Question

Answer the following question.
a. How do stars form?
b. why do stars evolve ?
c. What are the three end stages of stars ?
d. Why was the name black hole given ?
e. Which types of stars end their life as a neutron star?

Answer 1

a. Spaces between the stars present in a galaxy are occupied by huge clouds of gas and dust, known as interstellar clouds. Contractions happen in this cloud due to some natural disturbance. Because of this, the density and temperature in the cloud starts increasing and a dense sphere of hot gas is formed from the cloud. Nuclear energy (energy released due to fusion of atomic nuclei) is generated with the burning of fuel, like hydrogen and helium, in the sphere when it attains a sufficient amount of density and temperature at its centre. Due to this energy generation, the gas sphere becomes self luminous and thus a new star is formed.

b. Star loses its energy constantly due to continuous emission of light and energy. This means that the temperature of a star decreases continuously and hence its gas pressure which thereby decreases its stability. But we know a star is very stable. So, to maintain the stability of a star, its temperature is maintained constant with the help of energy released by the burning of fuel at its centre. This burning and therefore the decrease in the amount of fuel is the reason for the evolution in the stars.

c. Three end stages of stars are:

• White dwarf
  • Density of star is very high compared to that of Earth
  • Small sized and very stable
  • Appears white in colour
  • Initial mass < 8M_Sun
• Neutron star
  • Completely made up of neutrons
  • Stable in nature
  • Initial mass 8M_Sun < M_Star < 25M_Sun
• Black hole
  • Gravitation pull is very strong
  • Absorbs everything, even the light rays falling on it
  • Appears as black hole
  • Initial mass > 8M_Sun

​​d. All the light rays falling on black hole gets absorbed. Because of this we cannot see this star at all but can probably see a minute black hole at its place. Thus, the name black hole is given to this star.

e. The stars whose initial mass is between 8 to 25 times the mass of the Sun, end their life as a neutron star.