If you notice the stars for some time they seem to move across the sky. Like the sun they rise in the east and set in the west. But this deceptive movement of the stars is caused by the movement of the earth on its axis. If you ignore this deceptive movement of the stars you can see that they are in the same position, night after night, month after month and year after year. Our ancient forefathers called the stars ‘fixed stars’ in a celestial sphere. However nothing is fixed in the universe. The moons or the satellites, move around their planets, the planets move around their suns, the suns move around the centre of their Galaxies. All these celestial bodies move on their axis. Everything is constantly in motion and so are the stars.
The stars do not appear to move, for, they are so far away. The closer we are to a moving object the faster it appears to move.
The stars are not distributed equally throughout the universe. They are grouped together in great star systems called the Galaxies. The bright stars we see in the sky belong to the Milky Way Galaxy. Our sun forms part of this Galaxy. The Galaxy is believed to be a vast, flat rotating disc with spiral arms. At the centre of the disc is a great spherical mass of stars forming what is called the nucleus? Our galaxy is estimated to contain about one lakhs million stars. Astronomy is a very interesting subject of wonders and wonders.
The movement of a star towards or away from us is called its radial motion. Astronomers can detect and measure this motion using that invaluable tool, spectroscope. The spectrum of light from a star shows dark lines characteristic of the chemical elements it contains. The interesting thing is that when a star is moving towards or away from us the lines show a shift in position from where they would have been if the star were at rest. When the motion is towards us the lines shift towards the blue end of the spectrum. When the motion is away from us the lines shift towards the red end. The extent of the shift is a measure of the speed at which the star is traveling towards or away from us.