A galaxy is a huge collection of gas, dust, and billions of stars and their solar systems, all held together by gravity.
There are billions of galaxies in the universe. Galaxies are labeled according to their shape, size, colour and composition. There are three types of galaxies that we find in the universe.


These types of galaxies are like flattened balls of old stars and contain very little gas. It also includes the most massive galaxies containing a trillion stars.


Spiral galaxies have a flattened the shape. They have a bulge in the centre composed of old stars surrounded by a disk of young stars and are arranged in spiral arms.
Spiral Galaxy


As its name suggests, Irregular Galaxies have no particular shape. There are billions of galaxies in the universe, the centre of the galaxy releases a huge amount of heat, radiation, radio waves and x-rays.

Important Galaxies

  • IC 1101 – One of the largest known galaxies
  • Milky way – IT is the galaxy that includes our Solar System
  • Andromeda
  • Cigar Galaxy
  • Pinwheel Galaxy


  1. Stars are heavenly bodies like the sun that are extremely hot and have light of their own. Stars are made up of vast clouds of hydrogen gas, some helium and dust.
  2. In all the stars (including the sun), hydrogen atoms are continuously being converted into helium atoms and a large amount of nuclear energy in the form of heat and light is released during this process.
  3. It is this heat and light which makes a star shine. Thus, a star is a hydrogen nuclear energy furnace, so big that it holds together by itself.
  4. The stars are classified according to their physical characteristics like size, colour, brightness and temperature.

Colour of a star

Colour of Stars
  1. The colour of a star is determined by its surface temperature.
  2. The stars which have comparatively low surface temperature are red, the star having high surface temperature are white whereas those stars which have very high surface temperature are blue on color.
  3. Some of the important examples of the stars are: Pole (or Polaris), Sirius, Vega, Capella, Alpha centauri, Beta centauri, Proxima centauri, Spica, Regulus, Pleiades, Aldebaram, Arcturus, Betelgeuse, and of course, the Sun

Why all the stars (except the pole star) appear to move from east to west in the night sky?

All the stars (except the pole star) appear to move from east to west in the night sky. Because the earth itself rotates on its axis from west to east. So, when the earth rotates on its axis from west to east, the stars appear to move in the opposite direction, from east to west.
Thus, the apparent motion of the stars in the sky is due to the rotation of the earth on its axis. Since we are ourselves on the earth, the earth appears to be stationary to us but the stars appear to be moving in the sky. Thus, it is due to the rotation of earth on its axis that we see the stars changing their positions in the sky as the night progresses.


  1. The raw material for the formation of a star is mainly hydrogen gas and some helium gas. The life cycle of a star begins with the gathering of hydrogen gas and helium gas present in the galaxies to form dense clouds of these gases. The stars are then formed by the gravitational collapse of these over-dense clouds of gases in the galaxy.

Stages in the formation of star

1. Formation of a Protostar

When the gas particles in the molecular cloud run into each other, heat energy is produced. This results in the formation of a warm clump of molecules referred to as the Protostar. The creation of Protostars can be seen through infrared vision as the Protostars are warmer than other materials in the molecular cloud. Several Protostars can be formed in one cloud, depending on the size of the molecular cloud.

2. Formation of a Star from Protostar

The protostar is a highly dense gaseous mass, which continues to contract further due to tremendous gravitational force. As the protostar begins to contract further, the hydrogen atoms present in gas cloud collide with one another more frequently. These collisions of hydrogen atoms raise the temperature of protostar more and more. The process of contraction of protostar continues for about a million years during which the inner temperature in the protostar increases from a mere, -173°C in the beginning to about 107°C. At this extremely high temperature, nuclear fusion reactions of hydrogen start taking place. In this process, four small hydrogen nuclei fuse to produce a bigger helium nucleus and a tremendous amount of energy is produced in the form of heat and light. The energy produced during the fusion of hydrogen to form helium makes the protostar glow and it becomes a star.

3. Red- Giant

A star converts hydrogen atoms into helium over its course of life at its core. Eventually, the hydrogen fuel runs out, and the internal reaction stops. Without the reactions occurring at the core, a star contracts inward through gravity causing it to expand. As it expands, the star first becomes a subgiant star and then a red giant. Red giants have cooler surfaces than the main-sequence star, and because of this, they appear red than yellow.

Chandrasekhar Limit

A great Indian scientist Chandrasekhar made a detailed study of the stars which end their lives by becoming white dwarf stars. Chandrasekhar concluded that the star having a mass less than 1.44 times the solar mass (or sun’s mass) would end up as white dwarf stars. The maximum limit of 1.44 times the solar mass (for a star to end its life as a white dwarf) is known as the ‘Chandrasekhar Limit’.


The solar system extends much beyond at the edge of the solar system, there are billions of very small objects called ‘comets’ these comets were formed very early from the same gas cloud from which other members of the collar system were made. These comets are so far off that normally they cannot be seen. They keep on revolving around the Sun, unknown to the world.
The tail of a comet always points away from the sun. Comets revolve around the sun like planets. The period of revolution of comets around the sun is, however, very large. For example, Halley’s Comet has a period of about 76 years. Halley’s Comet last appeared in the inner Solar System in 1986 and will next appear in mid-2061.

Important Comets

  • Shoemaker
  • Hale Bopp


  • Asteroids are very small planets of rock and metal which revolve around the sun mainly between the orbits of Mars and Jupiter.
  • The biggest asteroid called ‘ceres’ has a diameter of about 800 kilometers whereas the smallest asteroid is as small as a pebble.

Important Asteroids

  • Vesta
  • Pallas
  • Juno
  • Camilla


  • Many times we see a streak of light in the sky during the night which disappears within seconds. It is called a meteor or shooting star.
  • Meteors are the heavenly bodies from the sky which we see as a bright streak of light that flashes for a moment across the sky.
  • The meteors are also called shooting stars.
  • Some meteors are the dust particles left behind by comets and others are the pieces of the asteroid which have collided.
  • When a meteor enters the mesosphere with a high speed, a lot of heat is produced due to the resistance of air. This heat burns the meteor and the burning meteor is seen in the form of a streak of light shooting down the sky, and it falls on the earth in the form of dust.
  • If a meteor is big, a part of it may reach the earth’s surface without being burned up in the air. This fragment is called a meteorite. Thus, a meteor which does not burn completely on entering the earth’s atmosphere and lands on earth is known as a meteorite. Meteorites are a sort of stones from the sky.

Important Meteorites

  • Allende meteorite – Mexico
  • Hoba Meteorite – Namibia
  • Ahnighito Meteorite – Greenland
  • Tunguska Meteorite – Siberia
  • Chenchuri Meteorite – Australia
  • Byanjan – USA