomy","Solar System"],"category_ids":[12,6,4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/253/"},{"term_name":"Planet Formation","term_definition":"When a cosmic gas cloud collapses to form a star, that nascent star is surrounded by a swirling disk of gas and dust. This is a protoplanetary disk, where planets form: Ice-covered dust particles will stick to each other, forming slightly larger clumps, which will continue to grow. There are still open questions about how the next stages happen: What, for instance, is the role of turbulent gas motion in bringing those clumps closer together? Eventually, so-called planetesimals more than about a kilometer in size are formed. Some of those are pulled together by their own gravity to form larger planets, others remain behind as the first asteroids. Some protoplanets manage to draw large amounts of gas towards them to become gas giants. Some other protoplanets in cold regions far from the central star will accrete large amounts of frozen material along with gas, becoming ice giants. Others, with less gas, become terrestrial planets.","term_approval_level":"A","language_code":"en","term_number":254,"term_in_english":"Planet Formation","based_on_current_english_version":null,"linked_terms":[81,85,125,153,253,264,354],"alternate_terms":[],"categories":["Exoplanets & Astrobiology"],"category_ids":[6],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/254/"},{"term_name":"Planetarium","term_definition":"A planetarium is a theater with specialized equipment that can be used for visualizing astronomical phenomena. Most planetaria consist of specialized projection equipment and a hemispherical dome. When stars and planets are projected onto the dome, the audience is sitting under an artificial night sky. Traditional, opto-mechanical, projectors are built specifically for simulating the night sky. Digital planetariums instead use one or more \"specialized beamers\", which can project not only the night sky, but any kind of moving image. This allows for \"full dome movies\" not only on astronomy, but on other topics – the audience can travel in a virtual spaceship, but also explore the micro-world of human DNA, for example.","term_approval_level":"A","language_code":"en","term_number":255,"term_in_english":"Planetarium","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":[],"categories":["Naked Eye Astronomy"],"category_ids":[4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/255/"},{"term_name":"Planetary Nebula","term_definition":"A planetary nebula is a cloud of gas and dust, coming from the external layers of a dying star and expanding into the general interstellar medium. The gas, energized by ultraviolet light from the dying star, glows with an emission-line spectrum. Some planetary nebulae are approximately spherical, and may look like a planet in a small telescope but their nature is completely different. Other planetary nebulae are not spherical, due to the star's rotation, magnetic field, or binarity.","term_approval_level":"A","language_code":"en","term_number":256,"term_in_english":"Planetary Nebula","based_on_current_english_version":null,"linked_terms":[85,124,211,334,455,473],"alternate_terms":[],"categories":["Chemistry","Stars"],"category_ids":[12,2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/256/"},{"term_name":"Planetary Science","term_definition":"Planetary science is the branch of science which deals with the nature, origin, and evolution of planets in our Solar System and beyond.","term_approval_level":"A","language_code":"en","term_number":257,"term_in_english":"Planetary Science","based_on_current_english_version":null,"linked_terms":[314],"alternate_terms":[],"categories":["Chemistry","Exoplanets & Astrobiology","Solar System","Space Exploration"],"category_ids":[12,6,1,10],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/257/"},{"term_name":"Plasma","term_definition":"Plasma is a quasi-neutral (i.e. charge balanced)  gas where a significant fraction of the particles are positive ions and negative electrons, often mixed with molecules and neutral atoms. The properties of plasma are so different from those of ordinary neutral gases that plasma is considered a \"fourth state of matter\". The positive ions and negative electrons allow a plasma to conduct electricity. Plasma can form at high temperatures (as in stars) or by photoionization (as in interstellar gas). Plasma is found in stars, the interstellar medium, in the space between the planets in the Solar System in the form of the solar wind, within the planetary magnetosphere, and in the space between galaxies. It is estimated that more than 99% of the ordinary matter in the observable Universe (excluding dark matter and dark energy) is in the plasma state.","term_approval_level":"A","language_code":"en","term_number":258,"term_in_english":"Plasma","based_on_current_english_version":null,"linked_terms":[75,76,124,419,441,450],"alternate_terms":[],"categories":["The Sun"],"category_ids":[5],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/258/"},{"term_name":"Pluto","term_definition":"Pluto is one of the celestial bodies in our Solar System. It is located in the Kuiper Belt, beyond the orbit of Neptune. It used to be known as the ninth Solar System planet, but in 2006 it was reclassified as a dwarf planet. It was discovered in 1930 by Clyde Tombaugh. Pluto's average distance from the Sun is 6 billion kilometers (km), and its radius is 1185 km (smaller than the Earth's moon). One year on Pluto is equivalent to 247.9 Earth years and its day is equivalent to 6.4 Earth days.\r\n\r\nNASA's New Horizon mission was the first spacecraft to fly by Pluto, in 2015, and provided the first ever detailed view of its surface and its atmosphere. The surface of Pluto is so cold that very little of the hydrogen can exist in gaseous form. What little atmosphere Pluto has is mainly nitrogen, and the surface has large plains of frozen nitrogen. Its atmosphere extends to a distance of 1600 km. Pluto is composed of rocks (70%) and ice (30%). It has five known moons: Charon, Styx, Nix, Kerberos, and Hydra.","term_approval_level":"A","language_code":"en","term_number":259,"term_in_english":"Pluto","based_on_current_english_version":null,"linked_terms":[49,87,170,212,314],"alternate_terms":[],"categories":["Solar System"],"category_ids":[1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/259/"},{"term_name":"Polar Axis","term_definition":"","term_approval_level":"N","language_code":"en","term_number":260,"term_in_english":"Polar Axis","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":[],"override_term_number":90,"categories":[],"category_ids":[],"override_url":"https://astro4edu.org/resources/glossary/term/90/","url":"https://astro4edu.org/resources/glossary/term/260/"},{"term_name":"Polaris","term_definition":"Polaris is the nearest bright star (within one degree) to the celestial North Pole. Its official designation is α Ursae Minoris, but it is commonly known as Polaris, the North Star, or the Pole Star. All the stars of the northern hemisphere appear to rotate around it, so it provides an excellent fixed point from which to perform measurements for navigation and astrometry. Its elevation above the horizon gives the approximate latitude of the observing site. However, its position on the celestial sphere is slowly changing due to the precession of the rotation axis of Earth, so in several centuries Polaris will not indicate the location of the celestial North Pole anymore.\r\n\r\nPolaris is a triple star system, composed of the primary, a yellow supergiant designated Polaris Aa, in orbit with a smaller companion, Polaris Ab, a very close main sequence star; the pair has a wide companion, Polaris B, a main sequence star orbiting at a distance of 2400 astronomical units. Polaris B can be resolved with a modest telescope. The Hubble Space Telescope was able to resolve all three members of the Polaris ternary system. The apparent visual magnitude of Polaris fluctuates because Polaris Aa is a Cepheid variable. The Polaris system is about 447 light years from Earth.","term_approval_level":"A","language_code":"en","term_number":261,"term_in_english":"Polaris","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":["alpha Ursae Minoris","North Star","Pole Star"],"categories":["Stars"],"category_ids":[2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/261/"},{"term_name":"Proton","term_definition":"A proton is a subatomic particle with positive electric charge. Protons are one of the main constituents of atomic nuclei, alongside neutrons. The number of protons is the \"atomic number\" of a nucleus, and each atom with a specific atomic number corresponds to a specific chemical element. The atomic nucleus of hydrogen is a single proton, and the most elementary nuclear fusion reaction in a star is when two protons collide, one of them decays, and a deuteron is formed, consisting of one proton and one neutron. Protons can also be found in the high-energy cosmic rays reaching us from outer space.","term_approval_level":"A","language_code":"en","term_number":263,"term_in_english":"Proton","based_on_current_english_version":null,"linked_terms":[31,70,149,221,224,435,441],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/263/"},{"term_name":"Protostar","term_definition":"A protostar is an early stage in the star formation process. It is a large mass of gas and dust formed as a result of the contraction of a giant molecular cloud in the interstellar medium. As the cloud collapses, gravitational energy is converted into heat, warming the still-forming protostar. This phase may take anywhere from 10⁵ to 10⁷ years, depending on the mass of the star, with more massive stars forming more quickly. It begins with an increase in density in the molecular cloud core and ends with the formation of a pre-main-sequence star. Pre-main-sequence stars of similar mass to the Sun are known as T-Tauri stars. Once hydrogen fusion ignites in the core of a star it begins producing energy and becomes a main sequence star.","term_approval_level":"A","language_code":"en","term_number":264,"term_in_english":"Protostar","based_on_current_english_version":null,"linked_terms":[186,333,450,517],"alternate_terms":[],"categories":["Stars"],"category_ids":[2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/264/"},{"term_name":"Proxima Centauri","term_definition":"Proxima Centauri, also known as Alpha Centauri C, is the nearest star to the Sun at a distance of 4.24 light years (1.302 parsecs), or about 40 trillion kilometers. It is a red dwarf star, smaller than the Sun in size and mass, and therefore too faint to be seen with the naked eye. At the time of writing, there is one confirmed planet that orbits Proxima Centauri. It has been designated Proxima Centauri b, and its orbit is in the so-called habitable zone of the star. Two additional candidates for planets, Proxima Centauri c and d, have not yet been confirmed. Together with Alpha Centauri A and B, Proxima Centauri forms the Alpha–Centauri star system, which is comprised of three gravitationally bound stars.","term_approval_level":"A","language_code":"en","term_number":265,"term_in_english":"Proxima Centauri","based_on_current_english_version":null,"linked_terms":[106,276,453],"alternate_terms":[],"categories":["Stars"],"category_ids":[2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/265/"},{"term_name":"Pulsar","term_definition":"A pulsar is a rapidly rotating neutron star whose intense magnetic fields produce two intense beams of radiation in opposite directions. This radiation can be detected as a brief pulse by radio telescopes as it sweeps past our line of sight. Pulsars are about 10–20 kilometers across with a typical mass of about one and a half times that of our Sun. They spin about once to several hundred times a second and can act as very precise celestial clocks. Some pulsars have been detected as gamma ray or X-ray sources. Over 3000 have been detected in our Galaxy. In addition, around 30 of them have also been detected outside the Milky Way in the Magellanic Clouds. Pulsars are useful as probes of the interstellar medium, as a test of general relativity, and potentially useful for detecting gravitational waves from black hole mergers.","term_approval_level":"A","language_code":"en","term_number":266,"term_in_english":"Pulsar","based_on_current_english_version":null,"linked_terms":[123,214,272,450,455,487],"alternate_terms":[],"categories":["Stars"],"category_ids":[2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/266/"},{"term_name":"Quarter Moon","term_definition":"","term_approval_level":"N","language_code":"en","term_number":267,"term_in_english":"Quarter Moon","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":[],"override_term_number":182,"categories":[],"category_ids":[],"override_url":"https://astro4edu.org/resources/glossary/term/182/","url":"https://astro4edu.org/resources/glossary/term/267/"},{"term_name":"Quasar","term_definition":"Short for quasi-stellar radio source, quasars are distant extragalactic sources. Quasars appear as stars (hence, quasi-stellar) and were initially detected with radio telescopes. Observations reveal that they are associated with the region around the most massive supermassive black holes at the center of galaxies. Quasars are a subclass of active galactic nuclei (AGN), which includes radio and Seyfert galaxies, blazars, and low-ionization nuclear emission-line regions (LINERs). Quasars can be as much as 100 times more luminous than their host galaxy. Some also have giant jets which interact with the gas around and within their host galaxy. Because of their high luminosities, quasars can be used to study distant galaxies, intervening galaxies, and the intergalactic medium. 3C 273 was the first quasar detected.","term_approval_level":"A","language_code":"en","term_number":268,"term_in_english":"Quasar","based_on_current_english_version":null,"linked_terms":[5,43,272],"alternate_terms":["quasi-stellar object (QSO)"],"categories":["Galaxies"],"category_ids":[8],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/268/"},{"term_name":"Radial Velocity","term_definition":"When astronomers observe a distant object, the radial velocity is the part of the object's motion along the observer's line of sight – taking the object directly away from or directly towards the observer. Radial velocities are measured using the Doppler effect: For astronomical objects moving towards us, spectral lines are shifted towards smaller wavelengths (blueshift); for objects moving away from us, the lines are shifted towards larger wavelengths (redshift). Radial velocity is always relative to an observer. Precision measurements often give their results in terms of radial motion relative to the center of mass of the Solar System (\"barycentric\"), subtracting the influence of Earth's motion around the Sun. Objects that are orbiting each other have radial velocities that vary over time. The effect is strongest when we happen to see the orbital plane edge-on. In that case, orbital motion will periodically, and by turns, take each of the objects directly away from us and directly towards us. Radial velocity measurements can be used to deduce such orbital motions even when the objects themselves are not separately visible. Since higher masses induce higher velocities, such measurements can  be used to estimate the orbiting objects' masses. This has been used to discover binary stars, or exoplanets orbiting stars, and for reconstructing the masses of star clusters and whole galaxies. A systematic analysis of galaxies' distances and radial velocities led to the discovery of cosmic expansion.","term_approval_level":"A","language_code":"en","term_number":269,"term_in_english":"Radial Velocity","based_on_current_english_version":null,"linked_terms":[84,328],"alternate_terms":[],"categories":["Exoplanets & Astrobiology"],"category_ids":[6],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/269/"},{"term_name":"Radiative Zone","term_definition":"Energy is released by nuclear fusion reactions in the core of a star, and eventually radiated away into space from the star's photosphere. There are several ways that energy is transported from the star's core to the photosphere. The radiative zone, radiative region, or radiation zone is the region within a star where the energy is transported outwards by means of radiation, with photons repeatedly scattering off nuclei and electrons, losing some energy in the process but also leading to the emission of new thermal-radiation photons. Due to frequent scattering, progress is slow; in our Sun, photons need thousands of years to cross the radiative zone. \r\n\r\nIn the Sun the radiative zone lies between the core and the convective zone. In more massive stars the core itself is convective with the radiative zone extending from the convective core to the star's photosphere. Below 0.3 solar masses, stars have no radiative zone at all and are entirely convective.","term_approval_level":"A","language_code":"en","term_number":271,"term_in_english":"Radiative Zone","based_on_current_english_version":null,"linked_terms":[312,331,338,433,441,478],"alternate_terms":["radiative envelope"],"categories":["Stars","The Sun"],"category_ids":[2,5],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/271/"},{"term_name":"Radio Astronomy","term_definition":"Radio astronomy is the branch of astronomy concerned with observations of radio waves, a special region in the electromagnetic spectrum. Earth's atmosphere has \"radio windows\": It allows radio waves in certain frequency (or wavelength) ranges to pass nearly unhindered. That fact allows for observations of such radio waves from astronomical objects with ground-based radio telescopes. Typical observing frequencies range from an upper limit of about 300 gigahertz (GHz) down to tens of megahertz (MHz). This corresponds to wavelengths of 1 millimeter (mm) to tens of meters, respectively. By going to particularly suitable dry, high-altitude locations, astronomers can even perform submillimeter observations, down to wavelengths of about 0.3 mm, corresponding to frequencies of up to 1 terahertz (THz). The lower frequency limit at about 10 MHz is due to Earth's so-called ionosphere. That high-altitude region within our atmosphere contains numerous charged particles, which reflect ultra-long radio waves right back into space. Radio astronomy enables us to observe the emission from cold gas in galaxies and the Milky Way, such as atomic hydrogen and molecular gas. In this way, astronomers can study the diffuse interstellar medium, as well as the regions and processes in which stars and planets are born. Radio astronomy also allows for the study of highly energetic objects such as pulsars and active galactic nuclei: In or around objects like those, electrons are accelerated in a strong magnetic field, leading to the emission of radio waves known as synchrotron radiation. Pulsars and the very bright active galactic nuclei known as quasars were discovered using radio astronomy, as was the remnant of our Universe's hot Big Bang phase, the cosmic microwave background (CMB). Although radio waves from space were first detected in the 1930s, radio astronomy only became a major branch of observational astronomy after 1950.","term_approval_level":"A","language_code":"en","term_number":272,"term_in_english":"Radio Astronomy","based_on_current_english_version":null,"linked_terms":[96,274],"alternate_terms":[],"categories":["Telescopes, Instruments and Observatories"],"category_ids":[3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/272/"},{"term_name":"Radio Telescope","term_definition":"Radio telescopes receive radio waves from space.  Depending on the observing wavelength they may have a parabola shape, similar to a satellite dish with a receiver at the focal point, or may have metal rod-like figures sometimes referred to as dipole antennas. The signals are then amplified and processed by computer. A radio telescope can be a single dish or a number of antennas linked together to form an interferometer where a special computer called a correlator combines signals from the different radio telescopes to yield information that can then be processed into an image. They mostly observe radio waves, with frequencies ranging from about 30 megahertz to 300 gigahertz, or 10 meters to 1 millimeter in wavelength. Individual telescopes and receivers are optimized for specific regions within this band. Some radio dishes are optimized to observe light with slightly shorter wavelength in a region of the electromagnetic spectrum known as the submillimeter.","term_approval_level":"A","language_code":"en","term_number":273,"term_in_english":"Radio Telescope","based_on_current_english_version":null,"linked_terms":[96,272,274],"alternate_terms":[],"categories":["Telescopes, Instruments and Observatories"],"category_ids":[3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/273/"},{"term_name":"Radio Waves","term_definition":"Radio waves are the longest wavelength, lowest frequency, and lowest energy part of the electromagnetic spectrum. They range from a shortest wavelength of about 1 millimeter and frequency of 300 gigahertz, to many kilometers in wavelength and megahertz in frequency. Our atmosphere is transparent for most of this waveband, allowing radio telescopes to be located on the ground.","term_approval_level":"A","language_code":"en","term_number":274,"term_in_english":"Radio Waves","based_on_current_english_version":null,"linked_terms":[96,198,273],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/274/"},{"term_name":"Rainbow","term_definition":"The huge arc or bow with concentric stripes in the colors violet, indigo, blue, green, yellow, orange, and red, spread out across the sky, usually visible after it has rained, is called a rainbow. When an observer sees a rainbow, the Sun is at the observer's back. A rainbow occurs because the small droplets of water in the air break up the white sunlight into the color spectrum through a process called dispersion due to refraction; this is similar to how a prism works. In a normal rainbow, light is reflected once within the water droplets as well as being dispersed through refraction.\r\n\r\nSometimes, two nested rainbows can be seen, where the colors in the second rainbow are in reverse order. The inner, brighter one is called the primary rainbow while the outer, fainter one is the secondary rainbow. This double rainbow phenomenon is relatively uncommon. The secondary rainbow occurs when light undergoes reflection twice within the water droplets in addition to refraction.","term_approval_level":"A","language_code":"en","term_number":275,"term_in_english":"Rainbow","based_on_current_english_version":null,"linked_terms":[282,328,378],"alternate_terms":[],"categories":["Naked Eye Astronomy"],"category_ids":[4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/275/"},{"term_name":"Red Dwarf","term_definition":"A red dwarf is a small, low-mass star with an effective temperature below 3900 kelvins (K) (~3600 degrees Celsius (°C)). These would appear redder than yellow Sun-like stars to the human eye, but there are no red dwarfs visible to the naked eye. Stars with masses between about 8% and 50% of the mass of the Sun spend the vast majority of their lives as red dwarfs. These stars fuse hydrogen at a much slower rate than stars like the Sun and are thus fainter, but can sustain hydrogen fusion for much longer. Many red dwarfs have very strong magnetic fields which result in more magnetic storms and a higher number of starspots than stars like the Sun. The majority of stars in the Milky Way are red dwarfs as are the majority of the stars within 10 parsecs of the Sun.","term_approval_level":"A","language_code":"en","term_number":276,"term_in_english":"Red Dwarf","based_on_current_english_version":null,"linked_terms":[186,199,238,331,440,453,455,479],"alternate_terms":["M dwarf"],"categories":["Stars"],"category_ids":[2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/276/"},{"term_name":"Red Giant","term_definition":"A red giant is a star that has a comparatively cool surface, but a diameter typically tens, or sometimes hundreds,  times larger than that of a main sequence star like our Sun. The comparatively low temperature gives the surface a reddish appearance, while the large diameter makes the star shine considerably brighter than our Sun. Red giants were originally main sequence stars, that is, Sun-like stars that burn hydrogen to helium in their cores via nuclear fusion. When such a star runs out of hydrogen fuel, it begins to burn helium into heavier elements. At that time,  the star expands, its surface cooling down in the process, and with its increased size the star becomes more luminous. The Sun will enter the red giant phase billions of years from now, at which time life on Earth will likely not survive. Examples of red giants are Arcturus in the constellation Boötes and Mira in the constellation Cetus. Red giants are unstable to pulsation (an oscillation where the star repeatedly becomes smaller and larger in turn) and, as they pulsate, can vary in brightness. The star Mira is an extreme example that can vary in brightness by a factor of 1000. In the spectral classification used by astronomers, red giants are mostly of spectral type M, and some are of spectral type K. Red giants are similar to so-called red supergiants, but with a lower mass.","term_approval_level":"A","language_code":"en","term_number":277,"term_in_english":"Red Giant","based_on_current_english_version":null,"linked_terms":[186,278,331,334,349,453],"alternate_terms":["M giant"],"categories":["Naked Eye Astronomy","Stars"],"category_ids":[4,2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/277/"},{"term_name":"Red Supergiant","term_definition":"A red supergiant is a star that has a comparatively cool surface but a radius typically several hundred or even thousands of times that of a main sequence star like our Sun. The lower temperature gives the surface a reddish appearance, while the large diameter makes the star shine considerably brighter than our Sun. \r\n\r\nRed supergiants are formed in the same way as red giants but from precursor stars that are considerably more massive.  Both types were originally main sequence stars, that is, stars like our Sun that burn hydrogen to helium in their cores via nuclear fusion (although with masses much higher than the Sun's in the case of red supergiants). When such a star runs out of hydrogen fuel, it begins to burn helium into heavier elements. At that time,  the star expands, its surface cooling down in the process, and with its increased size the star becomes more luminous. The remaining lifetimes of red supergiants amount to only a few tens of thousands of years. \r\n\r\nExamples of red supergiants are Betelgeuse, in Orion, and Antares, in Scorpius. Like red giants, red supergiants are prone to pulsation and mass loss. Most, possibly all, red supergiants end in a supernova explosion, ejecting much of their gas, with their cores collapsing to form neutron stars or black holes.","term_approval_level":"A","language_code":"en","term_number":278,"term_in_english":"Red Supergiant","based_on_current_english_version":null,"linked_terms":[186,277,325,331,334,349,453,467],"alternate_terms":["M supergiant"],"categories":["Naked Eye Astronomy","Stars"],"category_ids":[4,2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/278/"},{"term_name":"Reflecting Telescope","term_definition":"In a reflecting telescope or mirror telescope, the main optical element is a mirror, the \"primary mirror\", which gathers infalling light. Mirror telescopes are often characterized by primary mirror diameter, ranging from 10 centimeters for smaller amateur telescopes to eight meters for the largest solid mirrors used in professional telescopes. Still larger collecting areas can be obtained by combining several mirror segments, which then act in a way similar to that of a larger solid mirror. There are several types of mirror telescopes. For example, in a Newtonian telescope, the light coming from the primary mirror is reflected sideways by a smaller, flat mirror into an eyepiece or a camera. In a Cassegrain telescope, a smaller, convex secondary mirror reflects the light back through an opening in the main mirror.","term_approval_level":"A","language_code":"en","term_number":279,"term_in_english":"Reflecting Telescope","based_on_current_english_version":null,"linked_terms":[281,352,459],"alternate_terms":["mirror telescope",""],"categories":["Telescopes, Instruments and Observatories"],"category_ids":[3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/279/"},{"term_name":"Refracting Telescope","term_definition":"A refracting telescope is a telescope that uses a lens as its main light-gathering element. This is opposed to a reflecting telescope, which uses a mirror in this role. Refracting telescopes are still common as amateur telescopes, where a special combination of lenses that correct for unwanted color effects (\"achromatic telescopes\") can produce excellent image quality for visual observing and for astrophotography. In professional astronomy, refracting telescopes were largely displaced by mirror telescopes from the early 1900s onwards. Astronomers wanted ever-larger apertures (lens or mirror diameters), and it is difficult to make refracting telescopes beyond lens sizes of about one meter as a lens is only supported at the rim, leading to the heavy center of the lens sagging under gravity, distorting the lens's shape and optical properties.","term_approval_level":"A","language_code":"en","term_number":281,"term_in_english":"Refracting Telescope","based_on_current_english_version":null,"linked_terms":[122,279,282,352,451,459],"alternate_terms":[],"categories":["Telescopes, Instruments and Observatories"],"category_ids":[3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/281/"},{"term_name":"Refraction","term_definition":"Refraction is the way that a wave – and, specifically, light – changes direction as it passes from one medium into another. The magnitude and direction of the change depends on the \"indices of refraction\" of the two media, which in turn depend on the speed of light in each medium, a relation that is encoded mathematically in Snell's law of refraction. The way that light passes into a piece of glass can be used to create a lens, which bundles parallel light rays – such as the light of a distant star – falling onto the lens. This is the main effect used in constructing refracting telescopes. Refraction also depends on the wavelength of the infalling light, a fact that can be used as in a prism, to separate light by wavelength into its constituent elementary colors – which is important for documenting and examining spectra.","term_approval_level":"A","language_code":"en","term_number":282,"term_in_english":"Refraction","based_on_current_english_version":null,"linked_terms":[275,281,328,451],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/282/"},{"term_name":"Angular Resolution","term_definition":"Resolution, or angular resolution is the smallest angle between two close point-like objects that can be seen as just separated. It can also be thought of as the spread of a point-like object (such as a star), that is mainly due to optics of the telescope. This is a very important characteristic of telescopes, as telescopes with higher angular resolution enable us to visually separate stars seen very close to each other as well as to see finer details in extended objects such as nebulae and galaxies. Two stars with angular separation smaller than the resolution will appear as a single object. The resolution of a telescope can be improved by increasing the size of its light collecting mirror or lens. It also depends on the wavelength and becomes poorer as the wavelength increases.","term_approval_level":"A","language_code":"en","term_number":284,"term_in_english":"Angular Resolution","based_on_current_english_version":null,"linked_terms":[352,451,459],"alternate_terms":["angular resolution"],"categories":["Telescopes, Instruments and Observatories"],"category_ids":[3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/284/"},{"term_name":"Right Ascension (RA)","term_definition":"Right ascension is one of two coordinates in the equatorial coordinate system (the other being declination), which astronomers use to define the positions of celestial objects in the sky. As seen from Earth, all the various positions in the sky together form what appears to be a distant sphere with Earth at its center. The points in the sky  directly above Earth's equator form the celestial equator on that sphere. The point directly above Earth's geographic North Pole is the celestial North Pole, and that above Earth's South Pole, the celestial South Pole. Just like geographers define geographic longitude and latitude on Earth's surface, one can define longitude and latitude on the celestial sphere. If we were to choose a celestial object's longitude coordinate to be that of the location on Earth directly below, a star's coordinate value would change over time as the Earth turns. Instead, equatorial coordinates measure right ascension as a form of celestial longitude relative to a \"meridian\" in the sky that does not rotate with Earth, but instead is fixed relative to the fixed stars. That meridian, the analog of the Greenwich meridian on Earth, is defined by where it intersects the celestial equator: At the exact point where the Sun's apparent path crosses the celestial equator from the southern to the northern celestial hemisphere. This longitude is called right ascension. Its value increases towards the east. Look towards the celestial equator, and the longitude values will pass you by in the course of (roughly) 24 hours. That is why right ascension is typically stated as a time value, with 24 hours corresponding to the full 360 degrees. Declination, the second equatorial coordinate, corresponds to geographic latitude. A slight wobble in Earth's rotation axis known as precession makes the equatorial coordinate system, and with it the right ascension and declination of stars and other celestial objects, change over time, but only very slightly and very slowly.","term_approval_level":"A","language_code":"en","term_number":286,"term_in_english":"Right Ascension (RA)","based_on_current_english_version":null,"linked_terms":[50,78,171,179,193,496,497,498],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/286/"},{"term_name":"Ring","term_definition":"The four giant planets in our Solar System (Jupiter, Saturn, Uranus, and Neptune) – for exoplanets, we cannot yet tell – are surrounded by numerous small pieces of ice or rock, micrometers to meters in size, in the shape of large rings. The most spectacular rings are those surrounding Saturn: An intricate system of rings separated by gaps. Some of that structure comes about through interaction with Saturn's larger moons, and two gaps are opened up by tiny moons orbiting inside them. There are several hypotheses about how the rings formed, most of them involving a moon torn apart or stripped through Saturn's gravity. There are estimates that Saturn's rings will have dissolved in a few 100 million years – not a very long time by astronomical standards. Jupiter, Uranus, and Neptune have less pronounced ring systems.","term_approval_level":"A","language_code":"en","term_number":287,"term_in_english":"Ring","based_on_current_english_version":null,"linked_terms":[129,167,212,294,360,375],"alternate_terms":[],"categories":["Exoplanets & Astrobiology","Solar System"],"category_ids":[6,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/287/"},{"term_name":"Rocket","term_definition":"A rocket is a device that is commonly used to launch spacecraft from Earth's surface into outer space. To that end, a rocket consists of rocket engines and fuel tanks. Rocket engines are also used to control spacecraft motion in outer space, changing the speed or performing course corrections. The basic principle of a rocket engine is to produce a stream of high-speed particles, usually by burning rocket fuel. When such a stream is pointed in a specific direction the rocket engine is accelerated in the opposite direction – a consequence of a fundamental law of physics called momentum conservation. Note that the particle stream does not need to push against anything for this effect to occur: rockets work perfectly even in the near-vacuum of outer space.","term_approval_level":"A","language_code":"en","term_number":288,"term_in_english":"Rocket","based_on_current_english_version":null,"linked_terms":[324],"alternate_terms":[],"categories":["Space Exploration"],"category_ids":[10],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/288/"},{"term_name":"Rover","term_definition":"A rover is a human-made machine that is sent to the surface of another planet or moon on a spacecraft to study that planet up close, and send the data it collects back to Earth using a communication method. Most can be operated from Earth and move around the surface of the planet or moon, although the Lunar Roving Vehicles of the Apollo mission were driven by astronauts on the Moon. A rover can carry a lot of scientific instruments such as small drills, a sample collecting tool, cameras, and even a small laboratory to analyze its air and ground samples and send the results back.","term_approval_level":"A","language_code":"en","term_number":291,"term_in_english":"Rover","based_on_current_english_version":null,"linked_terms":[14,204,253,257,324],"alternate_terms":[],"categories":["Space Exploration"],"category_ids":[10],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/291/"},{"term_name":"Sagittarius A*","term_definition":"Observations of the motions of stars and gas around the center of the Milky Way provide indirect evidence of a supermassive black hole with a mass around 4.5 million times that of the Sun, and approximately 40 million kilometers across, located about 27,000 light years away from Earth. Sagittarius A* (Sagittarius A-star) is the compact radio source that is associated with the supermassive black hole. It has been observed in a range of wavelengths, most notably in radio. The name Sagittarius is because it is located in the constellation of Sagittarius as observed from Earth; the letter A is because it is the brightest and first extrasolar radio source discovered in the constellation; the asterisk is because in physics atoms in excited states are denoted by *, and Sagittarius A* was an exciting discovery. \r\n\r\nIn 2022, the Event Horizon Telescope Collaboration released the first ever image of the silhouette (\"shadow\") of the black hole associated with Sagittarius A*.","term_approval_level":"A","language_code":"en","term_number":292,"term_in_english":"Sagittarius A*","based_on_current_english_version":null,"linked_terms":[116,274,348,474],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/292/"},{"term_name":"Artificial Satellite","term_definition":"An artificial satellite is a human-made device that is sent into space, to orbit Earth or other Solar System objects, where gravity keeps it in orbit. Artificial satellites can be built to perform various tasks including taking aerial photographs of Earth that help meteorologists predict the weather, or taking pictures of astronomical bodies and distant galaxies, which helps scientists to better understand the cosmic system. Artificial satellites are also used primarily for communications around the world and for finding one's position, e.g. the Global Positioning System (GPS). The first artificial satellite was launched into space in 1957 by the Soviet Union, and it was called Sputnik 1.","term_approval_level":"A","language_code":"en","term_number":293,"term_in_english":"Artificial Satellite","based_on_current_english_version":null,"linked_terms":[204,412],"alternate_terms":[],"categories":["Astronomy and Society","Naked Eye Astronomy","Solar System","Space Exploration"],"category_ids":[11,4,1,10],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/293/"},{"term_name":"Saturn","term_definition":"Saturn is the sixth planet from the Sun, and the second largest of the planets in terms of size and mass. It is a gas giant with a diameter of 120,000 kilometers (km), 9.4 times the radius of the Earth. Saturn has the lowest density of the Solar System planets, less dense than water on Earth. It has a mass of 95 times the mass of the Earth.\r\n\r\nIts typical distance from the Sun is 1.4 billion km, about 9.5 astronomical units (Earth–Sun distances). Saturn takes 29.4 years to complete one orbit of the Sun. Astronomers have detected more than 140 moons or natural satellites orbiting Saturn. Among these moons, Titan is the largest, and is the only moon in the Solar System with a significant atmosphere.\r\n\r\nNamed after the Roman god of agriculture, Saturn is known as the jewel of the Solar System. It can be seen with the naked eye as a matte point of light in the sky. Even though it is over a billion kilometers away from Earth, the beautiful rings that surround it can be seen with a small telescope.","term_approval_level":"A","language_code":"en","term_number":294,"term_in_english":"Saturn","based_on_current_english_version":null,"linked_terms":[26,125,129,234,287,314,363],"alternate_terms":[],"categories":["Naked Eye Astronomy","Solar System"],"category_ids":[4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/294/"},{"term_name":"Science","term_definition":"The word science is derived from the Latin scientia, which means knowledge. However, in the modern context science refers to the disciplines and subdisciplines that develop knowledge by using data from experimentation and observation. Each scientific discipline may have its own methods and approaches, but they share similarities that include objectivity, logic, rational thinking, and providing evidence-based explanations of the underlying physical mechanisms for phenomena and objects. As such, science is not based upon belief or authority; science is self-correcting as revealed by experiments and observations. Science can also refer to the body of knowledge relating to a particular discipline.","term_approval_level":"A","language_code":"en","term_number":295,"term_in_english":"Science","based_on_current_english_version":null,"linked_terms":[296,297],"alternate_terms":[],"categories":["Astronomy and Society"],"category_ids":[11],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/295/"},{"term_name":"Scientific Method","term_definition":"Often inaccurately portrayed as a linear process, in reality the scientific method is dynamic, and layered. There is no universal or fixed scientific method, and the methods that scientists use to explore the physical world vary. Key aspects of a scientific approach include the importance and requirement of empirical data to support theories, and that the results should be reproducible and checked by other researchers. Scientific method encompasses a range of methodological approaches underpinned by philosophical principles, and may have some defining characteristics like experimentation, systematic observation, or evidence-based explanations, but is not limited to these. There is no single, universal way of doing science, and the aims of the research guide the method. Nature is complex, and one universal method of inquiry will not be able to adequately unravel its mysteries.","term_approval_level":"A","language_code":"en","term_number":296,"term_in_english":"Scientific Method","based_on_current_english_version":null,"linked_terms":[295,297],"alternate_terms":[],"categories":["Astronomy and Society"],"category_ids":[11],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/296/"},{"term_name":"Scientific Model","term_definition":"Scientific models can be physical, mathematical, conceptual, or analogic, and they aim to represent and/or explain some aspect(s) of a process, system, or phenomena. Scientific models can also be used to make predictions, although that does not mean models that do not make predictions are not scientific. Scientific models are not always meant to be \"factual\" representations of the world, but rather they are tools for allowing us to explore concepts that would otherwise be abstract, intangible, and challenging to understand. One example is the geocentric model: although it does not represent the reality of the Solar System, it is used when discussing ideas related to the celestial sphere. Sometimes multiple models may be required to fully explain a concept.","term_approval_level":"A","language_code":"en","term_number":297,"term_in_english":"Scientific Model","based_on_current_english_version":null,"linked_terms":[295,296],"alternate_terms":[],"categories":["Astronomy and Society"],"category_ids":[11],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/297/"},{"term_name":"Scorpius","term_definition":"Scorpius (the Scorpion, commonly known as Scorpio) is one of the 13 constellations of the Zodiac. The stars that make up this constellation are in the part of the sky that intersects with the ecliptic (the plane defined by the Earth's path around the Sun). In fact, all the constellations that comprise the Zodiac intersect the ecliptic. From Earth, we can regularly find the Sun, and also the other planets of the Solar System, in the constellation Scorpius. It is one of the 88 constellations recognized by the International Astronomical Union. Scorpius was also one of the original 48 constellations identified by Ptolemy; it was identified by the Sumerians over a thousand years prior to him. Other cultures have different names for Scorpius: it is known as Maui’s Hook in Māori and Polynesian cultures, and Indigenous Australian groups like the Yolngu people of Arnhem Land identify Scorpius as both a crocodile and a scorpion. The brightest star in the constellation is Antares (known as alpha Scorpii), a red giant star around 550 light years from Earth.","term_approval_level":"A","language_code":"en","term_number":299,"term_in_english":"Scorpius","based_on_current_english_version":null,"linked_terms":[66,138,158,391],"alternate_terms":["Scorpio"],"categories":["Naked Eye Astronomy"],"category_ids":[4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/299/"},{"term_name":"Seasons","term_definition":"Earth's axis is not perpendicular to Earth's orbit around the Sun, but inclined at an angle of 23.4 degrees. In consequence, the apparent position of the Sun in the sky at a particular time of day, varies over the year. Whenever the Sun is, on average, higher in the sky, more sunlight reaches a given area of ground. Over the year, this leads to warmer and cooler stretches of time, more pronounced for regions farther away from Earth's equator, which are called the seasons. Northern hemisphere seasons are opposite to southern seasons: Northern summer when the northern hemisphere is tilted maximally towards the Sun, is southern winter, with the southern hemisphere tilted away, and vice versa for southern summer. Many parts of the Earth close to the equator have seasons that are different from the summer and winter pattern seen at temperate and arctic latitudes. It should be noted that the duration, beginning, and end of each season can be influenced by cultural practices and time period.","term_approval_level":"A","language_code":"en","term_number":300,"term_in_english":"Seasons","based_on_current_english_version":null,"linked_terms":[34,102,171,317,439],"alternate_terms":[],"categories":["Astronomy and Society","Naked Eye Astronomy"],"category_ids":[11,4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/300/"},{"term_name":"Shooting Star","term_definition":"A shooting star (or meteor) is a fragment of an asteroid or comet, or a piece of space debris, entering the atmosphere of the Earth or another celestial body and catching fire due to heat generated by friction with the atmosphere. This heat generated by friction is similar to the way we warm our hands by rubbing them together when we are cold. Shooting stars are usually very small in size ranging from a few millimeters to a few centimeters. Astronomers call them meteors. Their direction, time of year we observe them, and color allow us to learn more about where they originated from and what they are made of. \r\n\r\nThe brightest meteors are called fireballs and they can often be seen in the sky, sometimes even during the day. Occasionally, observers have even reported hearing sounds as they burn and travel through the atmosphere. Shooting stars also causes the ionization of the atmosphere which, for Earth, can be observed with radar.\r\n\r\nOn Earth, shooting stars are visible throughout the year, but occasionally many shooting stars occur in short timescales such as on the same night. Many of these meteor showers recur on predictable yearly patterns and have been given names based on the constellation the meteors appear to originate (or radiate) from. Famous meteor showers include the Perseids and the Leonids.","term_approval_level":"A","language_code":"en","term_number":302,"term_in_english":"Shooting Star","based_on_current_english_version":null,"linked_terms":[49,196,197],"alternate_terms":["Fireball","meteor","meteor shower"],"categories":["Naked Eye Astronomy","Solar System"],"category_ids":[4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/302/"},{"term_name":"Sidereal Time","term_definition":"Sidereal time is a measure of time based on the Earth's rotation relative to distant stars in the night sky. At night, we can see the pattern of stars in the night sky rotate around us. If we pick a star and note when it reaches its highest position in the sky,  then exactly one sidereal day later, namely 23 hours, 56 minutes, and 4.1 seconds later as measured by our clocks, the star will again reach that highest position.\r\n\r\nThis is subtly different from a solar day, which is the time between local noon – defined as when the Sun reaches its highest point in the sky – on two consecutive days. The reason is that the Earth orbits the Sun. For an observer on the Earth, this introduces additional changes of the Sun's position in the sky over the course of one year. During the time it takes for the Earth to rotate once with respect to the distant stars, the Sun has moved and the Earth needs to rotate a little bit more to catch up. That is why a solar day is a little bit longer than a sidereal day.\r\n\r\nSidereal time is important for astronomers as it tells them which parts of the sky are overhead at a particular point during the day or night, and thus which objects can be observed. In a standard astronomical coordinate system, the equatorial system, the sidereal time at any place on Earth (excluding the poles) is an angle, namely the right ascension (one of the coordinates used to specify locations in the sky  in that system) of the point in the sky directly overhead. In practice, when observing, present-day astronomers make use of the time measured by highly precise atomic clocks, and then use computers to calculate which clock time corresponds to which sidereal time.","term_approval_level":"A","language_code":"en","term_number":303,"term_in_english":"Sidereal Time (Day Month Year)","based_on_current_english_version":null,"linked_terms":[90,286,309],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/303/"},{"term_name":"Sirius","term_definition":"Sirius, also called the Dog Star or Alpha Canis Majoris, is the star that appears brightest in the night sky to our eyes. It is located at a distance of 8.6 light years from us in the constellation Canis Major near Orion. The bright star that is visible to us is Sirius A, a star of spectral type A. Sirius A has a gravitationally bound companion, Sirius B, which is a white dwarf star that is very faint in visible light but very bright in X-rays. Sirius B is too faint and too close to Sirius A to be seen with the naked eye.","term_approval_level":"A","language_code":"en","term_number":304,"term_in_english":"Sirius","based_on_current_english_version":null,"linked_terms":[1,40,138,386,467,487],"alternate_terms":[],"categories":["Naked Eye Astronomy","Stars"],"category_ids":[4,2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/304/"},{"term_name":"Sky","term_definition":"The sky is what we see when we are outside, our view unencumbered by any structure on Earth, looking up or at least higher than the horizon that marks the boundary of what we can see of Earth and earthbound structures. When looking at a clear sky at night, we can see distant planets, stars, and even a few galaxies (the Andromeda galaxy in the northern and the Magellanic Clouds in the southern hemisphere). The twinkling of the stars is evidence that we are still looking through the gases of Earth's atmosphere. During the day, sunlight scattered by air molecules makes the sky shine with a blue color, blocking our view into the cosmos. Clouds or fog covering the sky will also keep us from observing any astronomical objects. ","term_approval_level":"A","language_code":"en","term_number":305,"term_in_english":"Sky","based_on_current_english_version":null,"linked_terms":[53,145,454,462],"alternate_terms":[],"categories":["Naked Eye Astronomy"],"category_ids":[4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/305/"},{"term_name":"Sky Map","term_definition":"A map that points out the position of astronomical objects (stars, planets, the Sun, the Moon, etc.) on the sky at a particular point in time. \r\n\r\nSky maps allow astronomers to find the locations of objects in the night sky and can be used for navigation. \r\n\r\nModern astronomical researchers make maps of the sky as part of astronomical surveys which record a wide variety of information about the objects they study.\r\n\r\nInteractive sky maps for the general public are available as computer programs or mobile apps.","term_approval_level":"A","language_code":"en","term_number":306,"term_in_english":"Sky Map","based_on_current_english_version":null,"linked_terms":[210,305,529],"alternate_terms":["Celestial map","Star chart"],"categories":["Astronomy and Society","Naked Eye Astronomy"],"category_ids":[11,4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/306/"},{"term_name":"Solar Constant","term_definition":"The solar constant is the average amount of electromagnetic radiation (light energy) from the Sun that is received per unit time per unit area (oriented normal to the Sun's direction) at the distance of one astronomical unit (the average distance from the Earth to the Sun). It is currently equal to about 1361 watts per square meter in vacuum (outside the Earth's atmosphere). While not a physical constant, it varies by less than 0.2% over hundreds of years. However, because Earth's orbit is elliptical the flux density at the distance of Earth varies by 7%. As the Sun evolves it is slowly getting brighter, so over billions of years this value will increase significantly.","term_approval_level":"A","language_code":"en","term_number":307,"term_in_english":"Solar Constant","based_on_current_english_version":null,"linked_terms":[96,342],"alternate_terms":[],"categories":["The Sun"],"category_ids":[5],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/307/"},{"term_name":"Solar Cycle","term_definition":"The solar cycle is the approximately 11-year cycle in the number of sunspots, solar flares, and other forms of activity on the Sun. This activity is generated by the Sun's general magnetic field. Since that magnetic field switches between north and south polarity every eleven years, it takes a full 22-year period for the Sun's magnetic poles to return to their original configuration.","term_approval_level":"A","language_code":"en","term_number":308,"term_in_english":"Solar Cycle","based_on_current_english_version":null,"linked_terms":[342,346,455,479],"alternate_terms":["Sunsport cycle"],"categories":["Stars","The Sun"],"category_ids":[2,5],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/308/"},{"term_name":"Solar Day","term_definition":"","term_approval_level":"A","language_code":"en","term_number":309,"term_in_english":"Solar Day","based_on_current_english_version":null,"linked_terms":[305,342],"alternate_terms":[],"override_term_number":436,"categories":[],"category_ids":[],"override_url":"https://astro4edu.org/resources/glossary/term/436/","url":"https://astro4edu.org/resources/glossary/term/309/"},{"term_name":"Solar Eclipse","term_definition":"A solar eclipse occurs when the Earth, Moon, and the Sun are arranged in a straight line, with the Moon between Earth and the Sun. When observed from the surface of the Earth, the disk of the Moon covers the disk of the Sun in the sky; from space we can see the shadow of the Moon moving across the sunlit side of the Earth. \r\n\r\nThere are different types of solar eclipses. Total, when the disk of the Moon completely covers the Sun; partial, when only a fraction of the solar disk is covered even at maximum eclipse; and annular, when the Moon is farther away than average and hence appears smaller than usual allowing a ring of the solar disk to remain visible even at the maximum extent of the eclipse. \r\n\r\nDuring a total solar eclipse, the darkest point of the shadow of the Moon on Earth is called the \"umbra\", and the edge of the shadow is called the \"penumbra\".  Observers in the umbra see a total eclipse while observers in the penumbra see a partial eclipse.","term_approval_level":"A","language_code":"en","term_number":310,"term_in_english":"Solar Eclipse","based_on_current_english_version":null,"linked_terms":[91,203,342,475],"alternate_terms":[],"categories":["Astronomy and Society","Naked Eye Astronomy","Solar System","The Sun"],"category_ids":[11,4,1,5],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/310/"},{"term_name":"Solar Flare","term_definition":"A solar flare is a brief brightening on one part of the Sun. \r\n\r\nThe Sun is surrounded by a complex magnetic field. Sometimes this magnetic field becomes unstable, shifting its structure and releasing huge amounts of stored energy. This causes a localized heating of the Sun's atmosphere and leads to a small fraction of the Sun's photosphere becoming brighter. This can also lead to huge streams of particles being flung out into space (known as a coronal mass ejection).  \r\n\r\nMost solar flares change the brightness of the Sun by amounts barely noticeable to the human eye and can only be seen by solar telescopes or by space telescopes monitoring the Sun. \r\n\r\nThe coronal mass ejections from the Sun can cause geomagnetic storms if they come close enough to the Earth to interact with the Earth's magnetic field.\r\n\r\nOther stars also have flares (known as stellar flares) but, as we see the stars as points of light, we only see these flares as brief brightenings of the star.","term_approval_level":"A","language_code":"en","term_number":311,"term_in_english":"Solar Flare","based_on_current_english_version":null,"linked_terms":[33,251,315,323,342,455,505],"alternate_terms":["stellar flare"],"categories":["Stars","The Sun"],"category_ids":[2,5],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/311/"},{"term_name":"Solar Mass","term_definition":"A solar mass is a unit of mass equal to the mass of the Sun: about 1.989 x 10³⁰ kilograms or 333,000 times the mass of Earth. Solar mass is commonly used to measure and compare brown dwarfs, stars, clusters, and galaxies.","term_approval_level":"A","language_code":"en","term_number":312,"term_in_english":"Solar Mass","based_on_current_english_version":null,"linked_terms":[190,342],"alternate_terms":[],"categories":["The Sun"],"category_ids":[5],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/312/"},{"term_name":"Solar Prominence","term_definition":"Solar prominences (sometimes referred to as filaments) are loops of plasma that form due to the magnetic fields around the Sun. These are temporary but may persist for weeks or months and can often be photographed during solar eclipses. A typical prominence extends over many thousands of kilometers (km); the largest on record was estimated at over 800,000 km long, roughly a solar radius. As with many solar phenomena, other stars are also thought to exhibit prominences.","term_approval_level":"A","language_code":"en","term_number":313,"term_in_english":"Solar Prominence","based_on_current_english_version":null,"linked_terms":[310,342,455],"alternate_terms":[],"categories":["Stars","The Sun"],"category_ids":[2,5],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/313/"},{"term_name":"Solar System","term_definition":"All objects that are under the gravitational influence of the Sun are members of the Solar System. Apart from the Sun at the center, the Solar System includes the eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), their moons, dwarf planets, asteroids, comets, meteoroids, and other objects in the Kuiper Belt and beyond, as well as artificial satellites and space probes. It is believed that the Solar System extends up to 100,000 astronomical units from the Sun in what is called the Oort Cloud, comprising of billions of icy objects.","term_approval_level":"A","language_code":"en","term_number":314,"term_in_english":"Solar System","based_on_current_english_version":null,"linked_terms":[17,26,62,87,89,167,170,189,192,197,204,212,294,342,375],"alternate_terms":[],"categories":["Naked Eye Astronomy","Solar System","The Sun"],"category_ids":[4,1,5],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/314/"},{"term_name":"Solar Wind","term_definition":"The solar wind is a stream of particles, primarily protons and electrons, flowing outwards from the Sun at up to 900 kilometers per second. The solar wind is essentially the hot solar corona expanding into interplanetary space.\r\n\r\nMany stars have winds; cooler, magnetically active stars like the Sun have winds driven by the hot coronae created by their magnetic fields. Some hotter stars have winds driven by their huge luminosities pushing particles out of their upper atmospheres by radiation pressure. Cool red giants and supergiants can also have winds driven by radiation pressure. The general term for a wind from a star is a stellar wind.\r\n\r\nThe bombardment of particles from a solar or stellar wind can be detrimental to any life that might be hosted by a planet. The Earth's magnetic field protects life on its surface from damaging effects of the solar wind. The interaction between the solar wind and Earth's magnetic field is the cause of aurorae close to the Earth's poles.","term_approval_level":"A","language_code":"en","term_number":315,"term_in_english":"Solar Wind","based_on_current_english_version":null,"linked_terms":[33,68,184,241,323,342,441,455],"alternate_terms":["Stellar wind"],"categories":["Solar System","Stars","The Sun"],"category_ids":[1,2,5],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/315/"},{"term_name":"Solar Year","term_definition":"","term_approval_level":"A","language_code":"en","term_number":316,"term_in_english":"Solar Year","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":["Tropical Year"],"override_term_number":389,"categories":[],"category_ids":[],"override_url":"https://astro4edu.org/resources/glossary/term/389/","url":"https://astro4edu.org/resources/glossary/term/316/"},{"term_name":"Solstice","term_definition":"Earth's axis is not perpendicular to Earth's orbit around the Sun, but inclined at an angle of 23.4 degrees from the perpendicular. In consequence, at different parts of its orbit, the angle between Earth's axis and our sightline to the Sun varies over one year. The practical effect is that for an observer on Earth, the highest point that the Sun will reach above the horizon in a given day will vary. The northern summer solstice, which is also the southern winter solstice, occurs around June 21 and is the time when the Sun is highest above the horizon in the northern hemisphere and at the same time lowest in the southern hemisphere. The southern summer solstice, which is also the northern winter solstice, occurs around December 21 and is the time the Sun is highest in the southern and at the same time lowest in the northern hemisphere.","term_approval_level":"A","language_code":"en","term_number":317,"term_in_english":"Solstice","based_on_current_english_version":null,"linked_terms":[34,90,92,145,342,436,439],"alternate_terms":[],"categories":["Astronomy and Society","Naked Eye Astronomy"],"category_ids":[11,4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/317/"},{"term_name":"Southern Cross","term_definition":"The Southern Cross is the common name for the southern hemisphere constellation Crux. It has five stars visible to the naked eye forming a long cross, is compact, and easily recognized. Crux covers the smallest area of the celestial sphere of any of the 88 official constellations. The brightest star, alpha Crucis is a triple star system whilst beta Crucis is a Cepheid variable. Crux also contains a stunning open cluster, the Jewel Box (NGC 4755). Crux can be used to help find south and the celestial South Pole. It is represented on the flags of Australia, Brazil, New Zealand, Papua New Guinea, and Samoa.","term_approval_level":"A","language_code":"en","term_number":318,"term_in_english":"Southern Cross","based_on_current_english_version":null,"linked_terms":[66,210],"alternate_terms":[],"categories":["Astronomy and Society","Naked Eye Astronomy"],"category_ids":[11,4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/318/"},{"term_name":"Space","term_definition":"Space has two definitions within astronomy: space (or \"outer space\") is all that exists beyond Earth; more technically, it is the dimensions of length, width, and depth within which all things exist (as opposed to time, which is sometimes considered as a fourth dimension).","term_approval_level":"A","language_code":"en","term_number":319,"term_in_english":"Space","based_on_current_english_version":null,"linked_terms":[235],"alternate_terms":[],"categories":["Space Exploration"],"category_ids":[10],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/319/"},{"term_name":"Space Debris","term_definition":"Space debris or space junk is the term for artificial objects, or the remnants of such objects, which orbit Earth but serve no useful function. Examples are decommissioned or faulty satellites, broken pieces resulting from the collision of satellites, or upper stages of rockets that were used in the launch of spacecraft or satellites and discarded after having fulfilled their purpose. The more space debris there is, the greater the chance of damaging collisions as a serious hazard for spacecraft. In consequence, space agencies are discussing, and have begun to test, ways of removing space debris, and are making an effort to plan missions so that objects that have fulfilled their purpose are made to re-enter, and burn up in, Earth's atmosphere.","term_approval_level":"A","language_code":"en","term_number":320,"term_in_english":"Space Debris","based_on_current_english_version":null,"linked_terms":[293,324],"alternate_terms":[],"categories":["Space Exploration"],"category_ids":[10],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/320/"},{"term_name":"Space Station","term_definition":"A space station is a spacecraft in an orbit for a substantial period of time that provides a long-term home for humans in space. Specifically, it provides suitable living conditions, from breathable air to well-regulated temperature. All space stations so far have been dependent on regular deliveries of food and water. So far, space stations are or have been in low Earth orbits, with the International Space Station orbiting around 420 kilometers above sea level. An orbiting station is in free fall, with the astronauts, equipment, and space station all experiencing the same acceleration from the Earth's gravity. As the astronauts and equipment inside the space station are accelerating at the same magnitude and direction as their surroundings in the space station they  experience the sensation of weightlessness, even though they are not actually weightless. This sensation is often called microgravity, although this name can be confusing as the Earth's gravitational pull is still substantial on the space station, astronauts, and equipment. Space stations are primarily used for research, notably on the effect of microgravity on human beings; an important preparation for further stages of space exploration.","term_approval_level":"A","language_code":"en","term_number":321,"term_in_english":"Space Station","based_on_current_english_version":null,"linked_terms":[324],"alternate_terms":[],"categories":["Naked Eye Astronomy","Space Exploration"],"category_ids":[4,10],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/321/"},{"term_name":"Space Telescope","term_definition":"Electromagnetic radiation from space has to first pass through the Earth's atmosphere before it can be collected by ground-based telescopes. Gamma rays, X-rays, ultraviolet, and some kinds of infrared radiation are filtered out completely by the atmosphere. To observe them, astronomers use space telescopes, which are sometimes also called space observatories: Automated satellites above the atmosphere that carry a telescope and instruments, plus the equipment needed to orient the spacecraft towards a specific target, receive commands, and transmit data back to Earth. As the twinkling of stars shows, even light that passes through the atmosphere is disturbed in the process. Here, too, space telescopes can help. However, space telescopes are difficult to repair. Most space telescopes are either in Earth orbit (such as the Hubble Space Telescope), or at the so-called Lagrange point L2 (such as the James Webb Space Telescope).","term_approval_level":"A","language_code":"en","term_number":322,"term_in_english":"Space Telescope","based_on_current_english_version":null,"linked_terms":[155,227,293,371,487],"alternate_terms":["Space Observatory"],"categories":["Space Exploration","Telescopes, Instruments and Observatories"],"category_ids":[10,3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/322/"},{"term_name":"Space Weather","term_definition":"Space weather describes time-varying conditions near Earth related to the presence of charged particles and the properties of Earth's magnetic field. Inclement space weather, with large numbers of charged particles arriving from outer space, can make radio communication more difficult and, in the case of a solar storm, go so far as to damage electronics in spacecraft or even on the ground. It is also associated with what are surely the most beautiful space weather phenomena: colorful polar lights. Factors that influence space weather are cosmic rays, the state of the Van Allen belts of charged particles in Earth's magnetic fields, the solar wind of charged particles, and events such as flares and coronal mass ejections launching charged particles in the direction of Earth.","term_approval_level":"A","language_code":"en","term_number":323,"term_in_english":"Space Weather","based_on_current_english_version":null,"linked_terms":[33,184,241,315,435,441,455,505],"alternate_terms":[],"categories":["Space Exploration"],"category_ids":[10],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/323/"},{"term_name":"Spacecraft","term_definition":"A spacecraft is a vehicle built to travel in outer space. Propulsion is usually via some kind of rocket engine, emitting gas or plasma in one direction in order to accelerate in the opposite direction. Spacecraft that do not carry humans are called uncrewed or robotic spacecraft; examples are various kinds of satellites, space telescopes, or space probes that are used for exploration. Crewed spacecraft, on the other hand, need to provide for the survival of their human occupants, with, for example, a pressurized atmosphere, radiation protection, and a regulated temperature. Spacecraft that can return to Earth are called recoverable; those that can be launched again after recovery, reusable. A spacecraft designed to provide a longer-term home base, e.g. in Earth orbit, is called a space station.","term_approval_level":"A","language_code":"en","term_number":324,"term_in_english":"Spacecraft","based_on_current_english_version":null,"linked_terms":[288,293,321,322],"alternate_terms":[],"categories":["Space Exploration"],"category_ids":[10],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/324/"},{"term_name":"Spectral Type","term_definition":"Stars are classified into spectral types according to the appearance of features in their spectra. \r\n\r\nFor most stars, the spectral type is based primarily on the temperature of the stellar surface and follows a sequence: O, B, A, F, G, K, and M, listed from hottest to coldest. This sequence has recently been extended to the cooler types L, T, and Y. These three represent mostly brown dwarfs, but some objects with spectral type L are stars rather than brown dwarfs.\r\n\r\nThere are also letters to classify special classes of star. Carbon stars are stars with strong spectral features from molecules containing carbon. These are classified as type C. S-type stars are intermediate between types K or M and C in that the surface abundances of oxygen and carbon are nearly equal. White dwarfs are divided into a series of different types based on features in their spectra; all these types begin with the letter D (DA, DB, etc.). Hot, massive stars with broad emission lines have a series of types beginning with W (WN, WC, WO).\r\n\r\nThe present notation is a legacy from the first modern classification attempt, undertaken at Harvard College Observatory. The classes, originally designated A–Q, alphabetically, were subsequently reordered as a temperature sequence, resulting in the main types still used today. The main spectral classes are subdivided, denoted by the numbers from zero to nine. The Sun is spectral type G2. Additional letters refer to special characteristics (such as e for stars with bright emission lines), and the luminosity class, denoted by Roman numerals, may also be specified.","term_approval_level":"A","language_code":"en","term_number":325,"term_in_english":"Spectral Type","based_on_current_english_version":null,"linked_terms":[1,37,110,168,328,331,342,442,453,463,503],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/325/"},{"term_name":"Spectrograph","term_definition":"","term_approval_level":"A","language_code":"en","term_number":326,"term_in_english":"Spectrograph","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":["Spectrometer","spectroscope"],"override_term_number":328,"categories":["Chemistry","Telescopes, Instruments and Observatories"],"category_ids":[12,3],"override_url":"https://astro4edu.org/resources/glossary/term/328/","url":"https://astro4edu.org/resources/glossary/term/326/"},{"term_name":"Spectroscopy","term_definition":"","term_approval_level":"N","language_code":"en","term_number":327,"term_in_english":"Spectroscopy","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":[],"override_term_number":328,"categories":["Chemistry"],"category_ids":[12],"override_url":"https://astro4edu.org/resources/glossary/term/328/","url":"https://astro4edu.org/resources/glossary/term/327/"}]}