The cosmic year is related to the motion of a galaxy. A galaxy is a large system of stars held together by gravitational force. There are millions of galaxies in the universe. The size of a galaxy is measured in Light Years. The diameters of different galaxies range from a few thousand Light Years to 500,000 Light Years.
There are three main types of galaxies depending on their shapes: spiral galaxies, elliptical galaxies and irregular galaxies. The spiral galaxies are generally disc-shaped with curving arms extending from a centre bulge. The elliptical galaxies are shaped like spheres or flattened spheres. The irregular galaxies have no definite shape.
The Milky Way or Akash Ganga as described by ancient Indian astronomers is a spiral galaxy. The Milky Way belongs to a cluster of some 24 galaxies called ‘The local group’. Our solar system belongs to this galaxy. Sun is one of about 100,000 million stars located in the Milky Way. The modern estimates place the Sun at a distance of about 32,000 Light Years from the centre of the galaxy.
The Milky Way is not stationary. It is rotating round an axis passing through its centre. This is the real reason for its flat disc-like shape. The whole disc of the galaxy rotates about its galactic centre —but not at a uniform speed. The rate of rotation decreases with increasing distance from the centre. The galaxy does not rotate like a solid wheel.
Each star revolves around the central nucleus in an elliptical or circular orbit, just like the planets revolve round the sun. The stars near the centre move faster than the stars farther away. Sun moves round the centre of galaxy at a speed of 250 km per sec. It takes about 225 million years to complete one revolution around the galactic nucleus. This period is known as the cosmic year or the galactic year.
There is evidence of galactic rotation in observations that in general stellar motions are not random, but in preferential directions. The Dutch astronomer J.C. Kapteyn first observed this effect in 1904.
