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Some people seem born under an unlucky star. But some stars are equally unlucky themselves. Astronomers have spotted a star in another galaxy plunging toward a giant black hole and being ripped to shreds, sparking a flare so brilliant that observers detected it from a distance of 2.1 billion light-years. By watching the flare brighten and fade, scientists have achieved the unprecedented feat of reconstructing the life story of the doomed sun. |
一些人好像出生于不幸的星座之下，但是有些恒星本身也是同样不幸的。天文学家们发现了其他星系中的一颗恒星正在陷入一个黑洞里，并且正在被撕成细条，闪出的光芒异常明亮，致使21亿光年之外的望远镜能够探测到光亮。通过观察这种闪光的逐渐变亮和逐渐变暗，研究人员取得了前所未有的成绩：他们重新构建了这颗注定要灭亡的恒星所走过的生命历程。 |
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Giant black holes occupy the centers of most large galaxies, including our own, whose central black hole is 4 million times as massive as the sun and swallows a star once every 10,000 to 100,000 years. Astronomers have recently seen black holes in several other galaxies rip stars apart. But the new drama is unique. "This is the first time where we're really seeing one of these events from start to finish," says astronomer Suvi Gezari of Johns Hopkins University in Baltimore, Maryland. "What was so spectacular was the fact that we actually could figure out what type of star was disrupted." |
巨型黑洞占据了多数大型星系的中心，包括我们银河系也有黑洞，其质量是太阳的400万倍，每1万年至10万年吞没一颗恒星。最近，天文学家们看到好几个其他星系的黑洞将恒星撕裂。但是，这个新的撕裂事件是独一无二的。“这确实是我们首次看到的一个自始至终的撕裂恒星事件，”马里兰州巴尔的摩市约翰霍普金斯大学的天文学家苏薇·格扎里说，“事实上，最引人关注的事情是，我们能够推测出被撕碎的是哪一种恒星。” |
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Astronomers first picked up a signal from the constellation Draco in May 2010, when the Pan-STARRS 1 telescope in Hawaii spotted a flare at visible and near-infrared wavelengths. The scientists calculate that the black hole's gravity had torn the star apart a month earlier by pulling harder on one side of the star than the other. As stellar debris funneled into the black hole, gravity and friction roasted the star's remains until they emitted ultraviolet radiation, which NASA's GALEX satellite detected in June. The flare peaked in July 2010, outshining all the stars in the galaxy put together, and then faded, but was still aglow a year later. |
2010年5月，天文学家们最初从天龙座采集到一个信号，当时夏威夷全景巡天望远镜及快速反应系统（Pan-STARRS）的1号望远镜探测到可见光和近红外光波长的闪光。科学家们经计算得知，该黑洞提前一个月将这颗恒星撕裂，主要是由于它对恒星一边的引力比另一边更大造成的。随着恒星的残骸漏进黑洞，引力和摩擦力烘烤着其残骸，直到释放出紫外线，这些紫外线被NASA的恒星演化探测（GALEX）卫星于6月份探测到。2010年7月份，闪光的亮度达到顶峰，其亮度超过了该星系中所有恒星加在一起的亮度，然后逐渐变暗，然而在一年之后仍然在发光。 |
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As Gezari's team reports online today in Nature, the black hole is about 3 million times more massive than our sun, slightly smaller than the Milky Way's central black hole, and marks the heart of a galaxy dimmer than our own. The black hole swallowed only about half the star, which lost a huge amount of energy by plummeting into the black hole. Because the laws of physics dictate that energy must be conserved, the rest of the star shot away from the black hole at enormous speed. |
今天，格扎里的研究小组在《自然》杂志在线版上报道说：这个黑洞大约是我们太阳的300多万倍，比银河系中心的黑洞稍微小一些；该黑洞标志着那个星系的中心位置，而那个星系要比我们自己的星系暗淡一些。黑洞只吞没了大约半个恒星，恒星由于跌落到黑洞中而损失了巨大的能量。因为物理定律表明能量一定是保留了下来的，所以恒星余下的部分以极高的速度从黑洞射出去。 |
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The team has also pieced together the life story of the unlucky star. Born roughly a billion years ago, it once generated energy as the sun does, converting hydrogen into helium at its core. When the core filled with helium, the star became a red giant—a helium core surrounded by a puffy outer layer of hydrogen. |
研究人员还将这颗不幸的恒星走过的生命历程拼凑起来。该恒星大约诞生于10亿年以前，一度像太阳那样产生能量，在恒星核处将氢转换为氦，当恒星核处充满了氦之后，该恒星变成了红巨星。红巨星是由氦构成的恒星核，恒星核的外围环绕着一层膨胀的氢。 |
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But the distant sun was doomed. Gezari doesn't know whether it was born in a bad orbit or another star's gravity kicked it into one, but the star began approaching the black hole. Before its fiery demise, when the star was about as far from its nemesis as Pluto is from the sun, the black hole stripped off its hydrogen envelope. That left the star with only its helium core, which was a third of the sun's diameter and bore a quarter of its mass. When the star ventured within 50 million kilometers, slightly closer than Mercury is to the sun, the black hole tore it to pieces. Gezari and her colleagues can infer its composition because they detect helium but no hydrogen in its glowing remains. |
但是，这颗远方的恒星难逃一劫。格扎里不知道这颗恒星是否生来就处于一条有缺陷的轨道之中，也不知道是否另外一颗恒星的引力将其踢进一条这样的轨道之中，然而这颗恒星开始朝着黑洞移动了。燃烧终止之前，当该恒星距离黑洞如同冥王星距离太阳那么远的时候，黑洞剥去了其外围的氢。这样，该恒星只剩下氦核了，这个氦核有太阳直径的三分之一，太阳质量的四分之一。当该恒星冒险前进到距离黑洞5,000万公里范围内的时候，这时比水星距离太阳稍微近一点，黑洞将其撕成碎片。格扎里及同事可以推知该恒星的构成成分，因为在他们在发光的残骸中探测到氦而没有探测到氢。 |
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"It's quite impressive," says astronomer Giuseppe Lodato of the University of Milan in Italy, who was not affiliated with the scientists who discovered the event. "They're able to infer quite a few details not just about the black hole, but also about the kind of star that has been disrupted." |
米兰大学的天文学家吉尤色比·罗达托说：“该研究很令人难忘——不仅有关黑洞，而且有关被撕碎的是何种恒星，他们都能够推知相当多的详细情况。”罗达托并不是做出该发现的科学家之一。 |
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Ironically, only in death could astronomers study the star's life: It was so distant that had it not been destroyed, no telescope could have seen it. |
具有讽刺意味的是：只有在该恒星死亡之时，天文学家们才能够研究该它一生的经历，因为它太遥远了，若不是被摧毁的话，没有任何望远镜可以探测到它。 |
