In the World of Stars | Class 7 | Science | Chapter 20 | Maharashtra State Board
Explore the Wonders of the Cosmos: Stars, Constellations, and Celestial Mechanics.
Join us on a stellar journey through the universe as we unravel the mysteries of the cosmos in this enlightening video. We delve into the heart of galaxies, witness the birth and lifecycle of stars, and navigate the celestial sphere with its constellations that have guided humanity for millennia.
Questions & Answers
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 meridian 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.
2. A star rises at 8 pm tonight. At what time will it rise after a month? Why?
Ans. The star will rise at 6 pm after a month. This is because each star rises or sets 4 minutes earlier every day. Thus, in a month (30 days), the total advancement in the rising time of a star = 4 minutes/day x 30 days = 120 minutes = 2 hours. So, if a star rises at 8 pm today, it will rise 2 hours earlier i.e., at 6 pm after a month.
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?
Ans. When we say “the sun enters a nakshatra”, it means that the particular nakshatra is behind the sun from our perspective on Earth. So, when it’s said that “in the rainy season the sun enters the Mrug nakshatra”, it means that during the rainy season, the Mrug nakshatra (or Orion) is behind the sun. This is due to the Earth’s change in position.
4. Answer the following questions.
(a) What is a constellation?
Ans. A constellation is a group of visible stars that form a perceived pattern or outline in the sky. These patterns are typically associated with mythological creatures, animals, or objects. The constellations we see depend on our location on Earth and the time of year. Today, there are 88 officially recognized constellations.
(b) What points should be considered before a sky watch?
Ans. Before a sky watch, consider the following points:
Location: Choose a place away from city lights for better visibility.
Moon Phase: Ideally, it should be a new moon night to minimize light pollution.
Equipment: Use binoculars or telescopes for enhanced viewing.
Reference Point: Identify the pole star in the north as it makes sky watching easier.
(c) Is it wrong to say that the planets, stars and nakshatras affect human life? Why?
Ans. Yes, it is incorrect to say that distant stars, planets, and nakshatras have a significant influence on human life. This is because they are too far away to noticeably affect us via gravity or electromagnetism. While some people believe in astrological predictions based on the positions of celestial bodies, there is no scientific evidence to support these claims. Therefore, from a scientific perspective, these celestial bodies do not have any direct impact on our daily lives.
5. Write a paragraph on the birth and lifecycle of stars using figure 20.1
Ans. Birth of Stars: Stars originate from nebulae, which are vast clouds of hydrogen gas and dust.
Gravitational forces cause these clouds to contract, leading to an increase in density and temperature at the core. This process ignites nuclear fusion, transforming the cloud into a glowing ordinary star.
Lifecycle Progression: Over time, stars undergo changes due to contraction, expansion, and temperature variations
An ordinary star may swell into a red giant or, if massive enough, a red supergiant. Eventually, it may shed its outer layers as a planetary nebula, leaving behind a white dwarf. The most massive stars end their lives in a spectacular supernova explosion, possibly resulting in a neutron star or even a black hole.
This lifecycle spans millions to billions of years, marking the dynamic and transformative journey of stellar evolution.
E X T R A
1. Fill in the blanks:
1. The different group of stars is known as constellation.
2. The pole star is North star.
3. The moon moves around the earth in about 27 days.
4. The celestial objects are the stars.
5. The star that lies close to the aris of rotation of the earth Pole star.
6. The definite elliptical path in which a planet revolves around the sun is called Orbit.
7. Our earth as well as the sun belongs to the galaxy which has a spiral shape called milky way.
8. Sharmishtha is made up of five bright stars which are distributed along the figure of the letter M.
9. The pole star has Saptarashi on one-side and Sharmishtha on the other.
10. The continuous empty space between the planets and stars in the sky is called space.
2. Name the following:
1. The brightest star in the nakshatra.
Ans. Yogatara
2. The stars forming a group that has a recognizable shape.
Ans. Constellation
3. Millions of stars and planets present in the sky forming a group.
Ans. Milky way
4. The clouds from which stars are born.
Ans. Nebulae
5. Saptarshi constellation in English.
Ans. Great Bear
6. The festival celebrated when sun enters Makar raashi.
Ans. Makara Sankranti
7. The brightest star in the Orion constellation.
Ans. Sirius
3. State whether the following statements are True or False. Correct the false statements:
1. The hydrogen gas and dust particles in nebulae are attracted towards each other by gravity.
Ans. True
2. While standing on the ground, the celestial sphere exactly below our feet is called the Zenith.
Ans. False. The point on the celestial sphere exactly above our head is called the Zenith
3. The circle describing the apparent motion of the earth around the sun is called the ecliptic.
Ans. False. Ecliptic is the apparent motion of sun around the earth
4. Vrushchik or Scorpio is a constellation with 10 to 12 stars.
Ans. True
5. Makar raashi is also known as Capricorn zodiac sign.
Ans. True
4. Answer in one line:
1. In which direction stars move in the sky except polar star?
Ans. When seen from earth, stars appear to move from east to west.
2. What activity does IUCAA carry out?
Ans. IUCAA which is present in PUNE carries out fundamental research in astronomy.
3. Define constellation.
Ans. A group of stars occupying a small portion of the celestial sphere is called constellation.
4. What is Nakshatra?
Ans. The moon completes one revolution around the earth in approximately 27 days. The portion celestial sphere traversed by the moon in one day is called a nakshatra.
5. What is yogatara?
Ans. A nakshatra is known from the brightest star that it contains. The brightest star is called the yogatara.
5. Define the following:
1. Horizon
Ans. Far away the sky seems to be touching the ground. The line at which they meet is called horizon.
2. Zenith
Ans. While standing on the ground the point on the celestial sphere exactly above our head is called the Zenith.
3. Nadir
Ans. While standing on the ground the point on the celestial sphere exactly below our feet is called the nadir.
4. Meridian
Ans. The great circle which passes through both the celestial poles and the observer’s zenith and nadir is called a meridian.
5. Celestial equator
Ans. If we uniformly expand earth’s equator in all directions indefinitely, it will penetrate the celestial sphere along a circle. This circle is known as the celestial equator.
6. Ecliptic
Ans. The earth moves around the sun, but seen from the earth, the sun appears to move along a circle on the celestial sphere. This circle describing the apparent motion of the sun around the ‘earth is called the ecliptic.
7. Zodiac sign
Ans. The ecliptic has been imagined to divided into 12 equal parts. Each part subtends 30 degrees at the centre of the celestial sphere. Each of these parts is called a raashi or zodiac sign.
8. Mrug Nakshatra or Orion
Ans. It has 7 – 8 starts of which four are at the comers of quadrangle. The line passing through the three middle stars of the constellation when extended meets a very bright star. This is Vyadh or Sirius.
6. Answer the following questions.
1. Why is the pole star important for sky watch?
Ans. 1. Identifying the Pole Star in the north makes the sky watch easier. Hence the pole star should be used as a reference point for sky watch.
2. If we extend one side of the quadrangle of Saptarshi, it reaches the Pole Star.The pole star has Saptarshi on one side and Sharmishtha on the other.
2.What is the relation between the pole star and the constellations Saptarshi and Sharmishtha?
Ans.
1. Saptarshi is in the shape of a quadrangle with a tail made up 3 stars resembling a kite. If we extend one side of the quadrangle it reaches the Pole Star.
2. The constellations of Saptarshi and Sharmishtha are useful in locating the Pole Star.
3. The perpendicular bisector of the line joining the third and fourth stars in Sharmishtha goes towards the Pole star.
4. The Pole Star has Saptarshi on one side and Sharmishtha on the other.
3. One Zodiac Sign = ………….. nakshatras
Ans. 27 nakshatras. Each nakshatra is divided in padas or charan. Every nakshatra has 4 padas. These 27 nakshatra complete the entire circle of 360° of zodiac. The zodiac comprises of 360°.
4. Is sun the only star present in our Milky Way galaxy?
Ans. No. Sun is not the only star present in the Milky Way. There are lakhs of stars in the Milky Way, some of them being many times bigger than our sun. Some of them have their own planetary systems with a great diversity in colour, brightness, as well as size.
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