Some of the most important Physics Topics include energy, motion, and force.
What are Celestial Objects in the Sky
The objects which exist in the sky (or in outer space) 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). In this Chapter we will learn about the various celestial objects which exist in the vast surrounding space called universe.
The Night Sky
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).
On a clear, moonless night, we can see about 3000 stars with naked eye. Many more stars can be seen with the help of a telescope. 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 (false show) 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 which we can see in the night sky. If we keep on looking at the night sky for a considerable time, we may see a bright streak of light flashing across the sky for a few seconds (and then disappearing). This is a meteor which is commonly known as a shooting star. Let us first discuss the stars.
Stars are the celestial objects (like the Sun) that are extremely hot and have light of their own. Stars emit heat and light continuously. Stars consist mostly of hydrogen gas. The heat and light of stars is 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. Stars are very, very large objects having the shape of a ball. The stars are much, much bigger than our Earth.
Many of the stars are even bigger than the Sun. The stars appear to be small because they are very, very far away from us. In our everyday language, we do not call the Sun a star. But in reality, the Sun is also a star. 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. Though 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 (as compared to the Sun). We can see the stars only at night.
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. There are about 10,000 billion stars in the Universe.
We know that 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. Same is the case with all other stars. Thus, the stars appear 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.
When the Earth moves (or rotates) on its axis from west to east, then the stars in the sky appear to move in the opposite direction from east to west. 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 always remains in the same position in the sky. Though all other stars appear to move from east to west, the Pole Star remains fixed at the same place in the sky in the North direction. The Pole Star appears to be stationary and does not change its position with time because it lies on the axis of rotation of Earth (which is fixed in space and does not change with time) (see Figure). Since Pole Star remains fixed (or stationary) in the sky, all other stars appear to revolve around the Pole Star. Please note that the Pole Star is not visible from the southern hemisphere of the Earth.
The fact that the Pole Star does not appear to move and remains stationary in the sky can be shown by performing an activity as follows: Take an umbrella and open it. Make about 10 small stars out of paper. Paste one star at the top position of the central rod of the umbrella and other stars at different places on the umbrella cloth, near the end of each metal ‘spoke’ (as shown in Figure).
Now rotate the umbrella by holding its central rod in your hand. Observe the paper stars on the rotating umbrella. We will find that all the stars on the spokes appear to move but there is one star which does not appear to move. The star which does not appear to move is located at the top end of central rod (which is being rotated). In this activity, the central rod of umbrella is like the axis of rotation of Earth and the star fixed at its top end is like the Pole Star.
Before we go further, we should know the meaning of a special unit of distance called Tight year’ which is used for measuring the extremely large distances between the various celestial objects (like stars and planets) in this universe. This is described below.
The Unit of Measuring Distances in the Universe: Light Year
On the Earth, we normally express the large distances in the unit of ‘kilometre’. The distances in the Universe are so large that ‘kilometre’ becomes an extremely small and inconvenient unit to express such large distances. For example, the various stars are millions of kilometres away, so ‘kilometre’ unit is not suitable to express the distances between the stars and other celestial objects. The distances between the various celestial objects (like the stars and planets) are expressed in the unit of Tight year’.
One Tight year’ is the ‘distance travelled by light in one year’. We can calculate the value of ‘one light year’ in ‘kilometres’ by multiplying the speed of light (which is 3,00,000 km/s) by the number of seconds in one year (which is 3,15,36,000 s). We will find that:
1 light year = 9,460,800,000,000 kilometres
This can be written in short as :
1 light year = 9.46 × 1012 kilometres
This shows that Tight year’ is an extremely big unit of distance. Please note that year is a unit of time, but light year is not a unit of time. Light year is a unit of distance. In addition to light year, light minute is also used as a unit of distance in some cases. 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. In other words, we can say that 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.
Light from the next nearest star, Proxima Centauri, takes about 4.3 years to reach the Earth. So, the distance of Proxima Centauri star from the Earth is 4.3 light years. In other words, we can say that Proxima Centauri star is about 4.3 light years away from the Earth. Thus, the light coming from this star which we will see tonight would have left the surface of this star about 4.3 years ago ! The star nearest to the Earth (after the Sun) is Proxima Centauri and not Alpha Centauri. Some of the stars in the Universe are so far off that light coming from them takes millions of years to reach the Earth. Thus, some of the stars are even millions of light years away from the Earth.
From the above discussion we conclude that the distances between stars and planets are expressed in the unit of ‘light year’ because these distances are so large that it is inconvenient to express them in the unit of ‘kilometres’. It is, however, very convenient to express extremely large distances in the unit of light year. Now, suppose a star is 6 light years away from the Earth. By saying that a star is 6 light years away from the earth, we mean that the distance of this star from the Earth is so much that light from the star takes 6 years to reach the Earth. We will now describe the star groups called constellations.