Life Cycle of Stars
Exercise
Q. 1 Search and you will find.
a. Our galaxy is called Milky Way or Mandakini.
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 billions of 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 barred 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 revolution.
l. The number of fuels used in the life of a star depends on its mass.
Q. 2 Who is telling lies?
a. Light year is used to measure time.
Ans. The statement is a lie as light year is used to measure distance.
b. End stage of a star depends on its initial mass.
Ans. The statement is true as 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.
Ans. It's a lie because a star ends its life as a neutron star when the pressure of its neutrons balances gravity.
d. Only light can emit from the blank hole.
Ans. The statement is a lie because nothing can be emitted from black hole.
e. The Sun will pass through the supergiant stage during its evolution.
Ans. The statement is true as the sun will pass through the supergiant stage during its evolution.
f. The Sun will end its life as a white dwarf.
Ans. It is true as the Sun will end its life as a white dwarf.
Q. 3 Answer the following question.
a. How do stars form?
Ans. 1. Huge clouds of gas and dust are present in the empty spaces between stars in a galaxy. These are called interstellar clouds.
2. 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.
3. 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. Because of this energy generation, the gas sphere becomes self luminous and a star is formed or we can say that a star is born.
b. Why do stars evolve?
Ans. 1. Stars evolve over billions of years due to fuel burning at their core.
2. maintaining temperature and stability. As fuel depletes, energy generation stops, causing temperature drop and star contraction.
3. Different fuels burn based on star mass. After all fuels exhaust, stars cool, lose balance, and face their end stages.
c. What are the three end stages of stars?
Ans. There are three ways of evolution of stars depending on their initial mass.
1. End stages of stars having initial mass less than 8 time 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. 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 can balance the gravitational force for ever. 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 for ever. 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 for ever. 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.
d. Why was the name black hole given?
Ans. The name "black hole" was given because it's a space area where gravity is super strong, pulling in everything, even light. Since no light escapes, it looks like a "black" spot in space. The term was made popular by physicist John Archibald Wheeler in 1967.
e. Which types of stars end their life as a neutron star?
Ans. The stars having mass between 8 and 25 time the mass of the Sun end their life as a neutron star.
Q. 4 A. If you are the Sun, write about your properties in your own words.
Ans. 1. As the Sun, I am a massive, hot ball of gas, emitting light and heat.
2. I am enormous, surpassing the combined size of all planets in our solar system.
3. Gravity is my force, keeping everything, from planets to asteroids, in orbit around me.
4. I provide vital light and warmth, supporting life on Earth and other planets.
5. My powerful gravitational forces shape the orbits and paths of objects in our solar system.
6. I undergo a life cycle, transitioning from a stable star to a red giant and ultimately forming a white dwarf.
B. Describe white dwarfs.
Ans. 1. White dwarfs are small, dense stars and end-stage of sun-like stars.
2. They're about the size of Earth but much heavier.
3. These stellar remnants glow faintly because they're hot.
4. White dwarfs happen when stars run out of fuel and collapse.
5. They're like stellar remnants, no longer burning bright. Despite being dim, they're crucial in understanding star life cycles.
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