The universe consists of everything that exists. The sun, the moon, the planets, the stars, and all matter, energy, and space that exist constitute the universe. The universe contains billions the galaxies, each containing millions or billions of stars. Billions of galaxy clusters around the universe are strung together like a giant spiderweb.
The universe is incredibly huge. No one knows the exact size of the universe. The universe has not always been the same size. Scientists believe that it began in a Big Bang (explosion) which took place nearly 15 billion years ago. Since then, the universe has been expanding outwardly at a very high speed. The galaxies are also moving further apart as the space between them expands. This is called the Big Bang theory.
Galaxies are the building blocks of the universe. A galaxy is a group of millions of stars held together by gravity. There are probably more than a hundred billion galaxies in the universe.
On the basis of their shapes, galaxies are classiﬁed as irregular, spiral, and elliptical.
Earth’s own star, the sun along with other planets and many other stars lie in the Milky Way galaxy. Milky Way is you look up at the nig band of light in the sky formed by millions of faint stars that together form the Milky Way galaxy (Akashganga). It consists of 100 billion stars including the sun.
A star is a huge ball of glowing gas held together by gravity. Stars are luminous bodies, i.e., they emit their own light. Stars are made up mostly of hydrogen and helium. Stars are very hot due to the burning of hydrogen into helium in a process called nuclear fusion. During this process, a huge amount of energy is released in the form of heat and light.
There are many different types of stars. The smallest stars are red and don‘t give off much glow. Medium size stars are yellow, like the sun. The largest stars are blue and are hugely bright.
The sun is the closest star to the earth, so it appears so big. Other stars are very far away, so they appear as points of light. The sun is 150 million km away from the earth.
Stars in the sky appear to move from east to west because the earth spins from west to east.
The Pole Star is a visible star whose position is aligned with the Earth’s axis of rotation, i.e., it lies close to the earth’s axis of rotation. It
appears to be stationary in the sky. In practice, the term Pole Star refers to Polaris or the North Star. It is called Dhruv Tara in Hindi, All the other stars in the sky appear to revolve around it from east to west.
The celestial bodies that revolve around the sun are called planets, They are “OH-luminous bodies, i.e., they do not have their own light but reflect the light of the sun. They are smaller in size. than stars. Our earth is a planet.
DISTANCES IN THE UNIVERSE
The heavenly bodies like stars, planets, satellites, etc. may appear very close to the earth but actually, they are very far away. These
distances cannot be measured in meters or kilometers as it would be inconvenient to write such large numbers.
Thus, the astronomers use special units like light year and parsec to measure distances between the heavenly bodies. One light-year is the distance traveled by light in one year. Speed of lights 3,00,000 km/s
Therefore, the distance of the sun from the earth is about 8 light minutes and the distance of Proxima Centauri is 4.3 light-years from the earth.
Groups of stars forming recognizable patterns. like shapes of animals etc, in the sky, are called constellations.
The constellation moves in the sky from east to west, but the shape of the constellation remains the same because the stars retain their positive with respect to each other. Astronomers have
recognized and named 88 constellations to date.
All the stars that make up a constellation are not at the same distance from the earth. They are just the same line of sight as the sky.
Some of the major constellations are:
Ursa Major or the Great Bear
Its seven stars make the shape of a spoon-like object known as a dipper, which was used in old days to drink water. Three stars form the handle and four form the bowl of the dipper. So, itis also called the Big Dipper. Along with several other faint stars, it forms a pattern resembling a bear, thus also called the Great Bear.
In Hindi, it is called the Saptarishi. You can locate the position of the Pole Star with the help of, Ursa Major.
If you observe Ursa Major 3-4 times at an interval of 2-3 hours and also note the position of the Pole Star, you will see that it revolves around the Pole Star. In fact, all the stars appear to revolve around the Pole Star.
The Pole Star is not visible from the southern hemisphere. Some other northern constellations may not be visible from the southern hemisphere.
Ursa Minor or Little Bear
The seven stars in this constellation are also arranged similar to those of Ursa Major, i.e… in the form of a dipper, but the stars are less bright and closer to each other. It is also called Little Dipper.
The last star in the handle of the Little Dipper is the Pole Star itself.
It is often called the Hunter and is one of the most common and easily identifiable constellations in the sky. It has seven bright stars and many faint stars. The three middle stars represent the belt of the Hunter and the four bright stars form a quadrilateral marking the shoulder and legs of the Hunter.
In Hindi, it is called the Mriga. It is clearly visible during winter in the late evenings, in the northern hemisphere.
The brightest star in the sky. Sirius can be located with the help of Orion. If you draw an imaginary straight line through the three middle stars of Orion, it leads to Sirius.
It also contains seven bright stars and forms the shape of a scorpion along with other faint stars. It can be seen during the summer season.
It contains ﬁve bright stars forming a distorted W or M. Along with other faint stars, an image of the Egyptian queen Cassiopeia is formed.
This constellation is seen clearly during winter in the early part of the night in the northern hemisphere.
THE SOLAR SYSTEM
The sun at the center, along with the heavenly bodies that revolve around it constitute the solar system.
These heavenly bodies include planets, comets, natural satellites, meteors, and asteroids. Let us study more about our solar system.
The sun is the closest star to the earth. Its distance from the earth is about 150 million km. It takes about 8 minutes 20 seconds for the light to reach the earth. The gravitational attraction of the sun causes the planets and other bodies to revolve around it. Being a star, the sun is also a huge ball of super-hot gases— mostly hydrogen and helium. The sun is about 4.5 billion years old. The diameter of the sun is 1.39 million km and its surface temperature is about 6000°C.
The sun is the ultimate source of all the energy on the earth and all other planets.
Until 2006, our solar system was believed to have nine planets. But in light of the latest discoveries, our solar system now consists of only eight planets in the order— Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
All the planets revolve around the sun in elliptical (oval) paths called orbits. They take different times to complete one revolution. The time taken by a planet to complete one revolution is called its period of revolution. The period of revolution increases as the distance of the planet increases from the sun.
The planets, besides revolving around the sun, also rotate about their axes. The time taken by a planet to complete one rotation is called its period of rotation.
The table given below gives the information about different planets:
The four rocky planets at the center of the solar stem— Mercury, Venus, Earth, and Mars are crown as the inner planets, while Jupiter, Saturn, Uranus, and Neptune composed mainly of gases, are known as the outer planets. They have ings around them.
Inner planets have very few or no satellites or moon. The outer planets have many satellites or moons. Now let us study each planet in detail.
Mercury is the smallest planet and is closest to the sun. Its surface is covered with many craters (bowl-shaped holes).
It is very hot during the day and very cold at night. It is very difficult to observe because it is hidden in the glare of the sun most of the time. However, it can be seen as a bright spot of light in the sky before sunrise and just after sunset, near the horizon.
It has no natural satellite of its own.
Venus is about the size of the earth. It appears as the brightest planet in the sky. It is the closest planet to the earth and the hottest planet of the solar system.
It can be seen before sunrise and after sunset. Hence, it is known as the Morning Star or the Evening Star (although, it is not a star). Venus
shows phases, just like the moon when it is observed from the earth.
Our earth is the third planet of the solar system and the only planet on which life exists. It has ideal conditions required for the existence and survival of life. It is the only planet that has
water in liquid form and oxygen. The right distance of the earth from the sun and the presence of the atmosphere and ozone layer, help to maintain the right temperature for life to survive.
From space, the earth appears blue-green as light is reflected from water and landmass on its surface. The earth takes 365,1/4 days to complete a revolution around the sun. this period is termed as one year. It takes 24 hours (or 1 day) to complete one rotation on its axis. The axis of rotation of the earth is not perpendicular to the plane of the orbit but tilted at an angle of 23,1/2°. This tilt is responsible for the change of seasons on the earth.
The earth has one natural satellite or moon.
Mars is about half the size of the earth and is seen as a reddish planet in the sky. It is commonly called the Red Planet. It is also referred to as the cold planet and has a very thin atmosphere. Its surface is covered with mountains and craters. It has two natural satellites.
Jupiter is the largest planet in our solar system. It’s a cold planet. With a good telescope, we can see the famous great red spot, which is a giant hurricane raging on Jupiter. Its mass is 318 times that of the earth. It has faint rings around it and rotates very rapidly on its axis. It has the maximum number of natural satellites, 63, and four of them can be seen from the earth using a telescope.
Saturn is the second-largest planet in our solar system. It is very cold. Its distinctive feature is the rings made of ice and dust that surround the planet. This is the reason why Saturn is also known as the ringed planet. It is yellowish in appearance. It has 33 natural satellites.
Uranus and Neptune
Being very far away, these planets can only be seen through a powerful telescope. These planets are very cold. Uranus is greenish in appearance. Link venus also rotates from east to west. Also, its rotational axis is highly tilted. So it appears to roll along its orbit. It has 27 natural satellites. Neptune looks like a bluish circle through a powerful telescope. It has 13 natural satellites.