Here are some images of clusters of galaxies.
这是一些星团的照片。
They're exactly what they sound like.
正如听上去一样。
They are these huge collections of galaxies,
它们是一堆星系，
bound together by their mutual gravity.
由于互相的引力而吸引在一起。
So most of the points that you see on the screen are not individual stars,
你在屏幕上看到的大部分点，不是单独的恒星，
but collections of stars, or galaxies.
而是一系列恒星，或者称为星系。
Now, by showing you some of these images,
现在通过展示这些图片，
I hope that you will quickly see that galaxy clusters are these beautiful objects,
希望你已经看到星团是很美丽的东西，
but more than that,
都不止于此，
I think galaxy clusters are mysterious,
我认为星团是神秘的，
they are surprising,
它们让人惊奇，
and they're useful.
而且它们很有用。
Useful as the universe's most massive laboratories.
作为宇宙里最大的实验室，它们很有用。
And as laboratories, to describe galaxy clusters is to describe the experiments that you can do with them.
就像描述实验室一样，要描述星团，就要去描述你能够通过它们做的实验。
And I think there are four major types,
我认为那包括四种主要类型，
and the first type that I want to describe is probing the very big.
我想描述的第一类是去探测“极大”。
So, how big?
那么，是多大？
Well, here is an image of a particular galaxy cluster.
这里是某个星团的一张照片。
It is so massive that the light passing through it is being bent, it's being distorted by the extreme gravity of this cluster.
它的质量非常大，以至于光在经过的时候，由于其引力的作用而变弯了。
And, in fact, if you look very carefully you'll be able to see rings around this cluster.
事实上，如果你仔细看，能够看到这个星团周围的光环。
Now, to give you a number,
具体到数字，
this particular galaxy cluster has a mass of over one million billion suns.
这个星团的质量有超过10的15次方个太阳那么大。
It's just mind-boggling how massive these systems can get.
这些系统质量之巨大简直让人瞠目结舌。
But more than their mass,
但除了巨大的质量之外，
they have this additional feature.
他们还有其他特殊之处。
They are essentially isolated systems,
它们基本上可以算是孤立的系统，
so if we like, we can think of them as a scaled-down version of the entire universe.
如果愿意的话，我们可以将它们想象为整个宇宙的缩小版本。
