Question 1 What is meant by celestial objects? Name few celestial objects?
Question 2 What is a star? Name the star nearest to the Earth?
Question 3 Why does pole star appear to be stationary in the sky?
Question 4 How much time does light takes to reach Earth from the Sun?
Question 5 Define the term light year?
Question 6 Why do stars twinkle at night whereas planets do not?
Question 7 Why is the distance between stars and planets expressed in light year?
The objects which exist in the sky are called celestial objects.
The stars (including the Sun), the planets (including the Earth), satellites (like the Moon), asteroids, comets and meteoroids are all celestial objects.
Celestial objects are also known as heavenly objects (or heavenly bodies).
During the daytime, we can see only the Sun in the sky. Because of the glare of sunlight, we cannot see any other celestial objects in the sky during the day. But as soon as the Sun sets in the evening and it becomes dark, we can see many celestial objects in the night sky. The various celestial objects which we can see easily in the night sky are stars, planets, moon and meteors (or shooting stars)
An important characteristic of stars is that they appear to twinkle in the sky. That is, their light increases and decreases continuously. The twinkling of stars is an illusion caused by the disturbance of star’s light by Earth’s atmosphere. We can also see some star-like objects in the night sky which do not twinkle. These are the planets which revolve around the Sun.
Planets do not twinkle because they are much more nearer to the Earth than the stars. Moon is the most prominent object we can see in the night sky.
(1) Stars are the celestial objects (like the Sun) that are extremely hot and have light of their own.
(2) Stars consist mostly of hydrogen produced by the nuclear fusion reactions taking place inside them all the time. In these fusion reactions hydrogen present inside the stars is converted into helium, with the release of a tremendous amount of heat and light.
(3) Stars are very, very large objects having the shape of a ball.
(4) The stars are much, much bigger than our Earth.
(5) The stars appear to be small because they are very, very far away from us.
(6) The Sun is the star which is nearest to the Earth. The Sun looks much bigger and brighter because it is much nearer to us than any other star.
(7) Many other stars are bigger and brighter than the Sun, but they look small and faint. This is because they are very far away from Earth.
(8) Though the stars are present in the sky even during the daytime, but we cannot see them during the daytime because of the bright light of the Sun.
The Sun (which is a star) rises up in the east in the morning, travels in the sky the whole day, and then sets in the west in the evening. So, the Sun appears to move in the sky from east to west direction. This apparent motion of the stars in the sky from east to west is due to the rotation of Earth from west to east on its axis.
There is one star which does not appear to move in the sky. It is called Pole Star. Thus, the star which appears stationary from the Earth is Pole Star. The Hindi name of Pole Star is Dhruva Tara. The Pole Star remains fixed at the same place in the sky in the North direction because it lies on the axis of rotation of Earth (which is fixed in space and does not change with time). Since Pole Star remains fixed (or stationary) in the sky, all other stars appear to revolve around the Pole Star.
The distances between the various celestial objects (like the stars and planets) are expressed in the unit of light year
One light year is the distance travelled by light in one year.
1 light year= 9,460,800,000,000 Km
This can be written in short as:
1 light year=9.46 x 1012 kilometres
One light minute is the distance travelled by light in one minute.
The distance between the Sun and the Earth is about 150,000,000 kilometres (which is 150 million km). Light from the Sun takes about 8 minutes to reach the Earth.
So, the distance of Sun from the Earth is 8 light minutes. The Sun is about 8 light minutes away from the Earth.
After the sun , the next nearest star to the Earth is Proxima Centauri. Proxima Centauri is at a distance of about 40,000,000,000,000 kilometres (which is 40,000,000 million km) from the Earth. So, the distance of Proxima Centauri star from the Earth is 4.3 light years.
Some of the stars are even millions of light years away from the Earth.