Glossary term: Nucleus

Description: 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.

Nuclei 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.

Nuclei 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.

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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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