英语听力：自然百科 穿越银河系的旅行 Through Milky Way—21

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What’s actually going on is the stars are orbiting around the center and that is what keeps them from falling in, in much the same way that the Earth is orbiting around the Sun. 

 

 

 

The planets in our solar system are in a delicate balance between gravity pulling them towards the Sun and their orbital velocity² wanting to pull them outwards³ into space. 

 

 

 

To keep in balance, planets / further from the Sun must orbit more slowly. If you go to / more distant planets at the edge of the solar system, they are going around the Sun much more slowly than the Earth is, and that’s because / the gravity is weaker. The same holds true for stars in the Milky⁴ Way. All of them are orbiting / the center of the galaxy. But the stars in the outer arm should be travelling more slowly then those closer to the galaxy’s heart. 

 

 

 

What’s interesting, that is not what’s going on. The stars in the outer parts of the galaxy are spinning around just as quickly as those in the inner parts. And they are not the only ones, it’s not just our galaxy, it is every galaxy we look at. Every galaxy we look at seems to be spinning too fast in its outer part. These speeding stars should be flying out of the galaxy altogether. But they are not. That is a puzzle. This means that there is a lot more mass there that we just can’t see. 

 

 

 

It’s this mass that must produce the gravity that hold these stars in their orbits. But when astronomers⁵ look for the mass, there appears to be nothing there. It’s led cosmologists like John Premark to an astounding⁶ conclusion.

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