Question

Fill in the blank:
The billions of stars which occur in the form of groups or clusters are commonly called ______.
(A) galaxies
(B) nebula
(C) plutons
(D) stellar

Answer 1

Hint
A system of millions or billions stars which are together with gasses or dust and they are all held together with the help of gravitational force of attraction is called a galaxy. In a galaxy billions of stars are seen together in the form of clusters.

Complete step by step answer
To solve this problem, we need to first know what all the terms which are given in the answer means. So let us go through them one by one,
Galaxy- A galaxy is a system of stars, stellar remnants, interstellar gas, dust and dark matter which are all held together with the gravitational force of attraction. Galaxies can be of different sizes, ranging from a few hundred million stars $\left( {{{10}^8}} \right)$ called as dwarfs, to a hundred trillion stars $\left( {{{10}^{14}}} \right)$ known as giants.
Nebula- A nebula is a giant cloud of dust and gas, such as hydrogen, helium and other ionised gasses, present in space. Most of the nebulas are very vast in size ranging up to a few hundred light years in diameter.
Plutons- Plutons are bodies of intrusive igneous rocks. They form from the magma which cools slowly and crystallizes below the surface of the earth.
Stellar- The word stellar means anything that is relating to stars or astral bodies.
So from the above definitions we can see that billions of stars which occur in the form of groups or clusters are commonly called galaxies.
Hence option A is correct.

Note
The galaxy in which our solar system is present is named as the Milky Way galaxy. It is one of many galaxies present in our universe. In the recent studies, after collecting data for over 20 years, it is concluded that there are over 2 trillion $\left( {2 \times {{10}^{12}}} \right)$ galaxies present in the observable universe.