天文学家首次拥有一张可靠的太空地图【第八十期】

Apr 26th 2018

FOR something so enormous, astronomers know remarkably little about the Milky Way, Earth’s home galaxy. They know its rough dimensions—somewhere between 100,000 and 180,000 light-years across. And they know that it contains 100bn stars—or perhaps 200bn, or maybe even twice that again.

对于如此巨大的天体，天文学家对银河系、地球所在的星系知之甚少，仅知道它的大致尺寸——大约在10万到18万光年之间，他们还知道，它包含了1000亿颗恒星——或许是2000亿颗，甚至可能是这一数字的两倍。

For part of the galaxy, though, things are about to become much clearer. On April 25th the European Space Agency’s Gaia satellite released one of the biggest chunks of data in the history of astronomy. Around 1.3bn stars, perhaps about 1% of the Milky Way’s total, have had their position, brightness and motion measured accurately for the first time.

然而，对于银河系的一部分，事情将变得更加清晰。4月25日，欧洲航天局的盖亚卫星发布了天文学历史上最大的数据集之一。大约13亿颗恒星，可能是银河系总数的1%，它们的位置、亮度和首次精确测量的运动。

The result is a stellar atlas of unprecedented size and accuracy—as well as great beauty. Gerry Gilmore, an astronomer at the University of Cambridge and one of Gaia’s scientists, showed an audience in London a slice of space so thick with stars that they looked like grains of icing sugar poured onto a sheet of black paper. And unlike an ordinary map, which is fixed and unchanging, Gaia’s map moves. In the five years since its launch, the satellite has taken dozens of pictures of every section of sky, which means the stars can be tracked as they float through space. A video, exaggerated to make the motions clear, showed the thousands of stars in Dr Gilmore’s image drifting across the heavens as they orbit around the centre of the galaxy.

这是一个规模和准确性都空前的恒星地图集，同时非常的美。剑桥大学(University of Cambridge)的天文学家格里·吉尔莫尔(Gerry Gilmore)和盖亚(Gaia)的一位科学家，在伦敦向观众展示了一片布满繁星的宇宙空间，他们看上去就像糖粉粒，倒在一张黑色的纸上。不像普通的固定不变的地图，盖亚的地图是移动。自发射以来的5年里，这颗卫星已经在天空每一部分都拍摄了几十张的照片，这意味着它可以追踪星星在太空中的漂浮。一段被夸大了的视频显示，吉尔莫博士的影像中数千颗星星在太空漂浮，就像数千颗恒星环绕银河系中心的轨道上漂浮。

Those movements will provide valuable clues about the forces that have shaped the structure of the galaxy, allowing astronomers to reconstruct its history. Thanks to a spectrometer aboard the satellite, many of Gaia’s stars can have their chemical compositions examined, which in turn will reveal their age.

这些运动轨迹将提供有价值的线索，来了解形成星系结构的力，让天文学家得以重建它的历史。多亏了这颗卫星上的光谱仪，盖亚的许多恒星都可以检测出它们的化学成分，而这些化学成分也会显示出它们的年龄。

The sheer number of observations should help almost every corner of astronomy, from the Milky Way’s companion dwarf galaxies (rich repositories of dark matter, a mysterious substance that makes up about a quarter of all the stuff in the universe) to supernovas. White dwarfs, for instance, are the shrunken, cooling and superdense remains of sun-sized stars that have run out of hydrogen to fuse. Until now, astronomers had reliable information for only a dozen or so. Gaia’s new data will boost that to 26,000. Similarly enormous hauls of data are expected for everything from exoplanets to Kuiper Belt objects, which are dark and distant piles of rubble left over from the formation of the solar system and which circle the sun beyond the orbit of Neptune.

从银河系的同伴矮星系(储存大量暗物质，构成宇宙中大约四分之一的神秘物质)到超新星，大量的观测可以帮助几乎天文学的每一个角落。例如，白矮星是缩小、冷却和高密度的太阳大小的恒星，是因熔化耗尽了氢的残骸。到目前为止，天文学家只有十几个星系的可靠信息。盖亚的新数据将使这一数字增加到2.6万。同样，从太阳系外的行星到柯伊伯带天体，也有同样巨大的数据，这些是黑暗遥远的，由太阳系形成后遗留下来的碎石堆成的天体，它围绕着海王星的轨道之外，围绕太阳运行。

The data could even help resolve some fundamental disputes. Dark energy is a mysterious force that seems to be accelerating the rate at which the universe is expanding. The speed of that expansion can be determined in two ways. One uses the cosmic microwave background radiation, which is the faint afterglow of the Big Bang. The other relies on measuring the speed at which distant objects recede. The two give answers that differ by a few percentage points. By improving the accuracy with which distances are known, Gaia might remove that discrepancy—or, even more intriguingly, confirm that it is real.

这些数据甚至可以帮助解决一些基本的争端。暗能量是一种神秘的力量，它似乎正在加速宇宙膨胀的速度。这种扩张的速度可以用两种方式来决定。一种是使用宇宙微波背景辐射，这是大爆炸的微弱余辉。另一种依赖于测量远处物体的后退速度。两者给出的答案相差几个百分点。通过已知距离提高距这两种方式的精确度，盖亚可能会消除这种差异——或者更进一步，确认它是真实的。

The present data dump is only the satellite’s second (the first, a much smaller release, took place in 2016). At least two more are planned over the next few years. Indeed, there is so much data that, rather than try to analyse it themselves—normal practice for any scientific project—Gaia’s controllers have made it all immediately available for anyone to use. The first data release, says Dr Gilmore, has been generating an average of one scientific paper a day for the past two years. And that was merely the rumble that heralds an avalanche.

目前的数据转储只是卫星的第二个数据(第一个是在2016年释放的更小的数据)。在接下来的几年里至少还有两个计划。事实上，这么多的数据，他们并没有试图分析分析这些数据，通常会用于任何科学项目——盖亚的管理者们让任何人都能立即使用这些数据。Gilmore博士说，在过去的两年里，第一个数据的发布，平均每天产生一份科学论文。巨变仅刚刚开始。

This article appeared in the Science and technology section of the print edition under the headline "Into the unknown"