Galaxy is a name given to the collection of stars. There are a no of galaxies present in the universe. Galaxies may or may not have regular shapes. The sizes of the galaxies also vary. The size of the galaxies depends upon the no of stars. As the no. of stars will increase the size of galaxy will also increase.
Two major categories of galaxies are:
1. Radio Galaxies
2. Normal Galaxies
Radio Galaxies: Those galaxies which emit radiations approximately million times than that of the normal galaxies. All these radiations are not emitted by the galaxy. It is supposed to be true that these radiations are coming from two radioactive sources i.e. S1 and S 2. These two sources are present on the either sides of the galaxy. The name given to the galaxy in the figure is G.
Normal Galaxies: The radiations emitted by these galaxies are very less as compared to the radiations emitted by radio galaxies. These galaxies appear to be bright at the centre. But as we move towards the edges the brightness goes on decreasing. There is a fade in effect at the edges of the galaxy.
The arrangement of stars in the galaxies is in the form of bands. Billions of stars are present in normal galaxies. Normal galaxies exist in different shapes and sizes. On the basis of shape three basic categories of the normal galaxies are formed.
1. Irregular galaxies
2. Elliptical galaxies
3. Spiral galaxies
Almost all the galaxies are having regular shapes. Nearly 2% of the galaxies have irregular shape. 80% of the galaxies are having spiral shape and the remaining 18% are elliptical in shape.
Two most common normal galaxies are Andromeda galaxy and Milky Way galaxy.
Milky Way galaxy: It is a large area looking like a twinkling belt formed by the combination of large no of stars. Our earth is also a part of Milky Way galaxy. On some clear night if we will look carefully then a large circle can be seen in the sky. In this circle a stretching white light like a foggy band can be seen. This is the Milky Way galaxy. The counting of stars in Milky Way galaxy is almost 1011 stars. Our sun is also part of Milky Way galaxy.
Its structure is somewhat strange. It is thin at its edges but is thick at the centre. The radius of Milky Way galaxy is near about 50000 light years.
It is nearly 5000 light years thick at the centre. The distance of sun from the centre of the galaxy is 3 x 104.
Through the nucleus of galaxy two arms are stretching towards the outside region. The width of each arm is near about 120 light years.
(ii) Milky Way galaxy rotates around its own axis which passes through its center. It looks like a flat lens. Most of the stars in a Milky Way galaxy are present at its centre. Stars present at its centre are nearly three times than that of the other regions of galaxy. The movement of sun in galaxy is also around its centre. The speed of sun during this motion is 250Km/s. The time taken by sun to complete its one rotation is 220 million years.
If we want to calculate the mass of Milky Way galaxy then the procedure is given below:
All suppositions are shown: M is the mass of the galaxy. This mass is supposed to be present at the centre. M is the mass of the sun. d is the distance between the sun and the centre of the galaxy. v is the orbital velocity. Then the formula to calculate the centripetal force on sun is:
Mv2 / r
This force was originated due to the gravitational pull of galaxy on the sun.
F= GMm / d2
Therefore, GMm / d2 = mv2 / d
Or
M = v2d / G
Substituting the values of V=250Km/s and d= 3×104light year.
Then the mass of the galaxy will be :
M = 3 x 1041Kg.
The average solar mass is equal to (1031Kg). So, the Milky Way galaxy consists of nearly 150 billion solar mass.