Neighboring galaxies

            A certain kind of variable star, or changing star, was helpful to astronomers in discovering galaxies beyond our own. this kind of variable star is very bright and follows a certain pattern. It is bright for a period, dim for a period, and then bright again. Such a pattern can be used to measure the distance to the star. Astronomers can determine the average absolute magnitude of the star by studying how much time passes between its bright and its dim periods. Then, by comparing the absolute magnitude with the apparent magnitude of the star, the astronomers can tell how far away the star is.
            In 1924, a famous astronomer, Edwin Hubble, was studying a distant cloud of dust and gas in space. In the cloud he found some of the variable stars of the kind just mentioned. These variable stars were known to be supergiants, and yet they appeared very faint. Hubble then guessed, correctly, that these stars were very far away. In fact, they were so far away that they could not be in our galaxy. A new galaxy had been discovered! It was named the Andromeda Galaxy. further studies showed that the Andromeda Galaxy is 2 million light-years away. This neighboring galaxy is about twice as large as the Milky Way but is very similar in shape.
            Many galaxies have since been discovered. Some of them are much closer than Andromeda. The Large and Small Magellanic Cloud galaxies are only about 200,000 light-years away. Studies revealed that our galaxy is part of a group of galaxies known as the Local Group. Twenty galaxies are presently thought to belong to the Local Group. There may be more. All galaxies in the Local Group are within 3 million light-years of our solar system.

More about galaxies

            Galaxies do not all have the same shape. Their different shapes may be the result of the speeds at which the galaxies began to rotate when they were formed. The amount of dust and gas in the galaxies may also affect their shape. Some astronomers think that all galaxies are about the same age but that the stars within them are of various ages.

New findings

            Within the last forty years, many new discoveries in astronomy have added to our understanding of the universe. One such discovery was that of quasars. "Quasar" is a short form for "Quasi-Stellar Radio Source." Stellar means starlike. The first quasar was detected by radio astronomers in 1963. The quasar seemed to be a starlike object. And yet its energy output in the form of radio waves was as large as that of a thousand galaxies. Something that small, giving off so much energy, had to be very far away.
            Since then, hundreds of quasars have been found. One discovered quasar is said to be 10 billion light-years from the earth. If that is so, the light reaching the earth now from that quasar left the quasar 5 billion years before our solar system was formed. Astronomers studying that quasar may be looking at something that existed at the beginning of the universe! This makes some scientists think that the universe may be 15 or 20 billion years old. They used to think it was about 10 billion years old.
            Another interesting discovery was that of pulsars. A pulsar is a very dense star that sends out radio signals in regular pulses. When the first pulsar was discovered in 1967, it was thought that its signals were from beings in another solar system. This theory is no longer believed to be true. A pulsar's signals seem to be due to the heat and pressure on the atomic particles that make up the star. Since pulsars emit energy in pulses of up to thirty per second, they are thought to be made up almost entirely of neutrons. Only neutrons are known to pulsate that fast.

Changes in the universe

            Soon after galaxies were discovered, astronomers noticed that most of the galaxies seem to be moving away from the earth. Later they found that quasars are also moving away at very great speeds. Astronomers concluded from this that the universe is expanding. Some believe that the universe will continue to expand indefinitely. Others think it will reach a certain point and then begin contracting to some original size.
            Many scientists use the idea of an expanding universe to support a theory about how the present universe may have begun. These scientists believe that the universe began as a huge fireball. They say the fireball was made of atomic particles that slowly formed some elements, such as hydrogen and helium. After millions of years of expanding and cooling, the matter in the fireball was spread throughout the universe. Then, perhaps as a billion years passed, separate clouds of gas and dust began to form from this matter. Gradually, galaxies and stars developed from these clouds. These galaxies are still expanding away from each other. Scientists call this the "big-bang" theory.

See also Universe.