Science textbook solutions for Class 7 science Chapter 20 In the World of Stars

Important points to remember :

1. Our solar system is a tiny part of a galaxy called  the Milky Way, which is many, many times larger than the solar system. 
2. There are lakhs of stars in the Milky Way, some of  them being many times  bigger than our  Sun.  
3. Some of  them have their own planetary systems. 
4. The line at which they meet is called the  horizon. 
5. The portion of  earth’s atmosphere and the portion beyond that which can be seen in the  form of a roof by our eyes while standing on the earth is called  the sky. 
6. The  continuous, empty space between the spheres (planets, stars, etc.)  in the  sky is called space. It may contain  gas and dust particles. 
7. Numerous star clusters have formed in space. The place for sky watching should be away from the  city  and, as far as  possible, it should be a new moon night.  
8. Binoculars or  telescopes should be used for sky watching. 
9. A  group of stars occupying  a small  portion of the  celestial sphere is  called a  constellation. 
10. This brightest  star is called the  yogatara. Which  nakshatra  we can  see during a sky watch depends upon the position of the earth along its orbit.
11. IUCAA  (Inter University Centre for Astronomy and Astrophysics) in Pune carries out fundamental  research in astronomy. 

Question 1: Write the proper words in the blanks.
(meridian, horizon, twelve, nine, apparent, celestial, ecliptic)

a) When seen from a great distance, the sky seems to be touching the ground along a circle. This circle is called the horizon.

b) The ecliptic is used while defining the zodiac signs.

c)  Classified according to seasons, one season will have nine nakshatras.

d) The rising of the sun in the east and its setting in the west is the apparent motion of the sun.

Question 2: A star rises at 8 pm tonight. At what time will it rise after a month? Why? 
Answer : The star will rise at 6 pm after a month. This is because each star rises or sets 4 min earlier everyday. Thus, in a month, the total advancement in the rising time of a star = 4×30=120 min=2 h4×30=120 min=2 h

So, if a star rises at 8 pm today, it will rise before 2 h i.e. at 6 pm after a month.

Question 3: What is meant by ‘The sun enters a nakshatra’ ? It is said that in the rainy season the sun enters the Mrug nakshatra. What does it mean?
Answer : A constellation or nakshatra is present behind the Sun, which gets hidden because of the bright sunlight. So, when we look up in the sky, we are actually seeing both the Sun and the constellation behind it. Thus, when the Earth changes position, it appears to an observer on Earth, as if the Sun has moved from its position and is entering a new constellation or nakshatra.
The Sun entering the Mrug nakshatra in rainy season means that Mrug nakshatra or Orion is behind the Sun.

Question 4: Answer the following question.
a) What is a constellation?
Answer : A group of stars forming a definite shape is known as constellation. Few examples of constellations are Orion, Big Dipper.

b) What points should be considered before a sky watch?
Answer : A sky watch is one of the most important part of the astronomical studies and research works.This helps in the observation of the stars and the other astronomical substances.
The points that should be considered before a sky watch:

1) proper arrangement of telescope and other equipments.

2) clear cloud free sky.

3) proper altitude and location of the observatory.

c) Is it wrong to say that the planets, stars and nakshatras affect human life? Why?
Answer : There is no credible evidence supporting astrology or any of its related pseudosciences. In fact, horoscopes are so generic that the newspaper zodiac listings can be scrambled and people will still think “their sign’s” horoscope applied to them (actual research!).Astrology is just another superstitious coping mechanism that pretends to help people live their lives.

Question 5: Write a paragraph on the birth and lifecycle of stars using figure 20.1. 
Answer : Nebula is made up of gas and dust and is the birthplace of stars. Now, from this Nubula, various forms of stars are formed. Some are massive stars whose size is much greater than the size of Sun and some are smaller stars whose size is smaller than the size of Sun. Very large, massive stars burn their fuel much faster than smaller stars and may only last a few hundred thousand years.

# let’s recall 
1. What is a galaxy ? What are the various components of a galaxy ? 
Answer : A spiral galaxy like the Milky Way has three basic components to its visible matter: the disk (containing the spiral arms), the halo, and the nucleus or central bulge.

2. What are the different  types of stars ?
Answer : Protostar: A protostar is what you have before a star forms.
T Tauri Star:
Main Sequence Star:
Red Giant Star:
White Dwarf Star:
Red Dwarf Star: Neutron Stars: Supergiant Stars:

2. Why is the Pole Star important  for sky watch ? 
Answer : Pole star is the brightest star in constellation Ursa Minor. – Position of pole star relative to earth is constant & thus it was the ideal star to look for direction in ancient times Hence, pole star is important tool for skywatch.

3. What is the relation  between  the Pole Star and the constellations Saptarshi and Sharmishtha ?
Answer : The position of the pole star can be located with the help of constellation Saptarishi and Sharmishtha. On one side of the pole star it has saptarishi and on the other side it has Sharmishtha. When Saptarishi rises, Sharmishtha sets. In ancient times constellations were devised to be able to recognise stars in the sky.

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