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Std 7 Chapter-In the world of stars

Title : Std 7 Science Chapter : In the world of stars

I. Sky watching II. Constellations III. Getting to know some constellations

Maharashtra state board solutions for class 7 SSC board science part STD 7- General science    In the world of stars

 Know everything about the world of stars Maharashtra state board class 7 SSC .Get detailed Questions and answers for chapter 7  Science I. Sky watching II. Constellations III. Getting to know some constellations

Important question and ansers

*Q.1) What is a galaxy?

 What are the various components of a galaxy?

Answer: A system of millions or billions of stars, together with gas and dust, held together by gravitational attraction is known as galaxy. The four main parts are the disk, the nuclear bulge, the halo, and the galactic corona. halo: the outermost part of a spiral galaxy, nearly spherical and lying beyond the spiral. globular cluster: a spherical group of up to hundreds of thousands of stars, found primarily in a galaxy's halo.

*Q.2) What are the different types of stars?

Answer: The Sun is a as a G2V type star, a yellow dwarf and a main sequence star. Stars are classified by their spectra (the elements that they absorb) and their temperature. There are seven main types of stars. In order of decreasing temperature, O, B, A, F, G, K, and M.

 **Q.3) What is a star? How are the stars formed?

 Answer: Stars are born out of nebulae. Nebulae are clouds made up mainly of hydrogen gas and dust particles. The particles in these clouds are attracted towards one another clue to the force of gravity. As a result, the cloud contracts and becomes dense and spherical in shape. At the same time, the pressure of the gas at the core of the cloud increases causing the temperature to rise tremendously and energy generation processes start there. Such a spherical cloud of hydrogen is called a ‘star’  Later, processes such as contraction, expansion, rise in temperature, etc. bring about changes in the nature of the star. These changes occur over a very long period of time and constitute the lifecycle of stars. The different forms of the stars at various stages during this lifecycle are identified as different types of stars.

Sky watching

Q.2) What is a celestial sphere?

 Answer: While turning around oneself, the horizon will be seen to form a circle and on looking up, the sky will appear to be a sphere based on this circle. The stars and planets moving in the sky appear to be moving on 3 this sphere. This Virtual sphere is called the celestial sphere. The circular horizon divides this sphere into two halves. Zenith and Nadir. Virtual sphere

**Q.3) What is Zenith?

 Answer: While standing on the ground, the point on the celestial sphere exactly above our head is called the zenith.

 **Q.4) What is Nadir?

 Answer: While standing on the ground, the point on the celestial sphere exactly below our feet is called the nadir.

*Q.9) What is a sky?

 Answer: Standing in an open space, if we look at the sky on a cloudless night, we see numerous stars against a dark background. 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.

*Q.10) What is a space?

Answer: The continuous, empty space between the spheres (planets, stars, etc.) in the sky is called space. It may contain gas and dust particles. Numerous star clusters have formed space.

 **Q.11) Why does the sun, the moon and the stars are seen to rise in the east and set in the west? Answer: The sun, the moon and the stars are seen to rise in the east and set in the west because the earth rotates from the west to the east. If we observe carefully, we will also notice that stars rise and set 4 minutes earlier every day. That is, if a star rises at 8 pm tonight, it will rise at 7:56 pm tomorrow. Against the background of stars, the sun and the moon appear to move from the west to the east, the sun moving through one degree every day and the moon through 12 to 13 degrees. This happens due to the motion of the earth around the sun and that of the moon around the earth

*Q.4) What points are necessary to remember while sky watching?

 Answer: 1. The place for sky watching should be away from the city and, as far as possible, it should be a new moon night.

 2. Binoculars or telescopes should be used for sky watching.

 3. 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.

4. As the stars in the west set early, sky watching should begin with stars in the west.

5. As in geographical maps, the east and west are shown to the right and left respectively in a sky map.

 6. On a sky map, the north and south are towards the bottom and top of the map respectively. This is because the sky map is to be held overhead. Hold the sky map in such way that the direction we face is at the bottom side.

**Q.5) One zodiac sign means how many nakshatra?

 Answer: One zodiac sign means 13 nakshatras. Getting to know some constellations

Constellations

 **Q.1) Write a note on Saptarshi.

Answer: During summer nights one can see a particular, arrangement of seven stars. We call them Saptarshi. In the month of February, this constellation rises around 8 pm in the north-east. It is on the meridian in the month of April and in the month of October, it sets around 8 pm. As the name suggests, Saptarshi is a group of seven bright stars. It is in the shape of a quadrangle with a tail made up of three stars. It thus resembles a kite and can be easily recognized. If we extend one side of the quadrangle, it reaches the Pole Star or Polaris. Different countries have different names for this constellation. In English it is called the Great Bear.

 **Q.2) How can you locate polestar?

 Answer: The constellations of Saptarshi and Sharmishtha or Cassiopeia are useful in locating the Pole Star. Sharmishtha is made up of five bright stars which are distributed along the figure of the letter M. The perpendicular bisector of the line joining the third and Sharmishtha or ' fourth stars in Sharmishtha goes towards the Pole Star. The Pole Star has Saptarshi on one side and Sharmishtha on the other. As Sharmishtha sets, Saptarshi rises. Thus, we can always use either one or the other as a reference point on any given night

. **Q.3) Write about Mrug nakshatra.

 Answer: Mrug nakshatra or Orion has very bright stars. On winter nights, they can be easily identified. It has seven-eight stars of which four are at the comers of a quadrangle. The line passing through the three middle stars of the constellation, when extended, meets a very bright star. This is Vyadh or Sirius. During the month of December, Mruga nakshatra rises at 8 pm on the eastern horizon. It is on the meridian during February and in June, it sets around 8 pm.

 **Q.4) Write about Vrushchik constellation.

Answer: Vrushchik or Scorpio is a constellation with 10 to 12 stars. Jyeshtha or Antares is the brightest among them. This constellation is below the equator, in the sky of the southern hemisphere. In the third week of April, it can be seen in the eastern sky a few hours after sunset.

 *Q.5) Why is the Pole Star important for sky watch?

 Answer: From our perspective on earth every star in the sky pivots around the pole star causing them to rise and set. Because of its position in the North Pole, it's the only star that doesn't appear to move.

 *Q.6) What is the relation between the Pole Star and the constellations Saptarshi and Sharmishtha? Answer: In ancient times constellations were devised to be able to recognize stars in the sky. Saptarshi is also known as the Big Dipper, the Great Bear. It can be seen in the summer sky during night. Sharmishtha is also known as Cassiopeia. It can be seen in the Northern sky during winters. Constellations are a group of stars forming some kind or recognizable figure in a pattern. In Indian terminology constellations are called Nakshatras.

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