英语听力：自然百科 穿越银河系的旅行 Through Milky Way—20（在线收听） |
They were once thought to mark the outer limits, but today astronomers believe the Milky Way galaxy is much bigger than what we can see. To understand why, we will travel to the stars of the outer galactic disc. Far from our usual place in space, we approach the edges of the galaxy where we may be able to discover what holds all the stars together to make our Milky Way.
Here our night sky looks a little different, the Milky Way is smaller and the sky is darker, but even though with tens of thousands of light years further away from the center of our galaxy, we’re still bound to it by the force of gravity.
Gravity is the force that makes any two objects on a move towards each other.
On earth, cities are built from iron girders in concrete beams. This largely invisible scaffold holds these buildings up against the pull of the gravity. Without this scaffold in, the/ skyscrapers would crumble, and the bridges would collapse.
But gravity not only governs us on earth, it governs throughout the universe. Its laws are straightforward: anything that has mass has a gravitational pull; the greater the mass, the stronger the pull.
With 200 billion stars, the Milky Way has a huge mass and tremendous gravitational attraction. So like buildings, our galaxy also needs propping up against the force of gravity. |
| 原文地址：http://www.tingroom.com/lesson/zrbaike/2011/260167.html |