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White Dwarf Star Enters Its Crystallization Era, Turning Into A 'Cosmic Diamond'
To us, stars may resemble cut jewels, glittering coldly against the velvet darkness of the night sky. And for some of them, that may actually be sort of true.
As a certain type of dead star cools, it gradually hardens and crystallizes. Astronomers have found one doing just that in our cosmic backyard, a white dwarf composed primarily of carbon and metallic oxygen just 104 light-years away, whose temperature-mass profile suggests that the center of the star is transforming into a dense, hard, 'cosmic diamond' made up of crystallized carbon and oxygen.
The discovery is detailed in a paper accepted into the Monthly Notices of the Royal Astronomical Society and available on preprint website arXiv.
"In this work we present the discovery of a new Sirius-like quadruple system at 32 parsecs distance, composed of a crystallizing white dwarf companion to the previously known triple HD 190412," write an international team of astronomers led by Alexander Venner of the University of Southern Queensland in Australia.
"By virtue of its association with these main sequence companions, this is the first crystallizing white dwarf whose total age can be externally constrained, a fact that we make use of by attempting to empirically measure a cooling delay caused by core crystallization in the white dwarf."
All things in the Universe must change. Every star that hangs in the firmament, shining brightly with the light generated by atomic fusion, will one day run out of fuel for their fires and evolve into something new.
For the vast majority of stars – those below about eight times the mass of the Sun, and including the Sun – that something is a white dwarf star.
When the fuel runs out, the star's outer material is shucked into the surrounding space, and the remaining core, no longer supported by the outward pressure supplied by fusion, will collapse down into an ultradense object, around the size of Earth (or the Moon!), but packing in as much mass as 1.4 Suns.
The matter in white dwarf stars is highly compressed, but it's prevented from collapsing further by something called electron degeneracy pressure. No two electrons can occupy identical states, and this keeps the white dwarf from becoming even denser, as seen in a neutron star or black hole.
White dwarf stars are dim, but they still shine with residual heat. Over time, they cool, and are expected to evolve into something called a black dwarf stars when they lose all their heat and become a cold lump of crystallized carbon.
Calculations suggest that this process takes a very long time, about a quadrillion years (that's a million billion years); since the Universe is only about 13.8 billion years old, we don't expect to find one anytime soon.
What we can do is identify the signs of crystallization starting in the cores of the white dwarfs we see around us.
During crystallization, the carbon and oxygen atoms inside the white dwarf stop moving about freely and form bonds, arranging themselves into a crystal lattice. Energy is released during this process, which dissipates in the form of heat.
This produces a sort of plateau or slowing in the cooling of white dwarf stars, which can be observed in the color and brightness of the star, making it appear younger than it actually is.
In order to accurately gauge the brightness of a star, you need to know with precision how far away it is, something that has been made much more possible in recent years due to the high-precision stellar mapping conducted by the Gaia mission.
This means that we can now identify crystallizing white dwarfs with a lot more confidence.
Venner and his colleagues were using the Gaia data to search for multiple star systems, by identifying stars whose association with others may have been unclear.
And they found that a recently discovered white dwarf star (remember these things are very dim) was gravitationally bound up with what had been thought to be a system of three stars, named HD 190412.
The discovery of the white dwarf, now named HD 190412 C, made the triplet a quadruplet, but there was more going on. Its properties suggest that it's undergoing the crystallization process.
Whether or not that white dwarf crystal is diamond is unknown; the density of white dwarfs is over around 1 million kilograms per cubic meter, while the density of diamond is about 3,500 kilograms per cubic meter. Denser allotropes of carbon do exist; on the other hand, there's plenty of diamond floating around out there in space.
The other three stars in the system allowed the team to externally constrain the white dwarf's age – something that hasn't been done before for a known crystallizing white dwarf.
The system's age is around 7.3 billion years. The white dwarf's age appears to be around 4.2 billion years. The discrepancy is 3.1 billion years, suggesting that the crystallization rate has slowed the cooling rate of the white dwarf by approximately 1 billion years, the researchers say.
In and of itself, the dating isn't sufficient to alter our models of white dwarf crystallization, but the discovery and its proximity to Earth suggest that there might be many more such systems out there that we can exploit for benchmarking this fascinating process.
"We propose that the discovery of this system at only 32 parsecs suggests that similar Sirius-like systems containing crystallizing white dwarfs are likely to be numerous. Future discoveries may therefore allow for stronger tests of white dwarf crystallization models," the researchers write.
"We conclude that the discovery of the HD 190412 system has opened up a new avenue for understanding crystallizing white dwarfs."
注释：
Crystallization: n
表示" 晶化；结晶"，means "the formation of crystals"，如：This process is called crystallization. 这过程被称为结晶。
velvet: n
表示" 天鹅绒；"，means "a cloth with a thick,short,soft pile on one side"，如：The lawn looks like green velvet. 那草坪看上去就像绿色的天鹅绒。
metallic: adj
表示"金属的；金属般的"，means "like a metal in appearance or sound"，如：This is a kind of metallic paint.这是一种有金属光泽的涂料。
preprint: v
表示" 预先印好"，means "print in advance"，如：Post a preprint on the online system. 在网上张贴预印本。
parsec: n
表示"表示天体间距离的单位,相当于2.262光年"，means "a unit of astronomical length based on the distance from Earth at which stellar parallax is 1 second of arc; equivalent to 3.262 light years"，如：Given that a parsec is a unit of measurement and not of time, it hardly seemed like a speed record. 由于秒差距并不是时间的衡量单位，这看来显然不是速度纪录。
By virtue of
表示"由于，因为；凭借，依靠"，如：He claimed a pension in virtue of his long illness. 由于长期生病，他申请救助金。
empirically：adv
表示"经验主义地"，means "in an empirical manner"，如：Most of the doctors used antibiotics empirically. 多数医生凭经验使用抗生素;
firmament: n
表示" 苍穹；天空"，means "the apparent surface of the imaginary sphere on which celestial bodies appear to be projected"，如：It will cross the entire firmament , and tomorrow the sun will shine once again. 它会穿过整个苍穹，明天太阳又会光照世界。
shuck: v
表示"欺骗; 剥皮"，means "remove the shucks from"，如：You should not shuck a little child. 你不应该去欺骗一个小孩。 Shuck these ears of corn and put them into the boiling water.把玉米叶剥下來后将玉米放到沸水里。
degeneracy：n
表示" 退步；退化"，means "moral perversion; impairment of virtue and moral principles;"，如：Moral degeneracy followed intellectual degeneration. 思想腐化之后紧接着就是道德沦丧。
neutron: n
表示" 中子"，means "an elementary particle with 0 charge and mass about equal to a proton; enters into the structure of the atomic nucleus"，如：Neutron has no charge, either positive or negative.中子不带电荷,既不带正电荷,也不带负电荷。
lump: n
表示" 块；团"，means "a mass of sth solid without a special size or shape; a hard swelling on the body"，如：The surgeon excised the lump from her breast.外科医生切除了她乳房的肿块。
quadrillion: n
表示" 千的五次方"，means "the number that is represented as a one followed by 15 zeros"，如：At more than 1 quadrillion mathematical calculations per second, it is about 55,000 times faster than your typical PC. 它的运算速度大于每秒钟100万亿次，大约比传统意义上的个人电脑快55000倍。
lattice: n
表示" 格子；"，如：Click the left mouse button corresponding lattice placed pawn. 鼠标左键点击相应的格子放置棋子。
dissipate: v
表示" 驱散；消散"，means " cause to disperse; go away"，如：The sun dissipated the mist. 太阳驱散了雾。
Gaia: n
表示" [希神]盖亚(大地女神)"，如：Some say it is depression caused by contemplation of Gaia's suffering. 有些人说这是由于盖亚母神的受苦而带来的消沉。
triplet: n
表示"三胞胎之一；三连音符"，means "one of three offspring born at the same time from the same pregnancy"，如：His wife gave birth to triplets. 他妻子生了三胞胎。
quadruplet: n
表示"四胞胎中之一；[音]四连音"，means "one of four children born at the same time from the same pregnancy"，如：Quadruplet pregnancy is an extremely race, curable and high risk pregnancy. 四胎妊娠是一种极其罕见、可以防治的高危妊娠。
allotrope: n
表示" 同素异形体"，means "a structurally different form of an element"，如：Charcoal and diamond are allotropes of carbon. 木炭和金刚石都是炭的同素异形物
discrepancy: n
表示"差异；不一致；"，means " difference; failure to agree"，如：We'd like to discuss the details of the contract, especially discrepancy and claim.我们想讨论合同的各项细目,特别是差异和索赔。
Sirius: n
表示" 天狼星"，如：Sirius is the brightest star in the sky. 天狼星是天空中最亮的一颗星。
中文简要说明：
著名的钻石广告词，以「手上的星星」来形容钻石，这可能是对的，天文研究发现，像是太阳这样的中型恒星，最后的结局可能会晶化成超巨大的宇宙钻石，它的质量不能以克拉来计算，而是几千亿吨。
科学警报(ScienceAlert)报导，我们太阳这种小质量恒星，最后并不会引发超新星大爆炸，而是在膨胀之后，中心缩小成白矮星，然后缓慢的冷却时。那么白矮星之后又是如何?科学家认为它们会逐渐变硬并结晶，而且天文学家可能发现了一颗进入晶化的白矮星。它是一颗主要由碳和金属氧组成的，距离我们仅 104 光年，其温度质量分布表明，恒星的中心正在转变为致密、坚硬的宇宙钻石。
澳洲南昆士兰大学(USQ )的天文学家亚历山大•凡纳 (Alexander Venner)在32秒差距处，发现了一个新的类天狼星4重星系统，这由一颗结晶化的白矮星伴星组成，与HD 190412的3重星相伴。
凡纳博士说：「这可能是第一颗走向结晶化白矮星，它的年龄可能受到其他伴星的影响，我们利用测量由白矮星核心结晶引起的冷却延迟。」
宇宙中的所有事物都不是一成不变的，悬在天空中的每一颗恒星，虽然看似永不变动，但它们发出的光芒是靠着核融合反应消耗能量，总有一天也会耗尽它们的燃料，并走向下一个阶段。
对于绝大多数恒星(包括太阳，以及质量低于太阳8倍的小恒星)，就会逐步成为白矮星。当燃料耗尽时，恒星的外层物质被吹向周围的太空，恒星变得愈来愈稀薄，最后剩下的核心，无法维持向外扩张的压力，就坍缩成一个密度超级高的小白点，其大小与地球（或月球）相当，这就叫白矮星。著名的印度裔天文学家钱卓西卡(Subrahmanyan Chandrasekhar)计算出，形成白矮星有其质量上限，不得超过太阳质量的1.44倍，这称为「钱卓西卡极限」(Chandrasekhar Limit)，一但超过该值，就会形成更狂暴的中子星。
白矮星暗淡无光，但它们仍会因余热而发光。随着时间的推移，它们还是会冷却，当它们失去所有热量，预计会演化成一种叫做「黑矮星」的东西。计算表明，这个过程非常的缓慢，大约是1兆年之后，这是难以想象的遥远未来，需知如今宇宙只有大约 138 亿年的历史，因此这个宇宙尚未出现任何一个黑矮星。
我们如今能做的是，从我们周围的白矮星核心去寻找是否出现理论里的结晶迹象。在结晶过程中，白矮星内部的碳和氧原子停止自由移动并形成键，排列成晶格。在此过程中释放能量，以热量的形式消散，这是白矮星的发热原理，可以从恒星的颜色和亮度中观察到。近年来，由于盖亚任务进行的高精度恒星测绘，使天文学家更有信心识别结晶化的白矮星，最近的成果就是HD 190412系统中的第4颗，该系统也从原本以为的3重星，修正为4重星。
该系统的年龄约为73亿年，那颗白矮星的年龄似乎约为 42 亿年。研究人员说，这个差异是 31 亿年，这表明结晶速度，使白矮星的冷却速度减慢了大约 10 亿年。
钻石是碳元素在压力下的终极形态，其密度约为每立方公尺3500公斤，而白矮星的密度超过每立方公尺100万公斤，因此它极可能形成宇宙中最硬、密度最大的钻石。
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