Maharashtra State Board – Standard IX

Chapter 19 – Life Cycle of Stars

1. Search and you will find.

a. Our galaxy is called …………

Ans: Milky Way (also called 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 km/s.

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

Ans: About 10¹¹ (100 billion) stars.

e. The end stage of the Sun will be ………

Ans: White dwarf.

f. Stars are born out of …….. clouds.

Ans: Interstellar clouds.

g. The Milky Way is a ……….. galaxy.

Ans: Spiral galaxy.

h. Stars are spheres of ……….. gas.

Ans: Hot gas.

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 and about 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: This statement is false.
A light year is used to measure distance, not time.

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

Ans: This statement is true.

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

Ans: This statement is false.
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 black hole.

Ans: This statement is false.
Light cannot escape from a black hole. It is absorbed due to strong gravity.

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

Ans: This statement is false.
The Sun will pass through the red giant stage during its evolution.

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

Ans: This statement is true.

3. Answer the following questions.

a. How do stars form?

Ans:

  1. Interstellar clouds contract due to disturbances.
  2. Their density and temperature increase, forming a dense sphere of hot gas.
  3. When the temperature and density at the centre become very high, nuclear fusion begins.
  4. Due to nuclear energy generation, the gas sphere becomes self-luminous, and a star is formed.

b. Why do stars evolve?

Ans:
Stars evolve because the nuclear fuel in their core gets used up gradually. As the fuel decreases, changes occur in the star’s structure, leading to different stages in its life cycle.

c. What are the three end stages of stars?

Ans: The three end stages of stars are:

  1. White dwarf
  2. Neutron star
  3. Black hole

d. Why was the name black hole given?

Ans:

  1. When massive stars undergo supernova explosions, their size reduces drastically and their gravitational force increases tremendously.
  2. The gravitational pull becomes so strong that even light cannot escape.
  3. Since no light is reflected, the object cannot be seen.
  4. It appears like a dark region in space.

Hence, it is called a black hole.

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

Ans: Stars with an initial mass between 8 and 25 times the mass 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:

  1. I am made up of hot gases. About 72% of my mass is hydrogen, 26% is helium, and the remaining 2% consists of heavier elements.
  2. My mass is about 2 × 10³⁰ kg, and my radius is about 6,95,700 km.
  3. My surface temperature is about 5800 K, and the temperature at my centre is about 1.5 × 10⁷ K.
  4. I am approximately 4.5 × 10⁹ years old.
  5. I produce my own heat and light through nuclear fusion.

4. B. Describe white dwarfs.

Ans:

  1. When a red giant star sheds its outer layers, the inner core contracts.
  2. Its size becomes similar to that of the Earth, but its mass remains very high.
  3. Due to this, its density becomes extremely high.
  4. The pressure of electrons balances the gravitational force.
  5. The star appears white and small in size, so it is called a white dwarf.
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