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1. What is a galaxy?
A galaxy is the overall aggregation of stars, singly and in groups and clusters; of nebulae, dust and gas; of possibly every kind of astronomical object.
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2. How many different kinds of galaxies are there?
There are three groups of galaxies - elliptical, spiral and irregular galaxies.
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3. What is an elliptical galaxy?
An elliptical galaxy is one in which no arm structure is found. The small companion of the great Spiral in Andromeda is a typical elliptical galaxy.
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4. What is a spiral galaxy?
A spiral galaxy is one in which arm structure is found.
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5. What is an irregular galaxy?
A galaxy which is not elliptical or spiral is an irregular galaxy. There are known very few irregular galaxies.
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6. How many different parts are there in a galaxy?
Galaxies are generally divided into the nucleus, which is the concentration of stars and gas at the centre of the galaxy; the disk, which is the flatter portion of galaxy extending out around the nucleus; and the arms which extend from the disk to the greatest diameter of the galaxy and belong to the disk.
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7. Why do some galaxies have arms and others do not?
The reason for this is not fully known. It is possible that there are arms in those galaxies where there is a rather slow rotation of the material out of which the galaxy is formed. In galaxies of slower rotation and more gas and dust, the arms must be formed of gas and dust.
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8. Is there any difference in the materials found in various parts of a galaxy?
The nucleus is composed mainly of stars and
does not contain very much dust or gas. The disk has greatest
variety of material of it - stars, dust, gas and so on, and the
greatest changes are taking place. The arms contain most
of the dust and gas of a galaxy - perhaps 50% of the mass
of the arm structure of a galaxy is dust and gas, but only about
10% of the total mass of a galaxy is dust and gas.
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9. What is meant by stars of Population I and Popuiation II?
The stars of Population I are found mainly in the arms and disks of galaxies. In the regions in which there are Population I stars there is much dust and gas. The brightest stars of Population I stars are blue. They are hotter stars.
Population II stars are found in the nuclei of galaxies where there is little or no dust and gas. They are red and rather cold stars.
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10. Is it possible to see the dust in distant galaxies?
Yes.
In most of the photographs of brighter galaxies, there are
definite dark streaks and spots which are dust.
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11. How many galaxies are there?
There is no way of knowing exactly. There must be several thousands of millions of galaxies in space. We cannot say how many galaxies there are because we do not know the extension of space. The Milky Way Galaxy in which we live is probably an average galaxy. It contains about 100 milliard stars. There are smaller galaxies and there are also larger galaxies.
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