# MSBSHSE Class 8 Science Chapter 19: Life Cycle of Stars Important Textbook Questions and Solutions

Our solar system is situated in a galaxy. A galaxy is a collection of billions of stars, their planetary systems and interstellar clouds, which are present in the empty spaces between stars. The universe is made up of innumerable such galaxies. Galaxies differ in structure and shape. We can divide them into three types: spiral, elliptical and irregular galaxies. Our galaxy is a spiral galaxy and is called the Milky Way and Mandakini. In this article, we have provided MSBSHSE Class 8 solution of Science for Chapter 19 which is solved by a team of subject experts. Students can score well in their exam by referring to these solutions.

### MSBSHSE Class 8 Science Chapter 19 Textbook Exercise Questions

Q1. Search and you will find.

a. Our galaxy is called………

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

c. The speed of light is ………… km/s.

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

e. The end stage of the Sun will be……

f. Stars are born out of ….. clouds.

g. Milky way is a ……….. galaxy.

h. Stars are spheres of ……….. gas.

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

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

from the moon.

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

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

Answer a: Our galaxy is called Milky Way galaxy.

Answer b: For measuring large distances kilometre is used as a unit.

Answer c: The speed of light is 3,00,000 km/s.

Answer e: The end stage of the Sun will be a red giant.

Answer f: Stars are born out of interstellar clouds.

Answer g: Milky way is a spiral galaxy.

Answer h: Stars are spheres of hydrogen and helium gas

Answer i: The masses of other stars are measured relative to the mass of the sun

Answer j: Light takes time to reach us from the Sun while it takes minutes to reach us

from the moon.

Answer k: The larger the mass of a star the faster is its evolution.

Answer l: The number of fuels used in the life of a star depends on its life cycle.

Q2. Who is telling lies?

a. Light year is used to measure time.

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

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

d. Only light can emit from the blank hole.

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

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

Answer a: The statement is a lie as light year is used to measure distance.

Answer b: The statement is true as end stage of a star depends on its initial mass.

Answer c: It’s a lie because a star ends its life as a neutron star when the pressure of its neutrons balances gravity.

Answer d: The statement is a lie because nothing can be emitted from black hole.

Answer e: The statement is true as the sun will pass through the supergiant stage during its evolution.

Answer f: It is true as the Sun will end its life as a white dwarf.

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 a: Huge clouds of gas and dust are present in the empty spaces between stars in a galaxy. These are called interstellar clouds. Due to some disturbance, these clouds start contracting. Because of the contraction, their density starts increasing and their temperature also starts to increase and a dense sphere of hot gas is formed from the cloud. Once the temperature and density at the centre of the sphere increase sufficiently, nuclear energy (energy generated through fusion of atomic nuclei) generation starts there. Owing to this energy generation, the gas sphere becomes self luminous and a star is formed or we can say that a star is born.

Answer b: Evolution of a star means change in its properties with time resulting in its passing through different stages. Stars evolve very slowly for most of their lifetime. As stars are continuously emitting energy, their energy is constantly decreasing. For their stability to remain intact i.e. for maintaining a balance between the gas pressure and the gravitational force, it is necessary that the temperature remains constant. For the temperature to remain constant, energy must be generated inside the star. This generation of energy occurs because of burning of fuel at the centre of the star. The reason for the evolution of stars is the burning of and therefore, the decrease in 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. How many fuels will be used depends on the mass of the star.

Higher the mass of the star higher is the number of fuels used. During this a lot of changes occur in the star. As a number of processes occur inside the star, it sometimes contracts and expands at other times and the star goes through different stages. When all possible fuels are exhausted, the energy generation finally stops and the temperature of the star starts decreasing. The balance between gravitational force and gas pressure cannot be maintained.

Answer c: There are three ways of evolution of stars depending on their initial mass.

1. End stages of stars having initial mass less than 8 times the mass of the Sun (Mstar < 8 MSun): Stars in this group undergo huge expansion and their radius increases by a factor of 100 to 200. In this stage they are called red giant stars. This name is given because of the large size and because of the fact that the stars look reddish due to their lower temperature. At the end of its evolution, these stars explode and their outer gas envelope is thrown out. The inner part contracts and its size becomes similar to the size of the earth. As the mass of the star is much higher than that of the earth and the size is similar to that of the earth, the density in the star becomes very high. In this state, the pressure due to the electrons in the star becomes independent of temperature and is able to balance the gravitational force forever. In this state, the star looks white and due to its small size it is called a white dwarf. After this its temperature keeps decreasing but its size and mass remain unchanged for ever and so white dwarf is the end stage of stars in this mass range.

2. End stages of stars having mass between 8 and 25 time the mass of the Sun (8 MSun < MStar < 25 MSun) : These stars also go through the red giant stage and later through the supergiant stage during which their size may increase 1000 times. The huge explosion, called the supernova explosion, which occurs at last is very powerful and so much energy is given off that we can see the star during the day also. The central portion which is left behind after the explosion, contracts and its size becomes as small as about 10 km. In this state, the stars are completely made up of neutrons and are called neutron stars. The pressure of these neutrons is independent of temperature and is capable of balancing the gravitational force forever. Neutron star is the end stage of these stars.

3. End stages of stars having mass larger than 25 times the mass of the Sun (Mstar > 25 MSun): These stars evolve like the stars in the second group but after the supernova explosion, no pressure is capable of balancing their huge gravitational force and they continue contracting forever. As their size becomes smaller, their density and their gravitational force increase tremendously. All nearby objects get attracted towards these stars and nothing can come out of them, not even light. Also, any light falling on these stars 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. This end stage of the star is therefore called a black hole. Thus, we have seen that, depending on mass, there are three paths of evolution and three end stages of stars.

Answer d: Stars having mass larger than 25 times the mass of the Sun (Mstar > 25 MSun) evolve like the stars in the second group but after the supernova explosion, no pressure is capable of balancing their huge gravitational force and they continue contracting forever. As their size becomes smaller, their density and their gravitational force increase tremendously. All nearby objects get attracted towards these stars and nothing can come out of them, not even light. Also, any light falling on these stars 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. This end stage of the star is therefore called a black hole.

Answer e: Stars having mass between 8 and 25 times the mass of the Sun (8 MSun < MStar < 25 MSun) go through the red giant stage and later through the supergiant stage during which their size may increase 1000 times. The huge explosion, called the supernova explosion, which occurs at last is very powerful and so much energy is given off that we can see the star during the day also. The central portion which is left behind after the explosion, contracts and its size becomes as small as about 10 km. In this state, the stars are completely made up of neutrons and are called neutron stars. The pressure of these neutrons is independent of temperature and is capable of balancing the gravitational force forever. Neutron star is the end stage of these stars.

Q4. A. Describe white dwarfs.

Answer: Stars having initial mass less than 8 times the mass of the Sun (Mstar < 8 MSun) undergo huge expansion and their radius increases by a factor of 100 to 200. In this stage they are called red giant stars. This name is given because of the large size and because of the fact that the stars look reddish due to their lower temperature. The size of a red giant star in comparison to other types of stars. At the end of its evolution, these stars explode and their outer gas envelope is thrown out. The inner part contracts and its size becomes similar to the size of the earth. As the mass of the star is much higher than that of the earth and the size is similar to that of the earth, the density in the star becomes very high. In this state, the pressure due to the electrons in the star becomes independent of temperature and is able to balance the gravitational force forever. In this state, the star looks white and due to its small size, it is called a white dwarf.

Q5. What is a galaxy?

Answer: A galaxy is a collection of billions of stars, their planetary systems and interstellar clouds which are present in the empty spaces between stars. The universe is made up of innumerable such galaxies. Galaxies differ in structure and shape. We can divide them into three types: spiral, elliptical and irregular galaxies.

Q6. What are the different constituents of our solar system?

Answer: The Solar System is our solar neighborhood which is held together by the Sun’s gravity. Our solar system consists of planets, moon or satellite, dwarf planets, comets, asteroids, meteoroids, stars, centaurs, transneptunian objects, etc.

Q7. What are the major differences between a star and a planet?

Answer: The major differences between a star and a planet are as follows:

 Stars Planets Stars are incredibly hot having high temperatures to them. Planets on the other hand have relatively low temperatures. They are objects that produce their own light and do not rely on an external source for the production of light Planets are incapable of producing their own light. Stars have a unique effect of twinkling in the sky. Planets do not exhibit the twinkling effect unlike stars The stars do not change their position at all. Planets in orbits on their own axis change their positions constantly. Stars have incredibly high temperatures to them. Planets on the other hand are relatively colder with lower temperatures.

Q8. What is a satellite?

Answer: An object orbiting around the sun, earth or any other colossal body is known as a satellite. There are two major types of categorization when it comes down to satellites, one is natural and the other is man-made.

Q9. Which is the star nearest to us?

Answer: Stars are celestial objects that can produce their own light. They are extremely hot and extremely large in size. They are mostly made up of hydrogen gas with a little helium in them. The sun is also a star which is nearest to us.

Q10. Define light year.

Answer: The distance travelled by light in one year is called a light year. It is a measure of distance for celestial objects.

Speed of light $= 3 \times 108 m/s$

1 light year $= 9.46 \times 1012 km$

The sun is 8 light minutes away from earth. The distance of Alpha Centauri is 4.3 light years away.

Q11. Define the Sun and the Planet.

Answer: The sun is the nearest star to earth. It is continuously emitting heat and light. It is the main source of heat and light energy for all planets in our solar system.

Planets are celestial bodies that do not emit heat or light of their own. They revolve around a star in fixed paths called orbits and the time it takes to go around the sun once is known as its period of revolution. A planet also spins on its own axis which is called a rotation.

Q12. What is stability of stars?

Answer: The gravitational force between the gas particles of the star keeps these particles together. If the gravitational force which is constantly trying to bring the gas particles close together and the pressure of the hot gas which is constantly trying to disperse the gas are balanced, then the star remains stable. The gravitational force is acting inwards, towards the centre of the star while the gas pressure is acting outwards, i.e. away from the centre of the star.

Q13. Define stars.

Answer: Stars are celestial bodies that emit their own light and heat. Our sun is an example of a star. Although stars are always present in the sky but are only visible at night due to the absence of sunlight. They appear as points as they are millions of kilometres away from us. Stars twinkle as their light gets refracted in our atmosphere. The position of stars keeps changing as seen from the earth’s surface, apart from the Polestar whose position is fixed.

Q14. What is solar system and how many planets are there in our solar system?

Answer: The sun and all other planets and celestial bodies that revolve around it are together called a solar system. Our solar system has 8 planets and an asteroid belt. Pluto is considered as a dwarf planet. All planets in the solar system revolve around the sun in fixed orbits. Planets nearer to the sun revolve faster as compared to the planets away from the sun.

Q15. Explain properties of stars.

Answer: The mass of the Sun is about 3.3 lakh times that of the earth and its radius is 100 times that of the earth. The age of the Sun and other stars, which is the time elapsed after their formation, can be between a few million years to a few billion years. If the properties of the Sun had changed in its lifetime, it would have caused changes in the properties of the earth and in the life on the earth. Detailed studies of the properties of the earth have led scientists to conclude that the properties of the Sun have remained unchanged over its lifetime i.e. the past 4.5 billion years. According to the studies made by astronomers, these properties will slowly change in further after 4.5 billion years. So, stars contain the above-mentioned properties.