英语听力：自然百科 穿越银河系的旅行 Through Milky Way—15
时间：2014-05-27 07:06:48
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（单词翻译）
Each tag is whipping around the center of the galaxy1, the particular, the most striking thing you will notice is the motion of SO2.So SO2 goes on an incredible roller-coaster ride. It comes whipping around and then back out.
Something with tremendous gravitational pull sends SO2 on a rapid orbit / around the center of the galaxy, something with a huge mass.
SO2 goes around once every 15 years. And what it tells us is that there is 4 million times the mass of the sun confined within its orbit.
And
astronomers2 know of only one contender for something that has such a giant mass and it's so small.
So that's incredible amount of mass inside a very small volume and that's the key to prevent a black hole.
So at the center of our galaxy lies a massive black hole, an object whose gravity is so strong that not even light can escape it.
This is an image of the center of our galaxy. We can't see the black hole, but we can see bright clouds of gas and dust spiraling towards it.
We are nearing the black hole, it's at the center of a stream of dust and gas, the entrails of stars
shredded3 after straying too close.
Black holes grow with time and that happens by material falling onto it,
accreting4 onto it. And that material can come in the form of either gas or stars that get torn apart by the black hole itself.
At the center, is the invisible black hole. What we were seeing is material it’s feeding on.
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