词条恒星演化

描述: 恒星演化描述了恒星的衰老以及它们在生命周期中的变化。与生物进化不同，恒星演化并不是指不同代恒星之间性状的变化。

恒星一生中的大部分时间都处于恒星演化的主序阶段，在其内核中将氢聚变成氦并释放能量。随着恒星年龄的增长，其内核中的氢开始耗尽，内核会收缩，并变得足够热以开始氦聚变。根据恒星的质量，这可能会导致其演化为巨星或超巨星。在一些巨星和超巨星中，核聚变会产生越来越重的元素。

初始质量介于0.5到8倍太阳质量之间的恒星最终会形成碳、氧和（或）氖的内核，而氢和氦的聚变会在内核周围的外壳中继续进行，从而形成洋葱状的分层结构。它们最终会失去外层，形成行星状星云，只剩下内核，成为一颗又小又亮的白矮星。

质量超过 8 个太阳质量的恒星会继续燃烧更重的元素，直到核心的原子核聚变成铁。此时，进一步的核聚变已无法释放额外的能量。这就引发了超新星爆发，留下的要么是一颗非常致密的中子星，要么是一颗质量非常大的黑洞。

行星状星云和超新星爆发都会把恒星中的物质喷射到星际介质中。在演化的某些阶段，许多恒星也会通过恒星风、极端脉动或爆发喷射出物质。由于核聚变，以及爆发时本身的核反应，喷出的物质富含重元素。这些富集的物质可能会进入下一代恒星中。

恒星在所有这些阶段的演化都可能因与多恒星系统中的伴星相互作用而改变。

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相关图表

赫罗图

图注： 这张图展示了不同恒星温度和亮度。每个点的大小代表恒星的半径，颜色代表人眼所看到的颜色。恒星的颜色从淡蓝色到淡橙红色不等，没有恒星具有像红、绿或蓝这样的纯颜色，因为恒星的光谱包含了许多不同颜色的光。然而，最红的恒星通常被称为红恒星，最蓝的恒星被称为蓝恒星。为了展示不同类型的恒星，制作这个图表的恒星样本选择上并没有反映出每种类型恒星的实际数量比例。从左上到右下是一条长长的恒星带，这些恒星在其核心燃烧氢气，这被称为主序。在这条线上，我们可以看到参宿三（Mintaka）、波江座α星（Achernar）、天狼星A（Sirius A）、太阳和比邻星（Proxima Centauri）等恒星。在主序线右下方的比邻星周围的天体被称为红矮星。在红矮星的右下方是Teide 1和Kelu-1 A。这两个天体是褐矮星，它们的质量太低，核心没有足够的热量来持续地进行氢融合。由于它们不燃烧氢，褐矮星不被认为是主序星。"褐矮星"这个名字与它们的颜色无关。在主序星的上方，我们发现次巨星、巨星和超巨星。这些是已经完成了核心的氢燃烧并演化成更大天体的恒星。恒星的亮度取决于其温度和大小，因此巨星比具有较小半径但相同温度的恒星更亮。随着时间的推移，这些天体将走向生命的尽头，经历行星状星云阶段或变成超新星。以行星状星云阶段结束生命的恒星会形成一种叫做白矮星的恒星残骸。这种天体比相同温度的恒星小得多，因此更暗淡，并且位于主序星带的显著下方。以超新星结束生命的恒星会成为黑洞或中子星。这些在这个图表上没有显示。
来源： IAU OAE/Niall Deacon

License: CC-BY-4.0 知识共享许可协议署名 4.0 国际 (CC BY 4.0) 图标

A diagram showing the evolutionary stages of five mass ranges of stars.

Stellar Evolution

图注： This diagram shows the life cycle of stars of different masses. The mass of the different types of star increases from bottom to top with time going from left to right. The life cycle of a star depends on its mass, with lower mass stars have longer lifetimes. All stars form from clouds of gas that collapse under their own gravity. As the star collapses, its core becomes hotter and denser. If the star has a mass greater than 0.08 solar masses (0.08 times the mass of the Sun), the pressure of the star’s mass pushing down on its core creates a high enough core temperature for hydrogen fusion to ignite. This burns hydrogen into helium in the star’s core, providing a heat source to power the star and to stop its core from collapsing further. If the collapsing object has a mass below 0.08 solar masses then it does not ignite hydrogen fusion in its core. It continues to cool and slowly contract. Such substellar objects are known as brown dwarfs, shown here in the lowest row. After stars have formed, they burn hydrogen in their cores and begin their so-called main sequence phase. The most massive stars (>25 solar masses, shown here at the top) have very high core temperatures and thus burn through their hydrogen fuel more quickly. This means they may only spend a few million years on the main sequence burning hydrogen in their cores. Once the hydrogen in the core is exhausted the star’s core contracts, becomes hotter and helium burning starts in the core. While the core contracts, the outer layers of the star expand and it becomes a supergiant. For the most massive stars strong stellar winds strip off the cooler outer layers, leading to the star being very large and very hot, a blue supergiant. Once helium is exhausted in the core, carbon is burned, and then heavier elements. Eventually the star ends with an iron core. Fusing iron into heavier elements does not generate energy so at this point fusion stops in the core. Once this core of iron is massive enough, it and the surrounding matter suddenly collapses to form a black hole and the outer layers are flung off in a supernova explosion. Slightly lower mass stars (between 8 and 25 solar masses, seen here second top) evolve in a similar way although they do not have strong enough winds to push their outer layers away and become blue supergiants, instead it evolves into a red supergiant. While such stars also collapse and create supernova explosions. The remnant of the star’s core is not massive enough to collapse into a black hole. Instead, its electrons and protons combine to form neutrons and it is supported by a quantum mechanical effect called neutron degeneracy pressure. This results in the remnant of the star being a tiny neutron star, several solar masses in mass but only a few kilometres across. For stars similar in mass to the Sun (between 0.4 and 8 solar masses, seen here in the middle row), the star burns hydrogen in its core until the hydrogen in its core is exhausted. At this point a hydrogen burning shell forms around the core. Eventually the core will become hot enough to burn helium into carbon and oxygen. After this the star is left with a carbon and oxygen core surrounded by shells burning helium and hydrogen. These shells are unstable producing thermal pulsations that convulse the star. Eventually these pulsations become so extreme that the star’s outer layers are thrown off. This leaves the carbon and oxygen core as a white dwarf supported by electron degeneracy pressure. The outer layers of the star form what is known as a planetary nebula (which doesn’t actually have anything to do with planets despite the name). The lowest mass stars (seen here in the second bottom row) are so low in mass that their evolutionary timescales are much longer than the age of the universe. This means that none have evolved beyond the main-sequence. Low mass stars are fully convective meaning material in the core is constantly being mixed with material above. This means that all the hydrogen in the star would eventually be burned in the core, but this will take trillions of years.
来源： Danielle Futselaar/IAU OAE

License: CC-BY-4.0 知识共享许可协议署名 4.0 国际 (CC BY 4.0) 图标