1) It is a cluster of billions of star.
2) In a galaxy, the stars do not form a definite pattern.
3) There are about 1011(100 billion) galaxy in the universe.
4) Galaxies are the building block of the universe.
5) Galaxies can be divided into two categories.
a) Normal Galaxies- These Galaxies emit more of the visible radiation as compared to radio radiation. These galaxies are divided into three groups.
i) Elliptical galaxies
ii) Spiral galaxies
iii) Irregular galaxies
The irregular galaxies are youngest, spiral galaxies are middle aged and elliptical galaxies are quite old.
Most massive elliptical galaxies are called Supergiant. A supergiant is 100 times heavier than our galaxy (i.e. Milky Way galaxy). Examples of normal galaxies are Milky Way and Andromeda galaxy.
b) Radio galaxies
These galaxies emit million times more of radio radiations as compared to visible radiations.
6) There are about 1011 stars in each galaxy. Thus total number of stars in the universe is 1022.
7) All the galaxies are moving away from each other, showing red shift in their spectra, indicating that the universe is expanding.

Milky way or Akash Ganga

It is the name of the galaxy to which our earth belongs. The Milky Way is the glowing belt of the sky formed by the combined light of very large number of stars. It is called Milky Way or Akash Ganga because the light from the various stars together gives the impression of a stream of milky flowing across the sky.
Milky Way is a spiral galaxy. Its mass is 150 solar masses (i.e. 3×1041kg).


These are quasi-stellar Radio sources. Quasars are situated at very large distance from our solar system and are further receding away with a speed of 0.9 times the speed of light.


There are radio sources which emit radio signals at extremely regular intervals of time. Their origin is due to neutron star.

Hubble’s law

It states that the red shift (z) is directly proportional to distance (r) of the galaxy from us i.e.
Z ∝ r but z= dλ/λ=V/c=speed of the galaxy/speed of light
Because V/c ∝ r or V ∝ r or V=Hr;
Where H is called Hubble’s constant. The quantity (1/H) has the dimensions of time. Hubble’s law helps us to estimate the age and evolution of the universe. The value of Hubble’s constant is 16 km/s per million light years.