Stellar Evolution

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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

词汇表: 黑洞 , 氢聚变 , 中子星 , 恒星形成 , 恒星演化 , 恒星残骸 , 超巨星 , 超新星 , 白矮星

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Sirius A with his faint white dwarf companion Sirius B

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图注： This Hubble Space Telescope image highlights Sirius, the brightest star in Earth’s night sky, appearing as an intensely luminous object at the center with prominent cross-shaped diffraction spikes. These spikes, along with the saturated glow around the main star, are caused by the Sirius' light being spread out by the telescope and camera used to make this image. Slightly below and to the left of the main star, a tiny point of light marks Sirius B, a much dimmer object captured thanks to Hubble’s high sensitivity. Sirius A is an A-type star, known for its high surface temperature and strong white-blue light, while Sirius B is a compact white dwarf, the dense remnant of a star that has exhausted its nuclear fuel. Together, they form a well-known Binary star system located about 8.6 light-years from Earth. Sirius B was originally a higher mass and brighter star that burned through its hydrogen fuel more quickly than Sirius A. This led to Sirius B evolving into a red giant and eventually ending its life as a planetary nebula, leaving only the remains of its core as a white dwarf orbiting Sirius A.
来源： NASA, ESA, H. Bond (STScI), and M. Barstow (University of Leicester)
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词汇表: A型星 , 双星 , 天狼星 , 白矮星
分类： 恒星

授权许可： 知识共享许可协议 署名 4.0 国际 (CC BY 4.0) 知识共享许可协议 署名 4.0 国际 (CC BY 4.0) 图标

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The Crab Nebula

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图注： The Crab Nebula is situated 6500 light years away in the constellation Taurus. This is remnant of a supernova explosion caused by a massive star at the end of its life. This happened several thousand years ago but the light from this explosion only reached the Earth in the year 1054. This celestial event was viewed by people across the world with many different societies noting it in their records. The force of the supernova explosion has pushed pushed the outer layers of the star out into the surrounding gas. Here we see this as the bubbly orange structure as the force of the explosions ploughs into the surrounding gas. At its heart lies the Crab Nebula Pulsar, a neutron star that is the remains of the exploded star. This neutron star has a strong magnetic field. Electrons in nebula dance in this magnetic field, emitting the diffuse blue light we see inside the orange bubble.
来源： NASA, ESA, J. Hester and A. Loll (Arizona State University)
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词汇表: 超新星遗迹
分类： 银河系与星际介质 , 恒星

授权许可： 公共领域 公共领域 图标

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蟹状星云脉冲星

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图注： 在距地球约6500光年、位于金牛座的蟹状星云中心，存在着蟹状星云脉冲星。它是一颗大质量恒星生命终结时爆发后留下的残骸。虽然恒星爆炸发生于数千年前，但这次爆发产生的光芒直到公元1054年才抵达地球。这次天体事件曾被全球多地民众观测到，许多文明都在其历史文献中留下了相关记录。 蟹状星云脉冲星每秒约旋转30周，并发射包括可见光在内的多波段光线。其质量约为太阳的1.5倍，但在它形成时的爆发产生了庞大力量，将如此巨大的质量压缩在半径仅约十公里的极小的空间内。 这张图像由美国夏威夷双子北座天文台的多张观测图像合成而成，脉冲星清晰可见于画面中央。构建图像的观测数据采集时间跨度达五年，其中2009年的数据以蓝色标示，2014年的数据以红色标示。在此期间，抛射物质持续远离脉冲星，形成了图中的彩色涟漪效果。需要说明的是，颜色并非真实色调，波纹显示的是冲击波远离脉冲星并撞击到周围气体时的位置。
来源： 国际双子座天文台/NOIRLab/NSF/AUR、Jen Miller、Travis Rector、Mahdi Zamani 和 Davide de Martin
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词汇表: 中子星 , 脉冲星 , 恒星残骸
分类： 银河系与星际介质 , 恒星

授权许可： 知识共享许可协议 署名 4.0 国际 (CC BY 4.0) 知识共享许可协议 署名 4.0 国际 (CC BY 4.0) 图标

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The red supergiant Betelgeuse

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图注： The image shows Betelgeuse, a red supergiant in the constellation Orion, observed by the Atacama Large Millimeter/submillimeter Array (ALMA). ALMA consists of many antennae spread across a plain in Northern Chile. The observations from all of these receivers is synthesised together by a central computer to form an image. The wide distances between the antennae mean that is can resolve very fine details. Most stars we observe are just seen as points of light, but Betelgeuse is so large (with a radius about 1,400 times larger than the Sun) and is sufficiently nearby that it is one of the few stars to have been resolved to show it as an extended object. Betelgeuse is a massive star, more than 14 times the mass of the Sun and is relatively young for a star (less than 14 million years old). However, its high mass led to it having a very hot core which burned through its hydrogen fuel quickly. It has since evolved through many stages and now appears as a red supergiant, it's final stage before exploding as a supernova. When such an explosion will happen is not known for certain, but it could be in around 100,000 years. Such an explosion would be visible from Earth, even during the day.
来源： ALMA (ESO/NAOJ/NRAO)/E. O’Gorman/P. Kervella
来源链接

词汇表: 巨星 , 红巨星 , 红超巨星 , 超巨星 , M 型星
分类： 恒星

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