Stellar Evolution

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Created for the OAE

Caption: 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.
Credit: Danielle Futselaar/IAU OAE

詞彙表術語: 黑洞 , 氫聚變 , 中子星 , 恆星形成 , 恆星演化 , 恆星殘骸 , 超巨星 , 超新星 , 白矮星

License: Creative Commons 姓名標示 4.0 國際 (CC BY 4.0) Creative Commons 姓名標示 4.0 國際 (CC BY 4.0) icons

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The Crab Nebula

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Caption: 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.
Credit: NASA, ESA, J. Hester and A. Loll (Arizona State University)
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詞彙表術語: 超新星遺跡
Categories: Milky Way and Interstellar Medium , Stars

License: Public Domain Public Domain icons

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蟹狀星雲脈衝星

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Caption: 在距地球約6500光年、位於金牛座的蟹狀星雲中心，存在著蟹狀星雲脈衝星。它是一顆大質量恆星生命終結時爆發後留下的殘骸。雖然恆星爆炸發生於數千年前，但這次爆發產生的光芒直到公元1054年才抵達地球。這次天體事件曾被全球多地民眾觀測到，許多文明都在其歷史文獻中留下了相關記錄。 蟹狀星雲脈衝星每秒約旋轉30周，並發射包括可見光在內的多波段光線。其質量約為太陽的1.5倍，但在它形成時的爆發產生了龐大力量，將如此巨大的質量壓縮在半徑僅約十公裡的極小的空間內。 這張圖像由美國夏威夷雙子北座天文臺的多張觀測圖像合成而成，脈衝星清晰可見於畫面中央。構建圖像的觀測數據採集時間跨度達五年，其中2009年的數據以藍色標示，2014年的數據以紅色標示。在此期間，拋射物質持續遠離脈衝星，形成了圖中的彩色漣漪效果。需要說明的是，顏色並非真實色調，波紋顯示的是沖擊波遠離脈衝星並撞擊到周圍氣體時的位置。
Credit: 國際雙子座天文臺/NOIRLab/NSF/AUR、Jen Miller、Travis Rector、Mahdi Zamani 和 Davide de Martin
Credit Link

詞彙表術語: 中子星 , 脈衝星 , 恆星殘骸
Categories: Milky Way and Interstellar Medium , Stars

License: Creative Commons 姓名標示 4.0 國際 (CC BY 4.0) Creative Commons 姓名標示 4.0 國際 (CC BY 4.0) icons

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Remnant of SN 1006

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Caption: This image shows the remnant of the supernova SN 1006. This was probably the result of a white dwarf that accreted so much material from a binary companion star that the white dwarf exploded (this is called a Type 1a supernova by astronomers). This explosion happened several thousand years ago, however it took time for the light from this event to reach Earth, only arriving in the year 1006. This bright explosion was noticed by observers across the Earth and its appearance was noted in the records of many different societies. Here we see the effect that supernova has had on its surroundings in the galaxy. The force of the explosion has blown a huge bubble in the surrounding interstellar gas with a hot shockwave at its edge. The image appears to be a simple color picture but it actually represents light far beyond what our eye can see. The blue is X-ray data from NASA's Chandra X-ray Observatory, the yellow and orange are data from optical telescopes and the red is detections in radio waves from the Very Large Array and the Green Bank Telescope. The bright blue of the outer shell shows the gas there is very hot and that the explosion produced energetic shock waves.
Credit: X-ray: NASA/CXC/Rutgers/G.Cassam-Chenai, Hughes et al.; Radio: NRAO/AUI/NSF/GBT/VLA/Dyer, Maddalena & Cornwell; Optical: Middlebury College/F.Winkler, NOAO/AURA/NSF/CTIO Schmidt & DSS
Credit Link

詞彙表術語: 恆星殘骸 , 超新星 , X 射線天文學 , 超新星遺跡
Categories: Milky Way and Interstellar Medium , Stars

License: Public Domain Public Domain icons

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What is a black hole?

astroEDU educational activity (links to astroEDU website)
Description: What are black holes and what would happen if the Sun was replaced by one of them?

詞彙表術語: 黑洞 , 萬有引力
Categories: Physics
Tags: Experiment , Investigation , Model
Age Ranges: 12-14
Education Level: Informal , Middle School
Areas of Learning: Discussion Groups , Interactive Lecture , Modelling , Observation based , Problem-solving , Structured-inquiry learning
Costs: Low Cost
Duration: 45 mins
Group Size: Group
Skills: Constructing explanations , Developing and using models , Engaging in argument from evidence , Planning and carrying out investigations
Authors: Tran Dong Thai Han, Thomas Russell

License: Creative Commons 姓名標示 4.0 國際 (CC BY 4.0) Creative Commons 姓名標示 4.0 國際 (CC BY 4.0) icons