Solution-Class 8-Science-Chapter-19-Life Cycle of Stars-Maharashtra Board

(a)  Our galaxy is called Milky way.

(b) For measuring large distances light year is used as a unit.

(c) The speed of light is 3,00,000 km/s

(d) There are about 100 billion stars in our galaxy.

(e) The end stage of the sun will be white dwarf.

(f) Stars are born out of interstellar clouds.

(g) Milky way is a spiral galaxy.

(h) Stars are spheres of hot gas.

(i) The masses of other stars are measured relative to the mass of the Sun.

(j) Light takes about 8 minutes to reach us from the Sun while it takes 1 second to reach us from the moon.

(k) The larger the mass of a star the faster is its rate of evolution.

(l) The number of fuels used in the life of a star depends on its mass.

False - Light year is used to measures distance.

True - End stage of a star is decided by its mass.

False- A star ends its life as a neutron star when the pressure of its neutrons balances its gravity.

False - Nothing can be emitted from black hole, not even light.

True - Sun will pass through the giant stage during its evolution. So, its end stage will be as white dwarf.

True - The Sun will end its life as a white dwarf

• Spaces between the stars present in a galaxy are occupied by huge clouds of gas and dust, known as interstellar clouds. The size of an interstellar cloud is about a few light years.
• 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. 
• A huge interstellar cloud can produce thousands of stars at a time.

• Although stars appear stable for quite a long period of time, their properties do change, though very slowly.
• A change in the properties of a star, leading to its passing through different stages, is called evolution of the star.
• Star loses its energy constantly due to continuous emission of light and energy.
• 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.
• Burning of the fuel at the centre of the star and a gradual decrease in its amount is the main reason of evolution of a star.

Three end stages of stars are:

• 1-Stars having initial mass less than 8 times the mass of the Sun ultimately become white dwarfs.
• 2-Stars having initial mass between 8 and 25 times the mass of the Sun ultimately become neutron stars.
• 3-Stars having initial mass larger than 25 times the mass of the Sun ultimately turn into black holes.

• When a star having initial mass larger than 25 times the mass of the Sun reaches its end stage, its gravitational force and density increase exponentially.
• All nearby objects get attracted towards the star and nothing can come out of it, not even light. All incident light is absorbed by the star.
• 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.

• Stars having initial mass between 8 and 25 times the mass of the Sun end up as neutron stars.
• When these stars pass through the supergiant stage, their size increases to 1000 times.
• Huge explosion that occurs in the last stage of these stars is very powerful and very high energy is given off.
• After the huge explosion, called the supernova explosion, their central portion contracts in size to about 10 km.
• In this stage, such stars are completely made up of neutrons and hence are called neutron stars.

There are billions of stars in the galaxy called the Milky Way and Mandakini.

I am one of the small stars called the Sun,

I have my own family called the solar system.

There are planets, satellites, asteroids, comets and meteors in my family.

• I am 4.5×10⁹ year old.
• I have my own heat and light.
• I am made up of hot gases. Hydrogen contributes to 72% of my mass while helium about 26%. The rest 2% is associated with elements heavier than helium.
• My mass is about 2×10³⁰ kg and and radius  is about 695700 km.
• My surface temperature is about 5800 K and temperature at the centre is 1.5 ×10⁷ K.

In the end stage of my life, I will turn into a red giant star. Thereafter, I will first explode and then contract to become as small as the earth.

I will appear small as well as white and hence, I will be called a White dwarf. This will be my last stage for ever.

• Depending on the initial mass, stars can reach one of the three end stages.
• White dwarf is one such stage of stars having initial mass less than 8 times the mass of the Sun.
• These stars undergo huge expansion and their radius increases by a factor of 100 to 200.
• These stars appear reddish because of their large size and lower temperature. Hence, these stars are also called red giant stars.
• At the end of their evolution, these stars explode, their outer gas envelope is thrown outward and the inner part contracts to the size of the earth. However, the density in the star becomes very high.
• In this stage, the pressure due to electrons becomes independent of temperature and sufficient to balance the gravitational force for ever.
• In this stage, such stars look white and due to their small size they are called white dwarfs.