The Earth and the Universe

Concept: Stars and other Celestial Bodies

What do we see in the night sky?

Due to the bright light of the Sun during the day, one can only see the Sun and nothing else. However, when the Sun goes down, the night sky becomes visible, and we can see different celestial bodies.

• One can see stars, constellations, planets, moons, asteroids, comets, and meteoroids in the night sky.
• All these bodies collectively are called celestial bodies.
• A telescope is used to study stars and the other heavenly bodies in the night sky.
• A person who studies different phenomena of these celestial bodies is called an astronomer.
• The study of heavenly bodies is called astronomy.

Stars:

• Stars are celestial bodies that emit their own light. Examples : Sun
• They emit heat and light continuously due to the nuclear reactions occurring there.
• Stars mainly comprise hydrogen gas with a small amount of helium gas.
• Stars are much bigger than the Earth but still appear small due to the considerable distance.
• The Sun is the star nearest to the Earth, and the next is the Proxima Centauri.
• Stars are there in the sky but are not visible during the day because of the glaring sunlight.
• The motion of stars in the night sky is from the east to the west, opposite to the rotation of the Earth.

Pole Star:

• The Pole Star appears to be stationary in the night sky i.e., it does not move from the east to the west direction.
• It happens because it is situated on the axis of the rotation of the Earth and is visible in the night sky towards the north.
• The Pole Star is not visible from the southern hemisphere of the Earth.
• It is called Dhruv Tara in Hindi.
• It was used in the earlier days to identify the directions. At present, a magnetic compass is used to determine directions.

Constellations:

• A group of stars is called a constellation.
• They are given specific names signifying an animal, human, or some other object depending on the shape of the arrangement of the stars.
• There are about 88 constellations that are known at present.
• Some stars in the constellations are so bright that they can be seen with the naked eye.

1. Ursa Major:
• It is also known as the Great Bear, Big Dipper or Plough.
• The Indian name of this constellation is Saptarishi.
• The constellation consists of seven bright stars arranged to resemble a big dipper.
• It is visible in the northern hemisphere during April.

2. Orion:
• It is named after the Greek hunter.
• The Indian name of this constellation is Mahavyadha.
• It comprises seven to eight bright stars arranged in a format that resembles a hunter in a kneeling position.
• It is visible in the night sky during winter in the late evenings.
• The brightest star in the night sky, called Sirius, is located close to the Orion constellation.

3. Leo:
• Leo stands for lion.
• It comprises nine main stars arranged in a pattern that resembles the outline of a giant lion.
• It is visible in the night sky during summer at early night.

Satellites:

• These are celestial bodies that revolve around a planet.
• Moon is the natural satellite of Earth.
• Different planets have different numbers of moons.
• The satellites revolve around the planets due to the gravitational force exerted by the planets.
• Satellites do not have light of their own.

1. Moon:
The Moon is the Earth’s natural satellite.
It is about 1/4th of the Earth’s diameter and revolves around the Earth and on its own axis.
The gravitational pull of the Moon is about 1/6th times that of the Earth.

The Moon’s surface is covered with rocks and loose dirt particles.
Life on the Moon is not possible due to the absence of water and air.
The Moon does not have a light of its own, and it shines when the light gets reflected on it from the Sun.
Neil Armstrong, an American astronaut, was the first man to land on Moon.

2. Phases of Moon:
• Due to the Moon’s motion around the Earth, different parts of the Moon is lit up by the light of the Sun due to which it keeps on changing its shape every day.
• The part of the Moon that is visible to us from the Earth on any particular night is called the Moon's phase.
1. New Moon Day:
• It is the day when the Moon is not visible in the night sky.
• The Moon lies between the Earth and the Sun on this day.
• The side of the Moon facing the Sun does not face the Earth, and hence we cannot see the Moon in the night sky.
2. Waxing Crescent Moon:
• It is the phase in which the Moon starts appearing in the form of a thin silver line in the sky from the next day of the New Moon Day.
• It marks the beginning of the first intermediate phase called the waxing crescent Moon.
3. First Quarter Moon:
• It is the phase where the Moon is 50% visible in the night sky.
4. Waxing Gibbous Moon:
• The word ‘waxing’ stands for getting bigger.
• After the first quarter, the Moon starts growing bigger each day.
• This phase is called the Waxing Gibbous Moon.
5. Full Moon:
• When the Sun and the Moon lie at opposite positions on the Earth, a full Moon is visible in the night sky.
• It occurs after 15 days of the New Moon might.
6. Waning Gibbous Moon:
• Once the Full Moon night is over, the Moon again starts decreasing in size with each consecutive night and reaches the New Moon day.
• This phase is called Waning Gibbous Moon.

Asteroids:

• The small celestial objects that revolve around the Sun between the orbits of Mars and Jupiter are called asteroids.
• They are smaller than planets and are often called minor planets or planetoids.
• Astronomers believe that these pieces of rocks were formed at the same time when planets and the solar system were formed.

Comets:

• These chunks of ice, gas, and dust revolve around the Sun in highly elliptical orbits and are visible only when they come close to Sun.
• They develop long tails when they move closer to the Sun.
• They do not last forever. Each time a comet passes near the Sun, some of its gases burn up, and ultimately only the dust particles are left behind.

Meteoroids: Meteors and Meteorites

• These celestial bodies vary in size. They can be small as a grain or as big as weighing 100 tonnes.
• They are formed by the disintegration of the asteroids and comets.
• Meteoroids keep on revolving around the Sun in different orbits, but in some cases, if they lose their orbit, they start falling towards the Earth.
• Once they enter the atmosphere of the Earth, due to the heat and resistance, they catch fire and burn. So these are called shooting stars or meteors.
• If a meteor does not burn completely while travelling towards the Earth and some of its remains reach its surface, it is termed a meteorite.

Artificial Satellites:

• A man-made object launched into the Earth’s orbit using a rocket is called an artificial satellite.
• The Earth's gravitational pull maintains the motion of the artificial satellite around the Earth.
• Artificial satellites are placed nearer as compared to the Moon.
• Their height can be adjusted according to the function they are designed for.
• Aryabhatta was the first man-made satellite of India.

Uses of artificial satellites:

• Remote sensing and communication through radios, televisions, internet.
• Weather forecasting and warning against natural disasters like floods and cyclones.
• Surveying the natural resources available on the Earth.
• In military operations.
• Studying about other planets, moons, stars and galaxies.

New Words

Did You Know?

• The artificial satellites or their parts that are no longer in use turn into space junk and continue revolving in their orbits.
• An asteroid equal to the size of a car enters the Earth’s atmosphere annually but cannot reach the surface as it burns and gets converted into ashes while crossing the layers of the atmosphere.
• There are a billion trillion stars in the universe, more than the grains of sand we have on beaches worldwide.
• A galaxy called Andromeda can be seen with naked eyes in the night sky.