One mark questions with answers

Q1. How are galaxies classified usually?
Ans1. Galaxies are usually classified on the basis of their shapes. There are three types of galaxies:

(i) Spiral galaxies, (ii) Elliptical galaxies, (iii) Irregular galaxies.

Q2. Name the most common shapes of galaxies. Mention their characteristics.

Ans2. Most of the galaxies are, either spiral or elliptical. Elliptical galaxies are very bright, while the spiral galaxies are usually much bigger.

Q3. Name the star which is nearest to the earth, excluding the sun.

Ans3. Alpha Centuari.

Q4. What is the relation between a light year and kilometer?

Ans4. 1 light year = 9.46 x 1012 km.

Q5. Name the physical quantity whose unit is Parsec?

Ans5. It is the unit of distance and is used for large distances.

Q6. Which of the following are constellations: Orion, Vega, Sirius, Scorpio?

Ans6. (i) Orion (ii) Scorpio.

Q7. Notice the pattern of stars shown in the diagram given below. Identify the constellation corresponding to it.

Ans7. This pattern of stars describes the constellation called Ursa Major (or Great Bear). Indian name of this constellation is Saptarishi.

Q8. Notice the pattern of stars shown in figure. Identify the constellation corresponding to it.

Ans8. The pattern of stars shown here is that of the constellation Orion (Hunter).

Q9. State the name of the gases and temperature of the cloud which forms a star.

Ans9. Hydrogen and Helium constitute clouds and its temperature is -173 ºC.

Two mark questions with answers
Q1. Draw the shape of our galaxy as seen from the front.

Ans1. Our galaxy is called Milky way. It is spiral in shape. Top and side (front) view of Milky sat galaxy is shown in figure. It appears as a band of stars faintly whitening the sky from north to south.

Q2. Write the International name and Indian name of four constellations.

Ans2.

Q3. Define a star. On which characteristic its colour depends?

Ans3. Stars are the heavenly bodies that are extremely hot and have light of their own. Every star is a huge mass of hot gases and big flames keep coming out of it. Its colour depends upon its temperatures.

Q4. What is a protostar and how is it formed?

Ans4. Protostar is a highly condensed cloud of hydrogen and helium gases. It is formed due to the gravitational contraction of gases present in the galaxy.

Q5. Why do the stars not collapse due to immense gravitational forces?

Ans5. Stars do not collapse because the gravitational force of attraction trying to compress it is less than the internal pressure generated by the energy released due to fusion process taking place in the interior of the sun.

Q6. Mention one point of difference between a red giant star and a white dwarf star.

Ans6. In the case of red giant star, hydrogen converts into helium in its outer shell only. But, in the case of white dwarf star, helium converts into carbon nucleus in its dense core. Both are definitely the fusion process.

Q7. State two major physical changes that would take place in the sun before it enters the red-giant phase.

Ans7. (i) Due to the drop in the pressure inside, the core of the sun would begin to contract.
(ii) The outer shell of the sun, would begin to expand.

Q3. Define a star. On which characteristic its colour depends?

Ans3. Stars are the heavenly bodies that are extremely hot and have light of their own. Every star is a huge mass of hot gases and big flames keep coming out of it. Its colour depends upon its temperatures.

Q4. What is a protostar and how is it formed?

Ans4. Protostar is a highly condensed cloud of hydrogen and helium gases. It is formed due to the gravitational contraction of gases present in the galaxy.

Q5. Why do the stars not collapse due to immense gravitational forces?

Ans5. Stars do not collapse because the gravitational force of attraction trying to compress it is less than the internal pressure generated by the energy released due to fusion process taking place in the interior of the sun.

Q6. Mention one point of difference between a red giant star and a white dwarf star.

Ans6. In the case of red giant star, hydrogen converts into helium in its outer shell only. But, in the case of white dwarf star, helium converts into carbon nucleus in its dense core. Both are definitely the fusion process.

Q7. State two major physical changes that would take place in the sun before it enters the red-giant phase.

Ans7. (i) Due to the drop in the pressure inside, the core of the sun would begin to contract.
(ii) The outer shell of the sun, would begin to expand.

Three mark questions with answers

Q1. Differentiate between a galaxy and a constellation.

Ans1.

Q2. Describe in detail the mechanism of formation of stars.

Ans2. (i) Hydrogen and helium gases present in the galaxies form small dense clouds. These clouds then start contracting under their own gravity at a temperature of about – 173ºC. A highly dense, contracting mass of gases is called protostar. A protostar does not emit light.

(ii) As the protostar begins to contract, the pressure of contracting mass begins to increase. This process of contraction continues for about a million years. The inside temperature then increases to about 10,000,000oC (10 million). At this temperature the conditions for nuclear fusion arise and hydrogen nuclei convert into helium. During fusion, a large amount of energy is released in the form of heat and light. This raises the temperature of the protostar further. The protostar now begins to glow and becomes a star.
The necessary reaction for the nuclear fusion is given below:
4 ¹H₁   Fusion >  ⁴He₂ + Tremendous energy

Q3. What is red-giant phase of a star?

Ans3. As the fusion continues in the interior of a star, more and more hydrogen is converted into helium. With this conversion the temperature of the star increases further but at the same time the amount of hydrogen in the core of a star starts decreasing and ultimately there is no hydrogen in the core. Now the fusion reaction stops in the core of the star. When this happens the pressure inside the core falls and it begins to contract again under its gravitational force. Now the gravitational force dominates the pressure due to the fusion reaction. Hence, the core contracts. The fusion reaction in the outer shell however continues. The outer shell then begins to expand. This increases its surface area, and reduces the intensity of the emitted light. The star now begins to change its colour and becomes red. This phase in the star's life is called the red giant phase and the star is called a red giant star.

Q4. What do you mean by Doppler effect in sound and in light?

Ans4. Doppler effect:
(a) In sound: Whenever there is a motion between the source and the listener such that the distance between them changes, the frequency of the sound heard by the observer appears to be changed. This change in the frequency of sound is called Doppler effect in sound.
(b) In light : When the wavelength of the light waves coming from a star is found different from the actual wavelength then this is called Doppler effect in light. This is due to the relative motion between the star and the earth.

Q5. Explain how sun has evolved from a dense cloud of gases?

Ans5. The over dense and cold cloud of gases (mainly hydrogen and helium) started contracting under their own gravity at very low temperature and a protostar was formed. Inside a protostar the temperature was increased greatly due to collision between the hydrogen molecules. Ultimately, hydrogen was converted into helium. In the process of nuclear fusion a large amount of energy was released due to which the protostar started emitting heat and light and became a star which is known as sun today. Depending upon its mass it is predicted that the sun would become a red-giant star and ultimately die out as white dwarf star after about 5000 million years.

Q6. How is the age of earth estimated? What is the approximate estimated age of the earth?

Ans6. The earth contains various types of rocks. Some of these rocks contain a radioactive substance called uranium. The uranium is unstable and gradually decomposes to form a stable element called lead. The rate at which the atoms of radioactive uranium decay to form non-radioactive lead is a constant, which has been determined by experiments. For example, it has been found that about one-third of uranium atoms present in a sample of uranium rock decay to form lead in every 4.5 billion years i.e. 4.5 x 109 years.
Thus, approximate age of the earth has been estimated 3700 million years.


Five mark questions with answers

Q1. What is a meteorite? How are meteorites dated?

Ans1. If a meteor is big, a part of it may reach the earth's surface without being burnt up in the air. This fragment is called a meteorite. Thus, a meteor, which does not burn completely on entering the earth’s atmosphere and lands on earth is known as a meteorite. Meteorites are a sort of stones from the sky. The meteorites range in size from small pebbles to big blocks many tonnes in weight. Some meteorites are made of stone and others are made of Iron. By studying the composition of a meteorites we can get valuable information about the nature of matter in outer space.

By the term "how are meteorites dated" we mean "how is the age of meteorites determined." The meteorites are dated by using the radiometric dating technique, which is based on the phenomenon of radioactivity shown by uranium element in meteorite samples. This can be explained as follows: When the meteorite was initially formed, then it contained a radioactive element uranium but had no lead in it. In due course of time, the radioactive uranium gradually disintegrated to form lead. So, the meteorites which fall to the earth contain some uranium and some lead (formed from the decomposition of uranium). The amount of lead present in the meteorite sample is found by experiments. Now, by knowing the amount of the lead present in a given sample of meteorite and by using the fact that one-third of radioactive uranium is converted into lead after every 4.5 billion years, the age of meteorites has been estimated to be 4500 million years. That is, the meteorites were created about 4500 million years ago.

Q2. Discuss how the sun and planets were formed.

Ans2. The solar system includes the sun, nine planets, their satellites and large number of other smaller heavenly bodies such as asteroids, comets etc. The formation of the sun and planets can be explained as follows:

(i) Explanation of the formation of Sun: The sun is a star. The sun was formed about 5000 million years ago from the clouds of mainly hydrogen gas, some helium and dust. The over dense and cold cloud of hydrogen gas contracted (shrank) under its own gravitational force to form a huge, dark ball of hydrogen called protostar. As the protostar went on contracting further, the hydrogen molecules present in it collided with one another more frequently. The collisions of hydrogen molecules in the protostar raised its temperature greatly which started the nuclear fusion reactions. The nuclear fusion reaction in which hydrogen was converted into helium, produced a tremendous amount of energy due to which protostar began emitting heat and light, and became the sun (which is a star).

(ii) Explanation of the formation of Planets: About 5000 million years ago, the newly formed sun was surrounded by a disc-shaped cloud of left-over gases (which were not utilized in the formation of sun). This left-over gas condensed to form small solid objects called planetesimals. These planetesimals started colliding with one another constantly during which some of them were shattered and then reassembled to form some larger and some smaller planetesimals. The larger planetesimals began to attract the smaller objects by their gravitational force and grew bigger and bigger. And ultimately they formed very large objects called planets.


 Q3. What are evidences, which explain the hypothesis that the universe began in a big-bang? Ans. (Try yourself). Q4. Explain the formation of a protostar and star. 

Ans. (Try yourself).

Q4. Explain the formation of a protostar and star.

Ans. (Try yourself).