Maharashtra Board Class 7 Science Chapter 20 In the World of Stars PDF Download

20. In The World Of Stars

Let's Recall

1. What is a galaxy? What are the various components of a galaxy?

2. What are the different types of stars?

We have already learnt about galaxies and stars as well as the solar system and its different components. 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 due 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.

Observe And Discuss

The following figure shows different stages in the lifecycle of stars after their birth from a nebula. Discuss these in the class.

20.1 Lifecycle Of Stars

Our solar system is a tiny part of a galaxy called the Milky Way, which is many, many times larger than the solar system. 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. Stars in the Milky Way show a great diversity in colour, brightness, as well as size. Some stars, which appear to be close to one another making a particular figure are together known as a constellation. We shall learn more about constellations in this chapter. But, before that, let us learn a few basic concepts related to sky watching.

Teacher's Note

Stars are born in space just like babies are born on Earth. They grow up, become old, and die. You can see this in the night sky if you look carefully with a telescope.

Exam Trick

Remember the word NEBULA means a cloud of gas and dust. Think of it like cotton candy that slowly gets compressed into a hard ball, which becomes a star.

Points To Remember

Stars are born from nebulae which are clouds of gas and dust.
Gravity pulls the gas and dust particles together tightly.
As the cloud gets denser, temperature rises very much.
Different types of stars are seen at different stages of their life.
Our Sun is just one small star in the Milky Way galaxy.

Sky Watching

Try This

Stand still in an open space and look into the distance. What do you notice about the ground and the sky? Now, still looking into the distance, turn around yourself and observe the ground and the sky as you do so.

Far away, the sky seems to be touching the ground. The line at which they meet is called the horizon. 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 this sphere. This virtual sphere is called the celestial sphere. The circular horizon divides this sphere into two halves.

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

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

3. Celestial Poles: If we extend the axis of rotation of the earth in the north and south directions it will penetrate the celestial sphere at points called the celestial North Pole and the celestial South Pole, respectively.

4. Meridian: In astronomy, the great circle which passes through both the celestial poles and the observer's zenith and nadir is called a meridian.

5. Celestial Equator: 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. It is in the same plane as the earth's equator.

6. Ecliptic: 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.

Teacher's Note

Imagine you are standing in the middle of a big empty field at night. The place where the sky touches the ground is the horizon. If you point straight up, that direction is called zenith.

Exam Trick

Remember: ZENITH = straight UP above your head. NADIR = straight DOWN below your feet. The horizon is where sky meets ground.

Points To Remember

The horizon is the circle where the sky seems to touch the ground.
The zenith is the point directly above your head in the sky.
The nadir is the point directly below your feet.
The celestial sphere is an imaginary ball that surrounds the Earth.
The ecliptic is the path of the sun as seen from Earth.

20.2 Virtual Sphere

Sky And Space

Sky: 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.

Space: 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 in space.

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 soon around the earth.

Constellations

A group of stars occupying a small portion of the celestial sphere is called a constellation. Some of these stars appear to form certain figures of animals, humans or objects. These figures have been named after certain events or beliefs of the times when the constellations were identified. In this way, western observers have divided the celestial sphere into 88 constellations. Similarly, ancient western astronomers put forward the idea of 12 zodiac signs, whereas Indian astronomers suggested the 27 nakshatras.

Zodiac Sign: The ecliptic has been imagined to be divided into 12 equal parts. Thus each part subtends 30 degrees at the centre of the celestial sphere. Each of these parts is called a raashi or zodiac sign. They are named Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces.

Nakshatra: The moon completes one revolution around the earth in approximately 27.3 days. The portion traversed by the moon in one day is called a nakshatra. So if we divide 360 degrees into 27 equal parts, each part is about 13 degrees and 20 minutes. A nakshatra is known from the brightest star that it contains. 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.

Teacher's Note

In India, we have 27 nakshatras which are different from the 12 zodiac signs used in Western countries. Each nakshatra is named after a bright star in it.

Exam Trick

Remember: 12 zodiac signs = Western system. 27 nakshatras = Indian system. The moon moves through one nakshatra every day.

Points To Remember

A constellation is a group of stars that form a picture.
The 12 zodiac signs divide the ecliptic into equal parts.
Each zodiac sign takes 30 degrees of the celestial sphere.
Nakshatras are the Indian way of dividing the moon's path.
The moon passes through one nakshatra in about one day.

Always Remember

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.

Find Out

Using a Marathi calendar, collect information about the 27 nakshatras and divide them into the following three categories:

Monsoon nakshatras

Winter nakshatras

Summer nakshatras

Use Your Brain Power

One zodiac sign = ..... nakshatras.

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