1. Search and you will find.

a. Our galaxy is called………

Ans: Milky Way and Mandakini

b. For measuring large distances …….. is used as a unit.

Ans: light year

c. The speed of light is ………… km/s.

Ans: 3,00,000

d. There are about ………. stars in our galaxy.

Ans: 10¹¹

e. The end stage of the Sun will be……

Ans: white dwarf 

f. Stars are born out of ….. clouds.

Ans: interstellar

g. Milky way is a ……….. galaxy.

Ans: spiral

h. Stars are spheres of ……….. gas.

Ans: hot

i. The masses of other stars are measured relative to the mass of the………….

Ans: sun

j. Light takes ……………. to reach us from the Sun while it takes……….. to reach us from the moon.

Ans: 8 minutes, 1 second

k. The larger the mass of a star the faster is its …….

Ans: rate of evolution

l. The number of fuels used in the life of a star depends on its…………

Ans: mass

2. Who is telling lies?

a. Light year is used to measure time.

Ans: It is lying as the light year is used to measure distance.

b. End stage of a star depends on its initial mass.

Ans: It is not lying. 

c. A star ends its life as a neutron star when the pressure of its electrons balances its gravity.

Ans: It is lying as a star ends its life as a neutron star when the pressure of its neutrons balances its gravity.

d. Only light can emit from the blank hole.

Ans:  It is lying as light does not get emitted from black hole but gets absorbed inside. 

e. The Sun will pass through the supergiant stage during its evolution.

Ans: It is lying as the Sun will pass through the red giant stage during its evolution.

f. The Sun will end its life as a white dwarf.

Ans: It is not lying. 

3. Answer the following question.

a. How do stars form?

Ans: 

(i) The interstellar clouds, due to some disturbance, undergo contraction.

(ii) As a result, their density and temperature begin to increase, and a dense sphere of hot gas is formed from the cloud.

(iii) When temperature and density at the centre of the sphere increase sufficiently, atomic nuclei undergo fusion reactions. Thus, at the centre of the sphere, the generation of nuclear energy begins. 

(iv) Due to this energy generation, the gas sphere becomes self-luminous, and a star is formed.

b. Why do stars evolve?

Ans: The reason for the evolution of stars is the burning of fuel inside the cores of the stars and, therefore, the decrease in the amount of fuel.

c. What are the three end stages of stars?

Ans: Three end stages of stars are white dwarf, neutron star and black hole.

d. Why was the name black hole given?

Ans: 

(i) When supergiant stars undergo supernova explosions, their size becomes smaller and their gravitational force increases tremendously.

(ii) The pull of gravitational force is so strong that any light falling on these stars cannot escape.

(iii) Hence, light is not reflected from these stars but is absorbed inside the stars.

(iv) As a result, the star is not seen at all. Instead, probably a minute black hole is observed in its place.

Hence, the name “black hole” was given to this stage of the star.

e. Which types of stars end their life as a neutron star?

Ans: Stars with an initial mass between 8 and 25 times that of the Sun end their lives as neutron stars.

4. A. If you are the Sun, write about your properties in your own words.

Ans: Following are the various properties of me as a sun:

(i) I am made up of hot gases. Hydrogen contributes to 72% of my mass, while helium makes up about 26%. The rest 2% is associated with elements heavier than helium.

(ii) My mass and radius are about 2 x 10³⁰ kg and 695700 km, respectively.

(iii) My surface temperature is about 5800 K, and the temperature at the centre is 1.5 x 10⁷ K.

(iv) I am 4.5 x 10⁹ years old.

(v) I have my own heat and light.

B. Describe white dwarfs.

Ans: 

(i) When a red giant star explodes, the inner part of the star contracts, reducing its size to a level similar to that of the earth.

(ii) At the same time, its mass is much higher than that of the earth. 

(iii) As a result, the density of a star becomes very high.

(iv) In this stage, pressure due to electrons in the star becomes independent of temperature and balances its gravitational force perpetually (forever). 

(v) The star looks white, and due to its small size it is called a white dwarf.