GALAXY-
>> A galaxy is a vast system of stars and a large number of gas clouds[mainly hydrogen gas] and dust, isolated in space from similar systems.
>> There are billions of galaxies having different sizes, regular and irregular shapes
>> Galaxies can be divided into two categories:[1.] NORMAL GALAXIES[2.]RADIO GALAXIES
1. NORMAL GALAXIES-
>>Emit a small number of radio radiations as compared to the total radiations emitted.
>>Bright from the centre and gradually dim towards the edges.
>>Contains billions of stars in the form of a band, travelling together in the Universe.
>>Divided into 3 groups depending upon their shapes;(1)Elliptical galaxies(2)Spiral galaxies(3)Irregular galaxies
>>Elliptical galaxies are quite old and about 18% , Spiral galaxies are middle-aged and about 80%, Irregular galaxies are youngest and about 2%.
2. RADIO GALAXIES-
>>Emit a million times more radio radiations than normal galaxies.
>>There are 170 billion galaxies in the observable Universe.
>>It is believed that the radio radiations do not come from the galaxy itself but come from two large radio sources
MILKY WAY-
>>If one looks at the sky on a clear night, a hazy band of white light stretching across a great circle is seen, which is called the milky way .
>>It is a SPIRAL GALAXY.
>>It is the home of earth and our solar system.
>>It was formed 5 billion years after the Big bang.
>>The first person to see this galaxy was GALILEO GALILEI.
>>Recently a supermassive black hole is known as Sagittarius A found in the core of the milky way.
>>ANDROMEDA is our nearest galaxy.