Gerhard Hüdepohl
德
国加尔兴(Garching)的科学家正在密切观测大约2.6万光年之外的一个罕见事件：一个超大的黑洞正在吞噬一团巨大的气体云。它让人类得以首次窥见黑洞是如何利用其巨大的引力吸引并吞噬星际物质的——对于这个现象，人类知之甚少。加尔兴马普地外物理研究所(Max Planck Institute for Extraterrestrial Physics)的斯特凡・吉勒森(Stefan Gillessen)说：“云团正在被撕裂。”2011年，吉勒森博士率先让世界注意到了这个事件。
当大型恒星死去，其物质塌缩成一个小得多的体积时，就可以形成黑洞。这相当于把地球的体积压缩成一个弹子球那么大，所形成的引力会强到光线都跑不掉。
现在对黑洞的了解基本上都是理论上的，科学家非常希望能采集到更多的现实数据。最好的望远镜都胜任不了，因为多数黑洞都在几百万光年之外。事实上，直到10年前科学家才确证有一个超大黑洞潜伏在我们附近，就在银河系里。
现在，良机显现，我们的这个黑洞邻居正在上演一些意想不到的太空大戏。这个银河系怪物将近100万年来一直在吸收一坨坨四处流浪的物质。紧绕黑洞飞行的星球证明它的引力相当于400万颗太阳。这种引力现在已经开始作用于那团长度有370亿英里（约595亿公里）左右的气体云。
借助于欧洲资助的、高耸在智利阿塔卡马沙漠(Atacama Desert)的甚大望远镜(Very Large Telescope)采集到的数据，吉勒森博士的团队最近推断，气体云前端运行速度较其尾端快了每秒310英里（约500公里）。云团已经有10%左右的部分被吸引到黑洞的另一侧。
科学家们希望观测这场宇宙大戏至少一年的时间。他们有很多疑问：云团是被撕成一小块一小块并被吞噬，还是会有很大一部分继续绕着黑洞外面的轨道飞行？黑洞会不会随着它的掠食而变得更亮一些？从未在如此极端条件下进行实验检验的爱因斯坦相对论，到时候站不站得住脚？
天体物理学家安德烈娅・盖(Andrea Ghez)在加州大学洛杉矶分校(University of California, Los Angeles)带了另外一个团队来观测这一事件。她说：“天文学领域很少有机会抓住正在发生的事件。我们根本就不知道会有什么事情发生。”
2006年，盖博士的团队注意到有一个物体正在银河系黑洞附近游荡。但她说，这个物体看上去不是特别有意思，所以“我们没去管它”。
五年过后，吉勒森博士已经在用智利的一架望远镜观测围绕黑洞飞行的那些星球。在研究旧数据的时候，他注意到盖博士看到的那个物体。吉勒森博士的同事、马普研究所理论物理学家安德烈亚斯・伯克持(Andreas Burkert)回忆说：“它只是一个小点——不是一个热星球，而是一个冷的东西。我说（观测）可能存在失误。”
2012年1月，马普研究所的团队在《自然》杂志(Nature)上发表了研究结果。他们认为，这个小点是一团来源不明的气体云，处在银河系黑洞的引力范围内。
直到几年前为止，科学家一直很难观测到银河系中心位置的物体，原因之一是地球大气层造成的模糊，也就是造成星星闪烁的那种效应。另外，由于银河系中心位置的尘埃将光线吸收殆尽，光学望远镜基本派不上用场。
吉勒森博士等人改用了以红外线观测银河系核心的望远镜。更晚近的一个创新，则是一项名叫自适应光学、可以纠正模糊效应的技术。研究人员今年已经提出40多套观测黑洞与气体云相遇的方案。
前不久的一天上午，吉勒森博士在他办公室用一台电脑展示了甚大望远镜拍摄到的一连串影像。2002年的时候，以蓝色小点标识的气体云似在进一步向银河黑洞靠近。吉勒森博士说：“它不是直线前进，意味著有个什么东西在牵引着它。”2008年的数据显示气体云在以每秒约800英里（约1,287公里）的速度前进，远快于每秒运行约19英里（约30公里）的地球。很能说明问题的是，最近的发现显示云团在以接近每秒1,250英里（约2010公里）的速度运行，说明黑洞的引力越来越强。
另一套名为“我们眼前的大混乱”(Total Disruption in Front of Our Eyes)的幻灯片显示，气体云的一部分至少已经被拉长到2004年长度的10倍。
然后吉勒森博士走进了一间实验室。在那里，他和马普研究所的其他同事正在组装一个大约两码（约1.8米）长的装置。这个装置名叫“引力机”(Gravity Machine)，造价接近2,700万美元。等2014年夏季组装完毕之后，这架机器将把甚大望远镜四个不同镜筒采集的光线信号集合到一起，从而大大提高测量精度。
黑洞时不时地闪耀出光芒，就像地球大气层在雷电照耀下变亮一样。吉勒森博士说，黑洞与气体云相遇可能会使黑洞的闪光更频繁或更剧烈。那样的话，引力机采集信号将更加容易，从而让科学家获得更多有关黑洞运行机制的信息。
和很多天文观测一样，这一次观测也有一丝超现实的感觉。银河系的黑洞在2.6万光年之外，意味着气体云事件实际上是在2.6万年之前发生的，光线到我们这里就得花这么久时间。盖博士说：“我们是在银河系的边缘地区，所以到现在才看到这部电影，但它是一部很惊艳的电影。”
Gautam Naik
（本文版权归道琼斯公司所有，未经许可不得翻译或转载。）
Scientists in Garching, Germany, are closely watching a rare event some 26,000 light years away: a supermassive black hole in the act of devouring a huge gas cloud. It's providing the first-ever glimpse of how a black hole uses its massive gravitational power to pull in and consume interstellar materials -- a little understood phenomenon.
'The cloud is being torn apart,' said Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics in Garching, who first brought the event to the world's attention in 2011.
A black hole can form when a large star dies and its matter collapses into a much smaller volume. It's as if the mass of the Earth were squeezed into a ball the size of a marble. The resulting gravity is so intense that even light cannot escape.
What is known about black holes is largely theoretical, and scientists are eager to gather more real-world data. The best telescopes aren't up to the job because most black holes are millions of light years away. Indeed, it was only a decade ago that scientists established that a supermassive black hole lurked surprisingly close by, in the Milky Way.
Now, by a stroke of fortune, our neighboring black hole is offering up some unexpected celestial fireworks. The Milky Way monster has drawn in bits of stray material for nearly a million years. Stars closely orbiting the black hole indicate that it has the gravitational pulling power of four million suns. That gravity is now starting to act on the gas cloud, which itself is about 37 billion miles long.
Using data from the European-funded Very Large Telescope, or VLT, perched high up in Chile's Atacama Desert, Dr. Gillessen's team recently concluded that the front of the gas cloud is traveling 310 miles per second faster than its tail. About 10% of the cloud has already been dragged to the far side of the black hole.
Scientists hope to observe the cosmic drama for at least a year. They have many questions: Will the cloud get shredded to bits and sucked in, or will much of it stay in orbit around the black hole? Will the black hole get brighter as it consumes its prey? Will Einstein's theory of relativity, which has never been experimentally tested under such extreme conditions, hold up?
'In astronomy, you rarely have the chance to catch something in the act,' said Andrea Ghez, an astrophysicist who leads a rival group observing the event at the University of California, Los Angeles. 'We just don't know what will happen.'
In 2006, Dr. Ghez's team noticed an object drifting in the vicinity of the Milky Way's black hole. But it didn't look especially interesting, 'so we didn't jump on it,' she said.
Five years later, Dr. Gillessen was using a telescope in Chile to observe stars circling the black hole. Studying old data, he stumbled upon the same object seen by Dr. Ghez. 'It was a little blob -- not a hot star, but something cold,' recalled Andreas Burkert, a theoretical physicist at Max Planck and a colleague of Dr. Gillessen. 'I said [the observation] was probably a mistake.'
In January 2012, the Max Planck team published its findings in the journal Nature. They concluded that the blob was a gas cloud, of unknown origin, in the gravitational clutches of the Milky Way's black hole.
Until a few years ago, scientists could barely observe objects in the center of our galaxy partly because of the blurring caused by the Earth's atmosphere, the same effect that causes stars to twinkle. And because dust at the center of the galaxy absorbs most of the optical light, optical telescopes are largely useless.
Dr. Gillessen and others instead rely on telescopes that view the galactic core in the infrared range of light. A more recent innovation is a technique known as adaptive optics, which can correct for the blurring effect. This year, researchers have made more than 40 proposals to observe the black hole's encounter with the gas cloud.
On a recent morning in his office, Dr. Gillessen used a computer to show a succession of images taken by the VLT. In 2002, the gas cloud, represented by a tiny blue blob, appears to be moving closer to the Milky Way's black hole. 'It's not going in a straight line, which means some object is pulling on it,' said Dr. Gillessen. Data from 2008 show the gas cloud traveling at about 800 miles per second -- a lot faster than Earth, which moves at about 19 miles per second. Tellingly, the most recent findings indicate that the cloud is hurtling at almost 1,250 miles per second. It means that the black hole's tug is getting stronger and stronger.
A separate slide, titled 'Total Disruption in Front of Our Eyes,' showed part of the gas cloud stretched out to at least 10 times its 2004 length.
Dr. Gillissen then headed to a lab where he and other Max Planck colleagues are assembling a roughly 2-yard-long device known as the Gravity Machine, at a cost of almost $27 million. When finished, by the summer of 2014, the machine will combine the light signals from VLT's four separate telescopes to provide far more accurate measurements.
Periodically, the black hole flares up, just as Earth's atmosphere briefly brightens from flashes of lighting. Dr. Gillessen says that the encounter with the gas cloud may cause flares from the black hole to become more frequent or violent. In that case, the gravity machine should pick up those signals more easily, giving scientists more information about how the black hole operates.
Like many astronomical observations, this one has a touch of the surreal. The Milky Way's black hole is 26,000 light years away. That means that the gas cloud event actually occurred 26,000 years ago, and it has simply taken that long for the light to reach us. 'We're out in the suburbs of the galaxy, so the movie only got to us now,' said Dr. Ghez. 'But it's an amazing movie.'
Gautam Naik
'The cloud is being torn apart,' said Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics in Garching, who first brought the event to the world's attention in 2011.
A black hole can form when a large star dies and its matter collapses into a much smaller volume. It's as if the mass of the Earth were squeezed into a ball the size of a marble. The resulting gravity is so intense that even light cannot escape.
What is known about black holes is largely theoretical, and scientists are eager to gather more real-world data. The best telescopes aren't up to the job because most black holes are millions of light years away. Indeed, it was only a decade ago that scientists established that a supermassive black hole lurked surprisingly close by, in the Milky Way.
Now, by a stroke of fortune, our neighboring black hole is offering up some unexpected celestial fireworks. The Milky Way monster has drawn in bits of stray material for nearly a million years. Stars closely orbiting the black hole indicate that it has the gravitational pulling power of four million suns. That gravity is now starting to act on the gas cloud, which itself is about 37 billion miles long.
Using data from the European-funded Very Large Telescope, or VLT, perched high up in Chile's Atacama Desert, Dr. Gillessen's team recently concluded that the front of the gas cloud is traveling 310 miles per second faster than its tail. About 10% of the cloud has already been dragged to the far side of the black hole.
Scientists hope to observe the cosmic drama for at least a year. They have many questions: Will the cloud get shredded to bits and sucked in, or will much of it stay in orbit around the black hole? Will the black hole get brighter as it consumes its prey? Will Einstein's theory of relativity, which has never been experimentally tested under such extreme conditions, hold up?
'In astronomy, you rarely have the chance to catch something in the act,' said Andrea Ghez, an astrophysicist who leads a rival group observing the event at the University of California, Los Angeles. 'We just don't know what will happen.'
In 2006, Dr. Ghez's team noticed an object drifting in the vicinity of the Milky Way's black hole. But it didn't look especially interesting, 'so we didn't jump on it,' she said.
Five years later, Dr. Gillessen was using a telescope in Chile to observe stars circling the black hole. Studying old data, he stumbled upon the same object seen by Dr. Ghez. 'It was a little blob -- not a hot star, but something cold,' recalled Andreas Burkert, a theoretical physicist at Max Planck and a colleague of Dr. Gillessen. 'I said [the observation] was probably a mistake.'
In January 2012, the Max Planck team published its findings in the journal Nature. They concluded that the blob was a gas cloud, of unknown origin, in the gravitational clutches of the Milky Way's black hole.
Until a few years ago, scientists could barely observe objects in the center of our galaxy partly because of the blurring caused by the Earth's atmosphere, the same effect that causes stars to twinkle. And because dust at the center of the galaxy absorbs most of the optical light, optical telescopes are largely useless.
Dr. Gillessen and others instead rely on telescopes that view the galactic core in the infrared range of light. A more recent innovation is a technique known as adaptive optics, which can correct for the blurring effect. This year, researchers have made more than 40 proposals to observe the black hole's encounter with the gas cloud.
On a recent morning in his office, Dr. Gillessen used a computer to show a succession of images taken by the VLT. In 2002, the gas cloud, represented by a tiny blue blob, appears to be moving closer to the Milky Way's black hole. 'It's not going in a straight line, which means some object is pulling on it,' said Dr. Gillessen. Data from 2008 show the gas cloud traveling at about 800 miles per second -- a lot faster than Earth, which moves at about 19 miles per second. Tellingly, the most recent findings indicate that the cloud is hurtling at almost 1,250 miles per second. It means that the black hole's tug is getting stronger and stronger.
A separate slide, titled 'Total Disruption in Front of Our Eyes,' showed part of the gas cloud stretched out to at least 10 times its 2004 length.
Dr. Gillissen then headed to a lab where he and other Max Planck colleagues are assembling a roughly 2-yard-long device known as the Gravity Machine, at a cost of almost $27 million. When finished, by the summer of 2014, the machine will combine the light signals from VLT's four separate telescopes to provide far more accurate measurements.
Periodically, the black hole flares up, just as Earth's atmosphere briefly brightens from flashes of lighting. Dr. Gillessen says that the encounter with the gas cloud may cause flares from the black hole to become more frequent or violent. In that case, the gravity machine should pick up those signals more easily, giving scientists more information about how the black hole operates.
Like many astronomical observations, this one has a touch of the surreal. The Milky Way's black hole is 26,000 light years away. That means that the gas cloud event actually occurred 26,000 years ago, and it has simply taken that long for the light to reach us. 'We're out in the suburbs of the galaxy, so the movie only got to us now,' said Dr. Ghez. 'But it's an amazing movie.'
Gautam Naik
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