Astronomy is the science that deals with the universe beyond the Earth's atmosphere. The word universe means 'everything that exists'. Naturally made object outside the Earth's atmosphere are called celestial bodies.

Celestial bodies include stars, planets, moons, asteroids and comets.

There are five types of celestial bodies:

1. Stars

2. Planets

3. Moons

4. Asteroids

5. Comets

A star is a celestial body that generates its own light and heat and consists of a mass of gas held together by its own gravity.

Most stars release light and heat due to nuclear reactions happening inside the star. The one star that you should be familiar with is the sun.

The sun is the brightest object in the Earth's sky. It is the most important source of energy for life on Earth. It is estimated that about 600 million tonnes of hydrogen are converted into helium every second by nuclear fusion happening in its center. Most of the objects seen in the night sky are distant stars.

Other that the sun, Proxima Centauri is the nearest star to Earth. It takes the light 4.2 years to travel from it to Earth!

A planet is a celestial body that is in orbit around a star.

You may sometimes see a planet in the night sky because they reflect light from the sun. There are three criteria for a celestial body to be called a planet. It must:

• Be in orbit around a star

• Have enough mass to form an almost round shape due to its own gravity.

• have 'cleared the region' around its orbit of other celestial bodies.

You may wonder what the term 'cleared the region' means. Planets are in orbit around a star. Over many millions of years, as the planet travelled through this orbit it pulled other large objects in the region towards itself. This force of gravity caused collisions. Over time, these collisions removed or cleared the orbit of the planet of all such objects except for another type of celestial body called a moon.

Pluto is no longer classified as a planet because it shares the region of its orbit with thousands of large objects, such as the dwarf planet Eris, which is a similar size to Pluto.

A moon is a natural satellite of a planet.

We are all familiar with the moon that orbits the Earth. Other planets also have their own moons. Earth has only one moon, whereas Jupiter and Saturn have more than 50 moons each. Uranus has more than 20 moons and Neptune has more than 10 moons. These numbers keep changing as further studies are carried out in astronomy. Our moon is a satellite of the Earth.

An asteroid is a small rocky body in space.

Asteroids are small celestial bodies that usually orbit a star. They are made of rock left over after the formation of the planets 4.5 billion years ago. They have diameters that are between a few meters to a few hundred kilometers.

Sometimes asteroids can be thrown off their orbit due to a collision with another asteroid. There are called 'rouge' asteroids and can mover through space. If they come close to another celestial body, such as a planet, the planet's gravity can pull them in towards the planet, causing a high-energy collision. A collision between a rogue asteroid and the Earth is thought o be responsible for the extinction of the dinosaurs.

A comet is a dusty snowball that orbits a star.

Comets are celestial objects made of ice and dust. They orbit the sun. However, unlike planets, their orbit is not a circular path with the sun in the middle. They spend most of their time in the outer reaches of the solar system. Sometimes they come closer to the sun. When near the sun, tails of gas and dust particles are emitted, each pointing away from the sun, making them visible in the night sky.

A solar system is a group of planets, asteroids, comets and other celestial bodies that are in orbit around a star.

Our solar system is an example of a planetary system. The size of the solar system is limited by the distance that the gravity of the star can attract other celestial bodies. Our solar system is only one of many millions of planetary systems in the universe.

Our solar system is composed of the sun, planets, moons, comets, asteroids and other celestial bodies under the influence of the sun's gravity. Gravity is often referred to as the 'glue of the universe'.

• The sun is a huge celestial body- a star at the centre of a system of eight planets. Our sun contains 99.9 per cent of the mass of the solar system. As already mentioned, it generates its own heat and light due to nuclear reactions. Due to its huge mass, the gravity of the sun attracts all other objects in the solar system.

• The eight planets in order of distance from the sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

  • My Mercury

Very Venus

Educated Earth

Mother Mars

Just Jupiter

Served Saturn

Us Uranus

Noodles Neptune

• The planets revolve around the sun in a nearly circular pathway called an orbit. The force of gravity between each planet and the sun helps the planets to remain in orbit. Due to the fact that the planets are orbiting around the sun, they are not pulled into the sun.

• Some planets have moons that orbit individual planets. As already discussed, a moon is any natural body that orbits a planet. Most moons in our solar system are named after mythological characters from a wide variety of cultures. Io, Europa, Ganymede and Callisto are all moons of Jupiter.

• Astronomers have discovered a large number of rocky objects between Mars and Jupiter. These are asteroids. Most of the asteroids in the solar system are clustered between Mars and Jupiter. This region is known as the steroid belt. However, some wander out of this region and are in danger of striking other planets.

• In 1908 an asteroid hit Tunguska, Siberia and knocked down 2,000km² of forest. Since the area was not populated, nobody died. Had it hit the Earth a few hours earlier, it would have caused devastation in London. It was predicted that the Earth would be hit by another asteroid, Apophis, in 2029, bit it was later discovered that it would miss the Earth.

• At the edges of the solar system, there still exists a region of gas clouds called the Kuiper belt. It contains billions of lumps of rocky ice up to 8 km in diameter in orbit around the sun. They are called comets.

• Occasionally the orbits of comets around the sun may be disturbed. They may begin to move towards the sun and may orbit the sun in a different orbit to the Kuiper belt. The sun melts some of the ice and a tail of gas, ice and dust is formed. The sunlight reflects off the tail when we see a comet. The most famous is Halley's Comet, which has settled in orbit around the sun. It revolves around the sun every 76 years.

A galaxy is a large system of stars held together by their gravity.

Our solar system is part of the Milky Way Galaxy. Galaxies are isolated from similar systems by vast regions of empty space.

• Our sun is just one of 100 billion stars in the Milky Way Galaxy. Many of these stars have their own planetary systems. Our nearest neighboring large galaxy is called Andromeda.

• Outer space, or simply space, is the empty volume that exists between celestial bodies. It is not completely empty but consists of a vacuum containing mostly hydrogen and helium, as well as dust and radiation.

• If ant two masses are placed near each other, they will be attracted to each other by a force of gravity.

• If the masses are small, the force of attraction is very small- too small to take into account.

• If one of the masses is very large, for example a moon, a planet or a star, the force of gravity is much larger

• The force of gravity is also affected by distance. The smaller the distance between the masses, the greater the force of attraction

• The force of gravity would be strongest if a body had a huge mass and was compressed into a very small volume wiht a small radius. An example of such an object is a neuron star. A neutron star is what is left over after a large bright star runs out of fuel and collapses. They have a mass comparable to that of our sun but compacted into a sphere with a diameter of only 10km. The density of material in a neutron star would be millions of tonnes per centimetre cubed!