Class 8 MAHARASHTRA STATE TEXTBOOK BUREAU Solutions Science Chapter 19: Life Cycle of Stars
Life Cycle of Stars Exercise Exercise
Solution 1
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 x105 km/s.
d. There are about 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 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.
Solution 2
a. This statement is lie. Light year is used to measure distance.
b. The statement is true as end stage of star depends on its initial mass.
c. It is a lie. Star ends its life as a neutron star when pressure of its neutrons balances gravity.
d. It is a lie. Nothing can be emitted from black whole.
e. The statement us true.
f. The statement is true.
Solution 3
a.
- The sizes of interstellar clouds are of a few light-years, and it takes few years for light to travel from one cloud to the other.
- Because of disturbances, these interstellar clouds contract resulting in an increase in density and temperature and form a hot, dense, gas sphere.
- When there is sufficient increase in the temperature and density at the centre of the sphere, nuclear energy is generated.
- This generation of energy makes the sphere self-luminous, and a star is born.
b. The reason for the evolution of stars is the burning of and therefore, the decrease is the amount of fuel in their centre.
When the fuel in the centre finishes, the energy generation stops.
As a result, the temperature of the star starts decreasing.
Due to the decrease in temperature, the gas pressure decreases and the balance between gas pressure and gravitational force cannot be maintained.
As the gravitational force is now higher than the gas pressure, the star starts contracting.
This causes another fuel to start burning e.g., when hydrogen at the centre is finished, helium starts undergoing fusion and energy generation starts again.
c. Three end stages of star are:
i. White dwarf -
- Mstar< 8 < MSun
- Density of star becomes high compared to Earth
- Small size and very stable
ii. Neutron Stars
- 8MSun< Mstar<25MSun
- Stable in nature
- The pressure of these neutrons is independent of temperature and is capable of balancing gravitational force.
iii. Black hole
- Mstar > 25MSun
- Light falling on such a star gets completely absorbed inside
- Gravitational pull is very strong
d. Any light falling on these blackhole does not get reflected and gets absorbed inside the star. Thus, we cannot see the star at all but can probably see a minute black hole at its place. Hence the name.
e. Stars having mass between 8 and 25 time the mass of the Sun end their life as neutron star.
Solution 4
A.
- Hydrogen makes up for 72% of the mass of the Sun while helium is
- 26%. The rest 2% is made up of elements heavier than Helium.
- Mass of the sun is 2 x 1030 kg while radius is 695700 km.
- Surface temperature of sun is 5800 K and temperature at the centre is 1.5 x 107 K.
- Age of the sun is 4.5 x 109 years.
B.
- White dwarfs are the end stage of low mass stars.
- They have density very high compared to that of Earth
- They are small in size and very stable.
- They appear white in colour.
- Their initial mass is less than 8 times the mass of the sun.