CBSE Class 8 Science Stars and the Solar System

CBSE Class 8 Science The Universe Chapter Notes. Learning the important concepts is very important for every student to get better marks in examinations. The concepts should be clear which will help in faster learning. The attached concepts made as per NCERT and CBSE pattern will help the student to understand the chapter and score better marks in the examinations.

THE UNIVERSE

INTRODUCTION
The universe is a huge space which contains everything that exists. The vast expanse of space that contains everything in it is known as the universe. It includes the sun, the planets, the Milky Way galaxy and all the other galaxies. The universe is continually expanding. Nobody knows how big the universe is. It is impossible to even have shown that the universe is getting bigger and bigger, like a balloon being blown up. The branch of science that deals with the study of heavenly bodies is called astronomy. People whostudy the universe are called astronomers. The most important instrument used by the astronomers is the telescope.

FACT FILE
* The universe is so vast in relation to the matter it contains that it can be compared with a building 20 miles long, 20 miles wide and 20 miles high that contains 1 grain of sand.
* The three most common elements in the universe are hydrogen, helium and oxygen.

NIGHT SKY
During the daytime you can see only the sun, which appears to move from East to West. After sunset, the night sky is dotted with bright stars. At night the moon and thousands of stars are seen twinkling in the sky. If you watch the sky through a telescope you may see millions of stars, some of which are brightly coloured. You see certain groups of stars which appear to be forming figures in clusters. They are called constellations. The moon is a natural satellite of earth whose size changes every day. Moon completes one revolution around the earth in 27 days, 7 hours and 43 minutes. Furthermore, you see an occasional comet, which appears as a ball of fire, having a tail. Then, there are shooting stars or meteors. They appear to fall from the sky. In addition to all these there are some bright heavenly bodies which do not twinkle. They are called planets. The natural bodies in the sky are called celestial bodies or heavenly bodies. Example– The sun, the earth, the moon, planets, stars, meteors, comets are called heavenly bodies or celestial bodies.

THEORIES OF THE ORIGIN OF UNIVERSE :-
The three main theories are :-

(1) Bigbang theory :- According to this theory the universe was born after a massive explosion called the bigbang. A long time after the big bang, stars like our sun were formed. At that time clouds of hot gases and particles revolved around the sun. Over time, many particles got stuck together to form large bodies. The bodies pulled in smaller objects near them by gravitational force. "This made them larger still. These bodies finally become the planets and this universe has been expanding ever since".

(2) Steady state Theory :- This theory states that the universe appears the same at all times and at all points. It is unchanging without a beginning or an end.

(3) Pulsating Theory :- This theory states that all matter is flying apart from a heavily compacted mass and will eventually slow down, begin to contract and become so condensed that it will explode again and the expansion will start again.

MEASUREMENT OF DISTANCES IN THE UNIVERSE
The distances between heavenly bodies in space are very large. It is very difficult to measure these distances in kilometre. That is why, they are expressed in terms of light year and parsec.

(a) LIGHT YEAR:A light year is the distance travelled by the light at a speed of 300,000 km/s. If a spaceship could travel at the speed of light (3 x 105 km/s), it would reach the moon in just over 1 second.
1 light year = Speed of light × 365 days = 3,00,000 km/s × 365 × 24 × 60 × 60 sec.
                                                           = 9,460,000, 000, 000 km
1 light year = 9.46 × 10¹² km (approx.) The sun, the closest star to earth, is about 8 light minutes away from it. It means that light from sun takes about 8 minutes to reach the Earth. The star closest to the Earth after the Sun is Alpha Centauri It is 4.3 light years away from Earth, i.e., it is at a distance of 4.3 × 9.46 × 1012 km from Earth. It also implies that light from this star, visible on Earth, at a particular night would have left its surface approximately 4.3 years ago.

(b) PARSEC: One Parsec (pc) is equal to 3.26 light years. The distance of Proxima Centauri which is a companion of, Alpha Centauri' is 1.3 parsec

STARS AND PLANETS
A star is a heavenly body which has light of its own. We are able to count 3000 stars in the sky with the naked eye. With the help of a small telescope, we can see 100,000 stars, Nearly 20 thousand million stars can be seen through a powerful telescope. The stars are very large in size and are several times bigger than the sun but some are smaller. They appear small because they are very far away.

Stars consist of large amount of hydrogen gas at very high temperature. They are so hot that they give out light. It is because of this light that stars glow at night. They move in the sky but their positions remain fixed in the sky with respect to other stars. They seem to twinkle due to air disturbances in the atmosphere of earth. The Sun is a star which gives light all the time.

"Planets are heavenly bodies which revolve around the Sun." They do not have their own light but reflect the light of the Sun that falls on them. Planets do not twinkle. They move with respect to the Earth as well as with respect to the sun. The Earth is also a planet. Till recently there were nine planets in the solar system but now the last, i.e., the ninth planet- Pluto has been removed from being called a planet.

TEST YOUR SELF

1. What is the universe?

2. What is a telescope?

3. Define astronomy.

4. What do you understand by the heavenly bodies?

5. What do astronomers study?

6. What is a light year?

7. Name the star closest to earth after sun.

8. What are stars?

9. Differentiate between stars and planets.

10. 1 parsec = .............. Iight years.

THE SOLAR SYSTEM

"The sun and all the bodies moving around it are together called the Solar System".

The solar system consists of the sun, the eight planets, moon, comets and asteroids which revolve around the sun in almost circular paths or orbits.

THE SUN

The closest star to the Earth is Sun. The average distance of the sun from the Earth is 150 million kilometre which is about 110 times the diameter of the Earth. The temperature at its centre is about 14 million degree Celsius; while its surface temperature is about 6000°C the temperature of Sun is very high because of very hot gases present in it. Hydrogen and helium are the gases which produce heat and light in the sun.

The sun is the brightest object in the sky. It is about 333000 times heavier than the earth and you could fit more than a million earth inside it. Its great mass cause a large gravitational force. This keep the sun and planets. The moons and some other smaller bodies together are as the sun family.

PLANETS

The solar system is the set of eight planets including Earth. All the planets move around the sun in elliptical orbits. At the same time they spin or rotate on their own axis within the solar system.

Each planet takes different time to complete one revolution around the sun. Earth takes about 365¼ days. Mercury which is closest to the Sun takes only 88 days to complete one revolution.

Beside revolving around the sun in 365¼ days, the Earth also rotates about its axis once every 24 hours. Other planets also rotate on their axes. The period of rotation is least for Jupiter, only 9 hours 55 minutes and the maximum for Venus which is 243 earth days. The planets are bright objects visible at night looking like stars. Some of them on being seen carefully, seem to be brighter and bigger than the stars. Some of the planets are known from ancient times while some were discovered only after telescope became available to observe the night sky.

After the sun planets are the largest bodies in our solar system. Scientists define a planet as a round body that orbits the sun and which has pulled in all objects near its orbit. Planets were formed when large bodies in space pulled in smaller bodies near it.

There are eight planets in the solar system. In order of their distance from Sun, they are described below:

(1) Mercury (2) Venus (3) Earth (4) Mars (5) Jupiter (6) Saturn (7) Uranus and (8) Neptune

1. MERCURY (BUDH) : It is the first and the nearest planet to the Sun. Being very close to the Sun, Mercury is very hot during the day (about 400°C) and extremely cold at night (about -200°C). It has no atmosphere because of its gravitational force. Mercury is covered with many bowl shaped holes called craters. Mercury is occasionally visible just before the sunrise or immediately after the sunset. It appears likes a very bright star and is also known as the morning or evening star. Although it is not a star it is called morning or evening star because of its brightness. Mercury also resembles moon in many ways. Both are nearly of the same size and mass. Both have no atmosphere and their surfaces are rocky too.

2. VENUS (SHUKRA) :

It is the second planet from the sun and its closest planet is the Earth. Venus is of about the same size as the Earth. It is a very hot planet (about 480°C), even hotter than Mercury. Though it is farther away from the sun, it appears to be brightest of all planets, moon and stars, visible in the night sky. It is due to its cloudy atmosphere that is reflects almost 3/4 of the sunlight that falls on it. It occasionally appears as an evening star just above the western horizon. It sometimes also appears as morning star in the eastern sky. As it is closer to the sun it is not visible throughout the year.

FACT FILE

 Venus rotates so slow that a typical day lasts approximately 244 Earth days (15,856 hours).

3. EARTH (PRITHVI) :

It is the third planet from the sun. The Earth is the only planet to have life on it. All the factors supporting life are know to be present on Earth only. The average diameter of Earth is about 12,800 kilometre.

The Earth as you know, revolves around the sun in 365.25 days known as one Year. It also rotates about an imaginary axis that passes through its North and south poles. This rotation causes day and night on the earth. The change in seasons on the earth takes place due to the tilting of its axis of rotation and the change in its position with respect to the sun. It is tilted at an angle of 23½ ° . As the tilting is always in the same direction therefore the tilting of the northern and southern hemisphere of the earth towards the sun keeps changing throughout the year. Figure given above shows the change of seasons on four dates 21 March, 21 June, 23 September and 21/22 December.

4. MARS (MANGAL) :
It is the fourth planet in the order of increasing distance from the  sun. It is seen as a reddish coloured object in the sky, hence it is sometimes also called the red planet. It is believed that Mars has a large amount of water in the form of ice, near its polar caps. The diameter of Mars is slightly more than half of that of the earth but its mass is only one tenth of that of the earth. It is believed to have lots of carbon dioxide on it. Astronomers are investigating the fact that it may have water and also it may have life in some form. It has a thin atmosphere which enables to view its surface from the earth.

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