Here are some images of clusters（串） of galaxies（星系）. They're exactly what they sound like. They're these huge collections（搜集） of galaxies（星系） bound together by their mutual（相互的） gravity. So most of the points that you see on the screen are not individual stars, but collections of stars or galaxies. Now, by showing you some of these images, I hope that you will quickly see that galaxy clusters are these beautiful objects. But more than that, I think galaxy clusters are mysterious（神秘的）, they're surprising, and they're useful, useful as the universe（ 宇宙）'s most massive（厚重的） laboratories（实验室）.

  And as laboratories, to describe galaxy clusters is to describe the experiments that you can do with them. And I think there are four major types, and the first type that I want to describe is probing（调查） the very big. So how big? Well, here is an image of a particular galaxy cluster. It is so massive that the light passing through it is being bent（弯曲的）. It's being distorted（扭曲） by the extreme gravity（地心引力） of this cluster（串）.

  And in fact, if you look very carefully, you'll be able to see rings around this cluster. Now, to give you a number, this particular galaxy cluster has a mass（块） of over one million billion suns. It's just mind-boggling how massive these systems can get. But more than their mass, they have this additional（附加的） feature. They are essentially（ 本质上） isolated（使隔离） systems. So if we like, we can think of them as a scaled（刻度）-down version（版本） of the entire（全部的） universe（ 宇宙）.

  And many of the questions that we might have about the universe at large scales, such as how does gravity work, might be answered by studying these systems. So that was very big. The second thing is very hot. OK, if I take an image of a galaxy cluster and I subtract（减） away all of the starlight, what I'm left with is this big blue blob. This is in false（不真实的） color. It's actually x-ray（射线） light that we're seeing.

  And the question is, if it's not galaxies, what is emitting（发出） this light? The answer is hot gas, million-degree gas. In fact, it's plasma（[等离子] 等离子体）. And the reason why it's so hot goes back to the previous（先的） slide（滑）.