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  • Stellar 演变

    Stellar evolution of high mass stars: the top 0.1%
    ::高质量恒星的斯特拉进化:最高0.1%

    Our world has risen from the ashes of massive stars. Those massive stars are rare: they make up just   0.1% of all stars . The synthesis of  heavy  elements   occurs  during the final 10% of the lifetime of   these  stars.  Except for hydrogen,   the  atoms  in our bodies were all formed by nucleosynthesis in stars. Without those stars, or specifically without the death of those stars, we would not have the material needed to build planets and needed for prebiotic and biological chemistry.   
    ::我们的世界已经从巨型恒星的灰烬中崛起。 这些巨型恒星是罕见的: 它们只占所有恒星的0.1%。 重元素的合成发生在这些恒星生命期最后10%的时间里。 除了氢外, 我们体内的原子都是由恒星的核子合成形成的。 没有这些恒星,或者特别是没有这些恒星的死亡,我们就不会有建造行星所需的材料,以及生物前化学和生物化学所需的材料。

    It bears repeating that the the mass of the star is what drives the rate of fusion reactions and therefore the evolution of a star.  The definition of "high mass" is generally taken to be a few times the mass of the Sun. The  evolutionary  path   is determined by the mass of the star; stars that are less than a few times the mass of the Sun are by far the most common. These lower mass stars take the high road in the    below and end their lives as compact white dwarfs, roughly the size of the Earth.  Higher mass stars follow the lower path in the below and end their lives as exotic neutron stars or black holes.   
    ::值得重复的是,恒星的质量是驱动恒星聚变反应速度和进化速度的动力。 “ 高质量” 的定义一般为太阳质量的几倍。 进化路径由恒星的质量决定; 远为最常见的恒星是太阳质量的不到几倍。 这些低质量恒星在下方走高路, 并结束其作为紧凑的白矮星的寿命, 大致相当于地球的大小。 更高质量恒星在下方走低路, 结束其作为奇特中子恒星或黑洞的生活。

     

     

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    The evolution and final state of a star are largely determined by the initial mass of the star. What will happen to our Sun after it is no longer able to carry out nucleosynthesis?
    ::恒星的进化和最终状态主要由恒星的初始质量决定。 当太阳不再能够进行核循环合成后,太阳会怎样呢?

     

    Stellar evolution of stars: the bottom 99.9%
    ::恒星的斯特拉星进化:底层99.9%

    Roughly  99.9% of the stars  are AFGKM spectral type stars. These stars fuse hydrogen in their cores on the main sequence and then evolve into red giants when the hydrogen is depleted. The red giant stars puff off about half of their mass (hydrogen enriched with helium, and trace metals like carbon, nitrogen, and oxygen), polluting the interstellar medium like an industrial chimney stack. As the outer   envelope of the   red giant (confusingly called a planetary nebula because astronomers were originally  uncertain  about these objects )   mix   with the interstellar medium,  the core of the remnant   red giant  collapses. If the core mass is less than 1.4 solar masses, it becomes a white dwarf. This is the ultimate fate of our Sun. 
    ::大约99.9%的恒星是AFGKM光谱型恒星。这些恒星在主序列中将氢聚入核心核心,然后在氢耗尽时演变成红巨星。红色巨星将约一半的恒星(含的氢和碳、氮和氧等微量金属)喷发,污染星际介质,如工业烟囱堆。红巨的外壳(统称为行星星云,因为天文学家最初对这些天体不确定)与星际介质(残余红巨星的核心)混合在一起。如果核心质量小于1.4太阳质量,它就会变成白矮星。这是我们太阳的最终命运。

    White dwarfs 
    ::白矮人

    A white dwarf is about the size of the Earth, and it is an incredibly dense object - one teaspoon of white dwarf material weighs several tons. Nuclear fusion is no longer taking place in the white dwarf (except for a short period of hydrogen fusion on the surface).   Now, electron degeneracy  supports the white dwarf against further gravitational collapse.   This supporting pressure  arises from the quantum mechanical nature of electrons.  Once the lowest spin energy state is occupied by an electron, the Pauli exclusion principle tells us that another electron cannot have that same spin energy state; the other electrons are forced into higher and higher (faster moving) energy states.  As long as the mass of the remnant white dwarf is less than about 1.4 times the mass of the Sun, electron degeneracy can support it against further collapse. 
    ::白矮星的大小与地球的大小有关,它是一个极其稠密的物体,一个白色矮星的茶匙重达数吨。核聚变不再发生在白矮星(除了表面短暂的氢聚变期之外 ) 。 现在, 电子衰变支持白矮星抵御进一步引力崩溃。 这种支持压力来自电子的量子机械性质。 一旦最小的旋转能量状态被电子占据, 保利排除原则告诉我们, 另一种电子不能拥有同样的旋转能量状态; 另一种电子被迫进入更高、 更高( 更快地移动) 的能量状态。 只要残余的白矮星的质量小于太阳质量的1.4倍左右, 电子衰变能可以支持它抵御进一步崩溃。

    Stellar evolution of stars: the  top  0.1%
    ::恒星的斯特拉进化:顶部0.1%

    As fragments of the cold molecular cloud begin to contract and form stars,  hundreds to thousands of  stars are born; however, only about 0.1% of the newborn stars will have enough mass to become O or B type stars.  These massive stars contract quickly and carry out hydrogen fusion at a furious pace for 1 or 2 million years. At that point they have burned through their endowment of hydrogen. The core collapses until helium fusion begins and the outer shell expands, forming a red super giant star. As described above, there are several cycles of fuel depletion, contraction, and re-ignition as the core of the red super giant develops an onion layer structure, with stratified shell burning of different elements.  Once the core contains iron, the star collapses again, but iron fusion does not produce energy and cannot support the star against gravitational collapse. The core of the star now hits a fork in the road:
    ::随着冷分子云的碎片开始收缩并形成恒星,成百上千颗恒星诞生;然而,只有大约0.1%的新生恒星将有足够的质量成为O型或B型恒星。这些巨型恒星迅速收缩,以迅猛的速度进行氢聚变,持续了一至二百万年。当它们通过氢的特性燃烧时,它们的核心破裂,直到聚开始,外壳膨胀,形成红色超级巨星。如上所述,作为红超级巨星核心的燃料耗竭、收缩和再点燃循环有几轮,形成一个红外层结构,而螺旋壳燃烧了不同元素。一旦核心内装铁,恒星再次崩溃,但铁聚不会产生能量,也无法支撑恒星反引力崩溃。恒星的核心现在在路上撞出一个叉子:

    • if the core mass is  between 1.4 and 3 solar masses, then the star becomes a neutron star
      ::如果核心质量介于1.4到3太阳质量之间,则恒星成为中子星
    • if the core is greater than 3 solar masses, then the star becomes a black hole. 
      ::如果核心大于3个太阳质量,那么恒星就变成黑洞。

    Betelgeuse is a red super giant star that is 12 - 20 times the mass of the Sun with a radius that is almost 900 times the radius of the Sun. Betelgeuse would sweep out almost to Jupiter if it were the center of our solar system. The star can be seen in the Orion constellation - at the shoulder of the famed hunter.
    ::贝特歇斯是一个红色超级巨星,是太阳质量的12-20倍,半径几乎是太阳半径的900倍。贝特歇斯将几乎扫向木星,如果木星是我们太阳系的中心。在猎户座星座上可以看到恒星,即著名的猎人的肩膀。

    This star has been in the news lately, because astronomers noticed that this red super giant  started dimming in October 2019. Is this pre-super-nova behavior that might be typical of other stars?  Or normal variable star behavior?  We have a ring-side seat to watch the evolution of this star, but it is impossible to know if  we will see this  happen next week, or over the next several  thousand  years.  Because Betelgeuse is 640 light years away, it is possible that the star has already gone super nova and we just haven't gotten the memo yet. 
    ::这颗恒星最近才出现在新闻中, 因为天文学家注意到这个红色超级巨星在2019年10月开始暗淡。 这种超超新星前的行为是否是其他恒星的典型行为? 或者正常的变星行为? 我们有一个环形座位来观察这个恒星的演进, 但是无法知道我们是否将在下周, 或者在接下来的几千年中看到它发生。 因为贝特尔吉斯已经过了640光年, 恒星可能已经变成超新星了, 我们只是还没有收到备忘录。

     

    Neutron stars 
    ::中子恒星

    When the mass of the remnant core is greater than 1.4 times the mass of the Sun, electron degeneracy can no longer support the core against gravitational collapse.  As the core collapses, electrons and protons squeeze together to form neutrons with a density   similar to  an atomic nucleus. The neutrons exert a resisting pressure to gravitational collapse that is similar to that of electrons. Objects supported by neutron degeneracy are called neutron stars. They are far more compact and far more dense than white dwarfs. The diameter of a neutron star is about the size of San Francisco, measuring roughly  10 kilometers ( or about  6 miles);  a teaspoon of neutron star material would weigh a billion tons .   Neutron degeneracy can support a star against further collapse as long as the total mass of the remnant is less than 2 or 3 times the mass of the Sun.
    ::当残余核心的质量大于太阳质量的1.4倍时,电子衰变能力无法再支持核心抵御引力崩溃。当核心崩溃、电子和质子挤在一起形成密度与原子核相似的中子时,中子对引力崩溃施加与电子相似的抗力压力。中子衰变支持的物体被称为中子衰变恒星。它们比白矮星要紧得多,密度也大得多。中子恒的直径大约在旧金山,约10公里(或约6英里);中子恒物质的茶匙重量将达10亿吨。中子衰变能支持恒星进一步崩溃,只要其总质量小于太阳质量的2或3倍。

    As the remnant core collapses into a neutron star, it spins up, conserving angular momentum. Neutron stars can rotate hundreds of times per second with a narrow beam of electromagnetic radiation   that spins with the star like a lighthouse . If the Earth happens to reside in the path of this beam of light, we see the neutron star as a rapidly blinking source - a pulsar.  Of course, most of the time, the synchrotron beam will not be so favorably aligned.  Pulsars were theoretically predicted going back to the 1930's. In 1968,    observed   radio emission  pulses   that confirmed the existence of neutron stars.   
    ::随着残余核心向中子星的崩溃,它会旋转,保持角动力。中子恒星可以每秒旋转数百次,以像灯塔一样与恒星旋转的狭窄电磁辐射束旋转。如果地球恰好位于光束的路径中,我们就会将中子恒视为闪烁迅速的来源 — — 脉冲星。当然,大部分时间,同步子波束不会如此吻合。理论上说,脉冲星可以追溯到1930年代。1968年,观察到的射电辐射脉冲证实了中子星的存在。

     

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    In 1967, the Irish astronomer, Jocelyn Bell, used radio telescope observations to measure periodic pulses from neutron stars. Why are the dishes of radio telescopes made from meshes rather than mirrors?
    ::1967年,爱尔兰天文学家Jocelyn Bell利用射电望远镜观测测量中子恒星的周期脉冲。 为什么射电望远镜的盘子是用乳胶而不是镜子制成的?

     

    Black holes
    ::黑洞

    Electron degeneracy can support a   stellar core (or white dwarf)  against collapse if the total mass is less than 1.4 M sun .  Neutron degeneracy can support against collapse if the remnant stellar core is between 1.4 and 2-3 M sun . If the mass of the remnant stellar core is greater than about 3 time the mass of the Sun, there is nothing that can stop the collapse, and a black hole is formed. Nothing escapes the black hole - not even light - so it is very difficult to find these stellar ghosts. But there are about a dozen candidate black holes in binary star systems where the second star is still visible.  We can measure the orbit of the visible star and deduce the presence of a massive, but invisible star, and in some cases we can see gas being funneled off the visible star and heated up to tens of millions of degrees as it spirals onto an accretion disk around something that cannot be seen. If it walks like a duck and it quacks like a duck.... 
    ::如果总质量小于1.4 Msun。如果残余恒星核心在1.4到2-3 Msun之间,则中子衰变能支持倒塌。如果残余恒星核心的质量大于太阳质量的大约3倍,则残余恒星核心的质量将无法阻止崩溃,并形成一个黑洞。黑洞将无处可逃,甚至光也无处可逃,因此很难找到这些恒星的幽灵。但是,在二星仍然可见的二星系统中,有十几个候选黑洞。我们可以测量可见恒星的轨道,推断出一个巨大但不可见的恒星的存在,在某些情况下,我们可以看到气体从可见的恒星上渗出,并加热到数千万度,因为气体会螺旋在无法看到的东西周围的倾角盘上。如果它像鸭子那样飘动,它会像鸭子一样振动。...

    Type Ia Supernovae
    ::Ia 型号超新星

    About half of stars like the Sun are members of binary systems - a gravitationally bound pair of stars.   T he fraction of binary systems is even higher for stars that are more massive than the Sun. This has interesting implications, especially if one star in a gravitationally bound system is more massive than the other. The more massive star in a binary system has a shorter lifetime - it will evolve first.  Let's say that the  more massive   star is originally 2 M Sun , it evolves through the red giant phase and ends up   as a white dwarf with a mass of  1.3 M Sun  (below the 1.4 M sun   ). Now, a billion years later, the other star (with a mass of 1.5 M sun ) in the binary system goes through a red giant phase.  As that star expands, it begins to dump some of its outer envelope onto the white dwarf. If the white dwarf crosses over the Chandrasekhar limit, it cannot be supported by electron degeneracy and it begins to collapse into a neutron star. During this process, the outermost shell of the white dwarf is blown off in a Type 1a supernova.  
    ::大约一半的恒星,如太阳,是二元系统的成员, 是一个重力结合的恒星。 对于比太阳大得多的恒星来说, 二元系统的一部分甚至更高。 这具有有趣的影响, 特别是当重力结合系统中的一颗恒星比其他恒星大得多时。 在二元系统中, 较大型的恒星的寿命期较短, 它会先演进。 比方说, 较大型的恒星最初是 2 个 MSun , 它会通过红色巨星阶段演变, 最终变成一个质量为1.3 MSun( 低于1.4 Mmun ) 的白矮星。 现在, 10亿年后, 另一颗恒星( 质量为1.5 Mmun ) 将经历一个红色巨星级。 随着恒星的膨胀, 它开始将一些外壳倾弃在白矮星身上。 如果白矮星在Chandrsekhar 极限上交叉, 它将无法被电解, 并开始崩溃成一个中子恒星。 在此过程中, 最外壳的外壳将在1型超级中爆炸 。

    The supernova explosion of a single massive star (called a Type II) supernova can   exhibit  a wide range of brightnesses. However, a Type 1a supernova is a carefully regulated process - mass from the evolving binary companion is funneled onto the white dwarf, and when the white dwarf hits the magic limit of about 1.4 M sun , the Type 1a supernova occurs. It's always the same physical process, so it is always the same brightness.  A Type 1a supernova is a very bright standard candle. 
    ::单颗巨星( 称为二类) 超新星的超新星爆炸 能够展示出各种亮度。 然而, 1型超新星是一个精心监管的过程 — 进化中的二进制伴侣的质量被渗入白矮星, 当白矮星达到1.4 兆子的魔力极限时, 1型超新星就会发生。 它总是相同的物理过程, 所以它总是相同的亮度。 1型超新星是一个非常亮的标准蜡烛。

    The     video   below is  a TedX talk about stellar evolution by Dame Jocelyn Bell Burnell
    ::以下影片由Jocelyn Bell Burnell 夫人拍摄,

    What fraction of stars will end up as neutron stars or black holes?  How might you estimate the total number of neutron stars or black holes in our galaxy?