天文學家發(fā)現(xiàn)了一顆被黑洞吞噬卻幸存下來的恒星

天文學家發(fā)現(xiàn)了一顆被黑洞吞噬卻幸存下來的恒星

When black holes swallow down massive amounts of matter from the space around them, they're not exactly subtle about it. They belch out tremendous flares of X-rays, generated by the material heating to intense temperatures as it's sucked towards the black hole, so bright we can detect them from Earth.

當黑洞吞噬它們周圍空間中的大量物質(zhì)時，它們并不是那么微妙。它們釋放出巨大的x射線耀斑，這種物質(zhì)在被吸向黑洞的過程中被加熱到極高的溫度，因此非常明亮，我們可以從地球上探測到它們。

This is normal black hole behaviour. What isn't normal is for those X-ray flares to spew forth with clockwork regularity, a puzzling behaviour reported last year from a supermassive black hole at the centre of a galaxy 250 million light-years away. Every nine hours, boom - X-ray flare.

這是正常的黑洞行為。不正常的是，這些X射線耀斑以時鐘規(guī)律性的方式噴發(fā)，這是去年從2.5億光年外星系中心的超大質(zhì)量黑洞所報告的令人費解的行為。每九個小時，爆炸X光閃光。

After careful study, astronomer Andrew King of the University of Leicester in the UK believes he has identified the cause - a dead star that's endured its brush with a black hole, trapped on a nine-hour, elliptical orbit around it. Every close pass, or periastron, the black hole slurps up more of the star's material.

英國萊斯特大學(University of Leicester)的天文學家安德魯·金(Andrew King)經(jīng)過仔細研究后認為，他已經(jīng)確定了原因——一顆死星與黑洞擦肩而過，被困在圍繞它的9小時橢圓軌道上。黑洞的每一次近距離飛行，或者說星周，都會吸食更多的恒星物質(zhì)。

"This white dwarf is locked into an elliptical orbit close to the black hole, orbiting every nine hours," King explained.

金解釋說：“這顆白矮星被鎖定在靠近黑洞的橢圓軌道上，每9小時繞軌道運行一次。”

"At its closest approach, about 15 times the radius of the black hole's event horizon, gas is pulled off the star into an accretion disk around the black hole, releasing X-rays, which the two spacecraft are detecting."

“在最接近黑洞視界半徑15倍的地方，氣體從恒星中被拉到黑洞周圍的吸積盤中，釋放出兩個航天器正在探測到的X射線。”

The black hole is the nucleus of a galaxy called GSN 069, and it's pretty lightweight as far as supermassive black holes go - only 400,000 times the mass of the Sun. Even so, it's active, surrounded by a hot disc of accretion material, feeding into and growing the black hole.

黑洞是一個名為GSN 069的星系的核心，就超大質(zhì)量黑洞的質(zhì)量而言，它是相當輕的——只有太陽質(zhì)量的40萬倍。盡管如此，它仍然是活躍的，被一個熱的吸積盤包圍著，吸積物質(zhì)進入黑洞并使其成長。

According to King's model, this black hole was just hanging out, doing its active accretion thing, when a red giant star - the final evolutionary stages of a Sun-like star - happened to wander a little too close. The black hole promptly divested the star of its outer layers, speeding its evolution into a white dwarf, the dead core that remains once the star has exhausted its nuclear fuel (white dwarfs shine with residual heat, not the fusion processes of living stars).

根據(jù)金的模型，當一顆紅巨星——一顆類似太陽的恒星的最后進化階段——碰巧離黑洞有點太近的時候，這個黑洞正在四處游蕩，進行著它活躍的吸積活動。黑洞迅速剝離了恒星的外層，加速了其演化為白矮星，也就是恒星耗盡核燃料后剩下的死核(白矮星發(fā)出余熱，而不是恒星的聚變過程)。

But rather than continuing on its journey, the white dwarf was captured in orbit around the black hole, and continued to feed into it.

但是，這顆白矮星并沒有繼續(xù)它的旅程，而是在環(huán)繞黑洞的軌道上被捕獲，并繼續(xù)向黑洞中進食。

Based on the magnitude of the X-ray flares, and our understanding of the flares that are produced by black hole mass transfer, and the star's orbit, King was able to constrain the mass of the star, too. He calculated that the white dwarf is around 0.21 times the mass of the Sun.

根據(jù)X射線耀斑的大小，以及我們對黑洞質(zhì)量轉(zhuǎn)移產(chǎn)生的耀斑和恒星軌道的理解，金能夠限制恒星的質(zhì)量。他計算出白矮星的質(zhì)量大約是太陽的0.21倍。

While on the lighter end of the scale, that's a pretty standard mass for a white dwarf. And if we assume the star is a white dwarf, we can also infer - based on our understanding of other white dwarfs and stellar evolution - that the star is rich in helium, having long ago run out of hydrogen.

而在天平的較輕一端，這對于白矮星來說是一個相當標準的質(zhì)量。如果我們假設(shè)這顆恒星是一顆白矮星，根據(jù)我們對其他白矮星和恒星演化的了解，我們也可以推斷出這顆恒星很久以前就已經(jīng)耗盡了氫。

"It's remarkable to think that the orbit, mass and composition of a tiny star 250 million light years away could be inferred," King said.

金說:“能夠推斷出2.5億光年外的一顆小恒星的軌道、質(zhì)量和組成，這是很了不起的。”

Based on these parameters, he also predicted that the star's orbit wobbles slightly, like a spinning top losing speed. This wobble should repeat every two days or so, and we may even be able to detect it, if we observe the system for long enough.

基于這些參數(shù)，他還預(yù)測到恒星的軌道會輕微擺動，就像旋轉(zhuǎn)的陀螺失去速度一樣。這種擺動應(yīng)該每兩天左右重復(fù)一次，如果我們觀察這個系統(tǒng)足夠長的時間，我們甚至可以探測到它。

This could be one mechanism whereby black holes grow more and more massive over time. But we'll need to study more such systems to confirm it, and they may not be easy to detect.

這可能是黑洞隨時間增長越來越大的一種機制。但我們需要研究更多這樣的系統(tǒng)來證實這一點，而且它們可能不容易被發(fā)現(xiàn)。

The research has been published in the Monthly Notices of the Royal Astronomical Society.

這項研究發(fā)表在《皇家天文學會月刊》上。