20. In the World of Stars
Complete solutions to the Exercises. Maharashtra State Board (Balbharati), Grade Seven General Science.
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Exercises
(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.
In one month (30 days): 30 × 4 = 120 minutes = 2 hours
8 pm − 2 hours = 6 pm
After a month, the star will rise at 6 pm.
Why: The earth revolves around the sun. Due to this, every star rises 4 minutes earlier each day. In one month, it rises 2 hours earlier.
The earth revolves around the sun. So the sun appears to move against the background of stars. When a nakshatra is behind the sun, we say that ‘the sun has entered that nakshatra’. The sun does not really move into it. It is only its apparent motion.
‘The sun enters the Mrug nakshatra’ means that in early June, the Mrug nakshatra is behind the sun. This is the beginning of the rainy season. Farmers start sowing at this time.
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(a) What is a constellation?
A group of stars in a small part of the sky is called a constellation. Some groups look like animals, humans or objects. They are named after them. The sky is divided into 88 constellations. Examples: Saptarshi, Sharmishtha, Mrug.
(b) What points should be considered before a sky watch?
- Choose a place away from city lights.
- Choose a clear new moon night.
- Keep a sky map and binoculars.
- First find the Pole Star in the north.
- Start observing from the west, as stars there set early.
- Go in a group with an elder.
(c) Is it wrong to say that the planets, stars and nakshatras affect human life? Why?
Yes, it is wrong. Science has proved that planets, stars and nakshatras do not affect human life. They are very far away from us. What happens in our life depends on our own efforts. So we should think scientifically and avoid such beliefs.
Stars are born from a nebula. A nebula is a huge cloud of hydrogen gas and dust. Due to gravity, the cloud contracts and becomes round. Its temperature increases a lot. Energy is produced and it starts glowing. It becomes a star.
An ordinary star like the sun becomes a red giant in old age. Its outer layer spreads out as a planetary nebula. Its core becomes a white dwarf.
A massive star becomes a red supergiant. It explodes in a supernova. Its remaining part becomes a neutron star or a black hole.
- Ordinary star: Nebula → Star → Red giant → Planetary nebula → White dwarf
- Massive star: Nebula → Star → Red supergiant → Supernova → Neutron star or Black hole
Nehru Planetariums are in New Delhi, Mumbai, Bengaluru and Allahabad. Note these points during the visit:
- How the night sky is shown on a dome.
- Constellations like Saptarshi and Mrug, and the Pole Star.
- Zodiac signs and nakshatras.
- Planets, comets, eclipses and life of stars.
Present this information with charts or slides on National Science Day (28th February).