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 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.
* 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.
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.
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