天文学家发现了一颗被黑洞吞噬却幸存下来的恒星

天文学家发现了一颗被黑洞吞噬却幸存下来的恒星

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.

当黑洞吞噬它们周围空间中的大量物质时，它们并不是那么微妙。它们释放出巨大的x射线耀斑，这种物质在被吸向黑洞的过程中被加热到极高的温度，因此非常明亮，我们可以从地球上探测到它们。

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.

这是正常的黑洞行为。不正常的是，这些X射线耀斑以时钟规律性的方式喷发，这是去年从2.5亿光年外星系中心的超大质量黑洞所报告的令人费解的行为。每九个小时，爆炸X光闪光。

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.

英国莱斯特大学(University of Leicester)的天文学家安德鲁·金(Andrew King)经过仔细研究后认为，他已经确定了原因——一颗死星与黑洞擦肩而过，被困在围绕它的9小时椭圆轨道上。黑洞的每一次近距离飞行，或者说星周，都会吸食更多的恒星物质。

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

金解释说：“这颗白矮星被锁定在靠近黑洞的椭圆轨道上，每9小时绕轨道运行一次。”

"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."

“在最接近黑洞视界半径15倍的地方，气体从恒星中被拉到黑洞周围的吸积盘中，释放出两个航天器正在探测到的X射线。”

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.

黑洞是一个名为GSN 069的星系的核心，就超大质量黑洞的质量而言，它是相当轻的——只有太阳质量的40万倍。尽管如此，它仍然是活跃的，被一个热的吸积盘包围着，吸积物质进入黑洞并使其成长。

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).

根据金的模型，当一颗红巨星——一颗类似太阳的恒星的最后进化阶段——碰巧离黑洞有点太近的时候，这个黑洞正在四处游荡，进行着它活跃的吸积活动。黑洞迅速剥离了恒星的外层，加速了其演化为白矮星，也就是恒星耗尽核燃料后剩下的死核(白矮星发出余热，而不是恒星的聚变过程)。

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，这颗白矮星并没有继续它的旅程，而是在环绕黑洞的轨道上被捕获，并继续向黑洞中进食。

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.

根据X射线耀斑的大小，以及我们对黑洞质量转移产生的耀斑和恒星轨道的理解，金能够限制恒星的质量。他计算出白矮星的质量大约是太阳的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.

而在天平的较轻一端，这对于白矮星来说是一个相当标准的质量。如果我们假设这颗恒星是一颗白矮星，根据我们对其他白矮星和恒星演化的了解，我们也可以推断出这颗恒星很久以前就已经耗尽了氢。

"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.

金说:“能够推断出2.5亿光年外的一颗小恒星的轨道、质量和组成，这是很了不起的。”

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.

基于这些参数，他还预测到恒星的轨道会轻微摆动，就像旋转的陀螺失去速度一样。这种摆动应该每两天左右重复一次，如果我们观察这个系统足够长的时间，我们甚至可以探测到它。

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.

这可能是黑洞随时间增长越来越大的一种机制。但我们需要研究更多这样的系统来证实这一点，而且它们可能不容易被发现。

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

这项研究发表在《皇家天文学会月刊》上。