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照亮黑洞(Illuminating Black Holes) 【中】

(2008-04-14 09:06:26)





分类: 谈天说地

The Keck Control Room is 20 miles from the telescope, in the ranching town of Waimea. To the researchers there, the spectacular laser is visible only as a wan beam in a live video feed on a computer monitor. The astronomers check their notebooks and watch screens full of telescope data, weather readings and the latest picture of the stars they're targeting. They use a video link to talk to the telescope operator, who will spend all night at the summit. Things are going so smoothy that there isn't much to do. The telescope will stay locked on the same spot in the sky for four hours; the laser's working fine, and a camera attached to the telescope takes one 15-minute exposure after another in an automated sequence. "This is just about the dullest kind of observing there is," university of California at Los Angeles astronomer Mark Morris says apologetically.

译文:Keck的控制室坐落在离望远镜20英里的Waimea的农场小镇。对于那里的研究人员来说,如此壮观的激光只能通过现场视频输入到电脑显示器上播放出来,而显现出来的只是一束苍白的光束。天文学家一边查看他们的笔记本,一边注视着显示屏,上面全是望远镜数据,天气信息和他们的目标星球的照片。他们通过视频与将在山顶待上一整夜的望远镜操作人员进行交谈。事情发展得非常顺利,因而没有多少事情要做。望远镜将连续四小时锁定在天空中的同一点,激光工作正常,附在望远镜上的照相机将自动而连续地进行每次十五分钟的曝光。“最枯燥的观测就是这样的。”加州大学洛杉矶分校(UCLA)的Mark Morris抱歉地说。


Even so, there is tension in the room. This team of astronomers, led by Andrea Ghez of UCLA, is in a heated competition whith astronomers at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. For more than a decade, Garching astrophysicist Reinhard Genzel and his colleagues have studied the black hole at the center of the Milky Way using the New Technology Telescope and the Very Large Telescope array in Chile. Ghez, 42, pushes her students to get the most out of each observation session at Keck. Four years ago she was elected to the National Academy of Sciences-- quite an honor for someone still in her 30s. "It's easy to be at the forefront of astronomy if you have access to the best telescopes in the world," she says.

译文:即使是这样,房间里仍然充满着紧张的气氛。由UCLA的Andrea Ghez 领导的这支天文学家团队正与德国Garching的Max Planck宇宙物理研究院进行着激烈的竞争。十多年的时间里,Garching的天体物理学家Reinhard Genzel和他的同事已经利用新技术望远镜和智利的大型望远镜序列对银河系中心的黑洞进行了研究。42岁的Ghez督促她的学生尽可能的从Keck的每一个观测得到更多的数据。四年前她被选入国家科学院,对于才三十多岁的她来说,那是一个很大的荣誉。“如果你能够接触到最好的望远镜,就很容易站在天文学的最前沿。”她说。


Several years ago the American and the German teams independently deduced that only a giant black hole could explain the behaviors of stars at the Milky Way's core. Stars circling a hefty mass-- whether a black hole or some large star--travel through space much faster than those circling a smaller mass. In visual terms, the larger mass creates a deeper funnel in the fabric of space around which the stars revolve; like leaves circling a whirlpool, the deeper the whirl-pool, the faster the leaves spin. Other astronomers had seen fast-moving stars and clouds of gas near the center of the Milky Way, so both Ghez and Genzel suspected a dense cluster of matter was hidden from view.



By painstakingly compiling infrared photographs taken months and years apart, the two teams tracked the innermost stars, those within one light-month of the galaxy's center. Combined, the images are like time-lapse movies of the motions. "Early on, it was clear there were a few stars that were just hauling," Ghez recalls, "Clearly , they were extremely close to the center." Something was trapping the stars in a deep whirlpool. A black hole made the most sense.



The clincher came in 2002, when both teams sharpened their images using adaptive optics, technology that compensates for the atmosphere's blur. The scientists followed stars that orbit perilously close to the galaxy's center and found that the fastest star's top speed was 3 percent of the speed of light-- about 20 million miles per hour. That's a startling speed for a globe of gas far bigger than our sun, and it convinced even the skeptics that a super massive black hole was responsible.


The blur of Earth's atmosphere has plagued telescope users since Galileo's first studies of Jupiter and Saturn nearly 400 years ago. Looking at a star through air is like looking at a penny on the bottom of a swimming pool. Air currents make the starlight jitter back and forth, just as a penny's image seems to dart around the pool's bottom.



In the 1990s, engineers learned to erase the distortions with a technology called adaptive optics. They had computers analyze the jittering patern of incoming starlight on a millisecond by millisecond basis, and used those calculations to drive a set of pistons on the back of a thin and pliable mirror. The pistons flexed the mirror hundreds of times each second, adjusting the surface to conteract the distortions and form a sharp central point instead of a fuzzy blob.



The technology had one severe limitation. The computers needed a strong, clear guiding light to track, as a kind of reference point. The system worked only if the telescope was aimed close to a bright star or planet, limiting astronomers to just 1 percent of the sky.



By creating an artificial guide star in any part of the sky, the Keck Observatory's laser removes that barrier. The laser beam is tuned to a frequency that lights up sodium atoms, which are left by disintegrating meteorites in a thin layer of the atmosphere. Keck's computers analyze the disortion in the column of air between the telescope mirror and the laser-created star.



Inside the telescope's 101-foot-tall dome, the laser system sits within a bus-size enclosure. The laser starts out with a jolting 50,000 watts of power, amplifying the light beam within a dye solution made from 190-proof ethanol. But by the time the light is adjusted to its correct color and its energy is channeled along a single path, its power dwindles to about 15 watts--still bright enough that the Federal Aviation Administration requires the obervatory to shut down the laser if an airplane flies near its path. From several hundred feet away the laser looks like a dim amber pencil beam. A bit farther and it isn't visible at all. As far as the rest of the island is concerned, there is no laser show at Mauna Kea.



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