Space And Beyond Galaxies

OUTER SPACE:

In the sky at night several bright clouds of light can be seen with the naked eye, and many more with telescopes. Some of these great clouds of gases that give off light are called nebulae. Other bright clouds are made up of thousands of other separate stars. Many of these star clouds belong to the Milky Way, of which our own solar system is a part. Others are much larger and quite separate and lie far out in space beyond the Milky Way. These are the galaxies. Each galaxy is millions of millions of miles away and contains millions of stars. There seems no limit to the number of galaxies-several thousand million of them can be seen through the largest telescopes, and they are scattered through space in all directions as far as the telescopes can reach.

A typical galaxy may be anything from 6,000 to 60,000 light years across and may contain a thousand million stars or more. Our own milky way is itself a galaxy, but it is a good deal larger than the average, and contains about a hundred thousand million stars. Besides these stars, galaxies contain a great deal of material in the form of fine dust and gases. Some galaxies consist more of this than stars.

ANDROMEDA GALAXY:

Apart from the Milky Way three other galaxies are visible to the naked eye. They are the great galaxy in Andromeda, and the two Magellanic clouds, the Large and Small. The Andromeda galaxy is about 15,00,000 light years away. These three, as well as our own milky way, all belong to a group of neighbouring galaxies called the 'local group ' or 'local cluster'. There are about 18 other galaxies in the local cluster together, but all the others are too faint to be seen without a telescope. The local cluster is over 40,00,000 light years across, and the Andromeda galaxy is the biggest galaxy in the local cluster.

Most galaxies belong to clusters, and it may be that all of them do. The next nearest cluster to our local cluster is a much larger one in the constellation Virgo. This cluster is about 75,00,000 light years away, and contains over500 galaxies. The most distant galaxies that have been photographed belong to clusters over 50,00,000 light years away.

Galaxies occur in three different shapes: they may be spiral, elliptical or irregular. A spiral galaxy is shaped like a Catherine wheel, with a bright collection of stars in the centre, and several arms of stars winding out from it. Spiral galaxies are among the grandest and most beautiful of all astronomical objects. Elliptical galaxies are rounded groups of stars shaped like rugby balls. Irregular galaxies, as their name suggests, have no definite shape.

The Andromeda galaxy is a spiral, while the Magellanic clouds are irregular galaxies. We see the Andromeda galaxy at an angle, so that it looks like a Catherine wheel seen from one side. Astronomers can tell it really has a spiral shape from careful observations of the positions and motions of the stars in it. Some other spirals are seen from the side,some face on and some edge on. There are several elliptical galaxies in the local cluster as well as the other spirals and irregulars.

MILKY WAY:

Our own Milky way is a spiral galaxy. The arms are almost hidden by dust clouds and cannot easily be seen through an ordinary telescope. They have been mapped out, however by radio telescopes which can penetrate the dust.

The galaxies in a cluster move about all the time, although the cluster as a whole keeps together. The individual stars practically never collide with one another because within each galaxy they are quite widely separated. Two galaxies can even pass right through each other, but astronomers think that some of the radio waves discovered by radio telescopes are caused by collisions between galaxies.

Astronomers have discovered that all the other clusters seem to be moving away from our local cluster. This is known as 'the expansion of the universe'. The size of the universe has not been ascertained. And it cannot be done shortly in any case. We do not even know whether ours is the only universe. But we can hope that in future the mystery of outer space can be unravelled.