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For use with Chapter 30
Universal Gravitation

Our Galaxy Has a Hole in It
Posted November 13th, 2000


Artist's concept of a massive black hole and surrounding accretion disk

The Milky Way is a spiral galaxy, which means the stars swirl around a bulging, massive center. Our sun is located in an outer region of one of the minor spiral arms. Decades ago, scientists thought that the center of the galaxy was just a grouping of stars.

Researchers at the University of California, Los Angeles, have confirmed that there is a black hole in the center of the Milky Way. This concept was speculated on for years, but it was only recently when astronomers were able to pinpoint the location of the black hole by observing the movement of orbiting stars. The strong gravitational pull of the black hole dictates the movements of neighboring stars.

A black hole is a highly dense object. It has a lot of mass packed into a small volume. Theoretically black holes are singularities, which are large amounts of mass packed into an infinitely small volume. With all of that mass in such a tiny spot, the gravitational pull is enormous so strong that not even light can escape.

Because black holes give off no light, they cannot be "seen" in the traditional sense. To observe the black hole, the researchers were able to locate the center of the galaxy, known as Sagittarius A, by detecting radio waves from that area. For years, it has been speculated that this source of radio signals was indeed a black hole. Then the researchers measured how other stars and gases interacted with Sagittarius A. Black holes can be detected by x-rays that are emitted as gas falls into the black hole and is excited.

Researchers were able to study the velocities and trajectories of the stars and gases surrounding Sagittarius A. They could also study their centripetal acceleration as they spun around the central point.

It is theorized that the black hole at Sagittarius A could be holding millions of stars within a very small volume roughly in an area that could fit within the orbit of Mars.

This research gives scientists new insights into the structure and formation of the galaxies. One of the main goals of using a powerful X-ray observatory is to determine the existence of black holes in other galaxies.

Activity
Use the Internet to investigate black holes and our galaxy. Record this information in your Science Journal and draw a picture depicting the black hole in the center of our galaxy.

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