A galaxy is a great collection of stars. They are formed by gravity. The stars are obliged to orbit around the center (nucleus) of the galaxy.
All stars move at various speeds. The reason is that they are placed in the galaxy at different distances. Those stars that are located relatively near the nucleus go faster and complete one orbit sooner than those located farther away, which naturally move at a slower pace and have much longer to travel.
Sun, being located in the outer region of our galaxy called the Milky Way (three-fourth of the way out, towards the edge),and carrying all planets and moons at 220 kilometers per second, takes 240 million years to complete one orbit around the center of the galaxy.
Clever boys and girls can work out for themselves the total distance Sun and the planets travel during one complete orbit (220 x 60 x 60 x 24 x 365 x 240,000,000 km)!
The Milky Way has been so named because the ancient Greeks called the wispy cloud running across the sky from one end to the other, galaxies kukolos (the Milky Circle). The Romans changed it a bit to via lactea (the Milky Way) which came to be adopted in our times. The Urdu or Persian word kehkashan is appropriate but only just. It simply means galaxy, any galaxy.
Billions of such star systems dot the skies. Their distances from the Solar System can be gauged from the astounding fact that the nearest of them from our own galaxy is 2.2 million light years (ly) away (300,000 x 60 x 60 x 24 x 365 x 2,200,000 km). That’s how far Andromeda is from the Milky Way!
Of course the two “lesser galaxies”, the so-called Magellanic Clouds (Large and Small) are small galaxies visible only in the southern skies (say from Australia, New Zealand, South Africa, South America, etc.). They are our satellite galaxies which somehow detached from the mother galaxy at the time they formed — some 12 billion years ago!
Unfortunately, we cannot see much of our own galaxy from our position inside it. Clouds of gas and dust (great clouds at that) block our view like dark thunder clouds block the view of the setting sun. We call these clouds, the “dark matter”. It abounds in the universe to such an extent that astronomers believe that only 10 per cent of all matter is luminous (i.e. the stars and galaxies) while the rest is dark matter; however, this quantity is not certain as yet.
All stars in a galaxy go by the perfectly balanced forces between the stars and the galactic center (nucleus). On the one hand, the nucleus tries to pull it inward towards itself, on the other, the star fuelled by the inexorable centrifugal forces, keeps to itself and continues to orbit around the nucleus. It is estimated that there are as many as 200 billion galaxies, each with billions of stars (often hundreds of billions), with the nearest of stars trillions of miles away from each other. More about galaxies soon! Then, about the types of galaxies! i
The writer is a professional astronomer and a former head of PIA Planetaria. He can be reached at email@example.com