CHAPTER-24
SPACE SCIENCE
(A) STARS AND GALAXIES
Text Book Question and Answers
I. Four alternatives are given to each of the following incomplete statement / question. Choose the right answer.
1. The raw material for the formation of protostar is
a. coal b.helium c.hydrogen d.uranium
2. In which of the following state is the sun ?
a. red giant b.white dwarf c. proto star d. steady state
3. The first element formed within few minutes after supernova is
a.helium b.hydrogen c. nitrogen d. oxygen
4. The heaviest element formed in the stellar evolution before supernove explosion is
a. calcium b. silicon c. neon d. iron
5. The diameter of Milky way is about
a. six lakh light year b.two lakh light year
c.one lakh light year d. ten lakh light year
II. Match the followings
1.The sun – yellow
2. The sirius – yellowish white
3. The Rigel – Bluish white
III. Answer the following questions
1. How protostar is formed?
Huge gaseous clouds mostly hydrogen exist in space. It is believed that the birth of a star begins when such a gaseous cloud contracts due to gravity. There will be increase in density which leads to increase in pressure. Gradually the cloud settles with a spherical mass at the centre of the cloud. The central portion has a mass of about 99% of the mass of the cloud. The sphere formed at the centre is called a protostar.
2. How steady state star is formed?
With the formation of a protostar contraction and heating up continues. Gradually the temperature increases to millions of degrees. When the temperature is high enough, fusion of hydrogen begins the energy released during nuclear fusion tries to expand the matter. Eventually the outward pressure due to the radiation generated balances the inward gravitational pull. This gives the star a stable state. A protostar thus reaches steady state.
3. Explain the evolution of a star whose mass is five times the mass of a star.
The evolution of all stars begins from protostar moves on to steady state and reaches red giant stage. Stars whose mass is five times the mass of the sun evolve differently after red giant stage. Their evolution has several stages of nuclear reaction. The carbon nuclei produced during the fusion of helium forms of carbon core. The carbon nuclei synthesize oxygen, reaction continues to form magnesium silicon and fusion cycle continues till iron is formed. At this stage the star explodes and the event is supernova. The remnant of supernova sometimes condenses to a core composed of tightly packed neutrons which is known as a neutron star.
4. Explain the ‘Big Bang’ theory.
At the beginning of the universe, all matter comprising galaxies and star and radiation were compressed into a fiery region, with extremely high temperature and enormous density. This is known as the “primordial fireball” The fireball exploded with a bang. There was an awesome explosion which launched the expansion of the universe. This is called the “Big Bang”. As matter and radiation cooled, stars and galaxies formed. Planets came into existence. Eventually life came into existence on earth. The radiation has gradually cooled. The existence of this radiation was detected in 1965 and this supports the ‘Big Bang’ theory.
5. State Hubble’s law
The velocity of recession of a celestial body is proportional to its distance from us. This is known as Hubble’s law.
Additional Questions
1. Name the nearest star to the earth.
Sun
2. How far away sun from the earth?
The distance of the sun from the earth is nearly 1.5 x 108 km
3. What is the mass of sun?
Mass of the sun is 2 x 1030 kg. This is more than 3x105 times that of the earth. Mass of the sun is represented by M.
4. What is the surface temperature of the sun?
Surface temperature of the sun is 5000 - 6000K
Matter in this region is in the plasma state and the temperature is of the order of a few million degrees.
5. List the temperature, colour of different stars.
| Temperature | Star name | Colour |
| 2500 - 3500K | Betelgeauae | Red |
| 3500 - 5000K | Arcturus | Orange yellow |
| 5000 - 6000K | Sun | Yellow |
| 6000 - 10,000K | Sirius | Yellowish white |
| 10,000 - 50,000K | Rigel | Bluish white |
6. Name the characteristics on the basis of which, sizes of stars are determined.
Sizes of stars are determined from its luminosity and temperature. Majority of stars have radii in the range of a tenth to twenty time’s solar radius.
7. What is a galaxy?
A huge group of stars is called a galaxy
8. Name the different types of galaxies.
Spiral galaxies, elliptical galaxies & irregular galaxies
9. What is stellar evolution
The process from the birth to the death of a star is called stellar evolution.
10. Define planatory nebula?
The outer envelope of Red giant get detached and thrown into space. It forms a cloud called planatory nebula .
11. What is Chandrashekar’s limit?
1.4 times mass of sun is known as Chandrashekar’s limit .
12. How red giant is formed?
In steady state of star hydrogen continues to fuse forming helium, a helium core forms at the centre. This is surrounded by hydrogen in the outer shell and hydrogen fuses at a rapid rate. The resulting radiation causes the star’s envelope to expand and cool. The colour of the star changes to red and the star is now called red giant.
13. How White dwarf is formed?
After Red giant stage if the star has a mass less than about 1.4 solar masses (known as Chandrasekhar limit), the star collapses under the action of gravity. As the temperature increases, the core develops pressure which prevents further collapse; the star becomes what is called White dwarf.
14. What is Super nova?
Stars which are 5 times or still more massive than the sun, evolve differently after the red-giant stage have several stages of nuclear reactions ignited one after the other. The carbon nuclei produced during the fusion of helium form a carbon core. This will get ignited to produce a core of oxygen. The chain continues leading to the production of heavier elements. Thus cores of oxygen, magnesium, silicon, etc are formed and the cores lie one inside the other like the layers of an onion. Iron is formed at the inner most core. At this stage, the star explodes and the event is called supernova
15. What are Pulsors?
Fast spinning of Neutron star emits radiation in pulses and are called pulsars.
16. What are Quassrs?
The objects in galaxies which emit radio waves are called Quasars.
17. Define black hole .
In the case of stars with masses about 30 times that of sun, the remnant at the end of supernova, has huge amount of matter compressed into a very small region. This is a region of intense gravitational field and is called a black hole.
18. What is the evidence for expanding universe?
The red shifts of galaxies show that they are moving away from us with enormous speeds. This demonstrates that the universe is expanding.
19. What is a Neutron star?
During supernova, most of the material is thrown out at speeds of about 104 km/s. A sphere of neutrons remains at the centre and this is called neutron star.
20. What are brown dwarfs?
If the mass of a contracting gaseous cloud is less than His ignition 0.1 M, it will not become a star. Such objects are called red giant brown dwarfs.
Jupiter may be called a brown dwarf.
21. What is the age of the Universe?
The age of the universe is estimated to be 10 to 20 billion years.
22. What is the age of our sun?
The age of our sun is estimated to be 4.5 to 5 billion years.
23. What is the name of our galaxy? Write its type.
Milky Way is the name of our galaxy. Milky Way is an example to Spiral galaxy.
24. List the different stages in life cycle of sun like stars & huge stars
| Stages | Sun like stars | Huge stars( mass more than 5 times that of sun) |
| 1 | Photo star | Photo star |
| 2 | Steady state star | Steady state star |
| 3 | Red giant | Red giant |
| 4 | White dwarf | Supernova |
| 5 | Black dwarf | Neutron star |
| 6 | Black hole |
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