Quasars, Pulsars, and Black Holes

Quasars: Neither Galaxies Nor Stars

Our galaxy has a diameter of about 100,000 light years, and contains about 100 billion stars. But there are other objects in the universe that emit up to 100 times as much energy as our galaxy, yet can be as small as our solar system. These enormous, high-energy emitting objects are called quasars. (The word "quasar" was taken from the expression "quasi-stellar radio source", which means an onject like a star that emits radio waves.)        Quasars are not galaxies or stars, but they have characteristics of both. If you look at a quasar through a normal telescope it looks like a normal, faint star. However, radio telescopes reveal quasars to be strong emitters of radio waves. Because quasars produce such high amounts of energy, yet appear only as faint points of light, scientists consider quasars to be the oldest, most distant, and most powerful sources of energy in the universe.

Pulsars

In 1967, an astronomer named Jocelyn Bell was making observations of stars with a large radio telescope. During one session, she detected short, rapid pulses of radio waves. Each pulse lasted about 1/100 of a second, and the time between pulses was one second. No star or observed galaxy had ever been observed that gave off signals like this. Bell and her co-workers thought they must be picking up signals from a recently launched satellite.        After two days it was clear the satellite explanation was wrong. The pulsing waves were coming from a source that did not move in the sky. Also, whatever the source was, it was located a very far way away from our solar system.        By the end of that year, Bell had found three more of these sources of pulsing radio waves. They were called pulsars because of the way they produced pulses of radio waves. Today, more then 400 pulsars have been identified. (Pulsars are also called neutron stars.)        What are pulsars and why do they send out pulses of energy? Astronomers have calculated that pulsars are very small, only about 20 km in diameter. The mass of a pulsar, however, is the same as the mass of a star. Astronomers think that pulsars must be rotating while giving off energy. This is like the rotating light of a lighthouse. It would appear as a pulse from someone far away. To sum things up, a pulsar is a dense, small, rotating object that emits pulsing radio waves. You wouldn't be able to see a pulsar even with a good telescope.

Black Holes

One of the most unusual objects in the universe is called a black hole. A black hole is an extremely small, dense core of a star. A black hole has such a strong force of gravity that it pulls in everything near it. Its pull is so strong that even light cannot escape from it, so it cannot be seen. This explains why we call it black.        If we cannot see black holes, why do astronomers think that they exist? The answer is that astronomers have discovered indirect evidence of black holes. For example, some material from another star may be pulled into the black hole. Just before it falls in, this material gives off high-energy X-rays that can be detected from Earth. Astronomers have discovered some X-ray sources that they think are caused by black holes.