The milky way
The universe consists of very large number of galaxies ie about 109 Each of them has a large number of stars approximately 1015 clustered in them The word Galaxy’ has been derived from the word “Gala” which means milk our galaxy is called the “Milky Way” The Milky Way in the form of a dice which is thicker near the center and thinner at the edges The diameter of the dice is about 106 light years and its thickness is about 5000 light years at the center and diminishes near the edges The sun which belongs to the galaxy is situated in its center plane at a distance of about 2 7×104 light years from the center of the galaxy which is called the Galactic When see through a telescope, it gives a cloudy
appearance due to presence of very large number of stars. On a clear night, we see several dark regions in the band of the Milky Way The regions look dark due to the presence of dust clouds which remain hidden from our view, but the stars lay behind them The space between stars in the Milky Way is filled with dust and gas. Some times, the dust gets illuminated by a hot star and it begins to shine by light reflected from it, giving rise to what is called Nebula. Orion is a bright Nebula. About 80% of the stellar matter is composed of Hydrogen. The smaller assembl of in a galaxy are known as Galactic There are two types of cluster, namely the clusters and the Globular clusters Galactic clusters are the groups of 100 to 1000 stars held in position by mutual gravitational forces. The cluster moves whole around the Galaxy. Globular clusters are the groups of about 10000 stars packed in a relatively small region. About 1000 globular clusters are known; all of them are more than 20000 light years away from the sun. The disc of the Milky Way has a spiral structure, ordinary optical telescope can not be used to study detailed structure because the field of view is blocked by interstellar dust clouds. But radio waves can penetrate the clouds of dust Therefore, the radio telescope has recently been used to study the detailed structure Their studies have revealed that the central part of our Galaxy
looks like an amorphous spheroid outside the central nucleus there the spiral arms which are about 1200 light yean wide and are separated by gapes of about 500 light years. The sun is situated near the edge ofone such arm A remarkable feature of the Milky Way is its rotation about an axis passing through its center This rotation is not a rigid rotator. Each star of the Galaxy is moving in its orbit with certain speed that is determined by the gravitational attraction of the stars enclosed within the orbit. The sun is one of about 150 billion stars in the Milky Way. All the stars revolve around the Galactic center The sun moves at the speed of 250kms-1, revolves around the center in about 25 0 million years. The mass of the Milky Way can be estimated as follows as under is the distance of the sun from the Galactic center and ‘v’ is speed around its orbit. Centripetal acceleration V2 /r. If M is the mass of Milky Way, the gravitational acceleration of the sun will be GM/2 Substituting for V 2.5X105 m/s 1, r 2.7 X104 light years G 6.67X 1011 Nm2Kg 2 Then, we get M 3X1041 Kg, which is about 150 million solar masses, From this, one may conclude the part of our Milky Way within the orbits of the sun consists of around 150 billion stars.