CBSE Class 6 Science The Universe, Stars, the Sun and Planets Notes Set 01

The Universe

The Universe is everything that exists in space: all the stars, planets, moons, comets, dust and gases, together with the empty space between them. It is so vast that nobody has seen its edge. Most scientists think it began with a huge explosion called the Big Bang about 13.8 billion years ago, and it has been expanding ever since.

The objects in the sky are called celestial or heavenly bodies. Stars, planets, satellites (moons), asteroids, comets and meteors are all celestial bodies.

Stars

A star is a huge ball of very hot gases that produces its own heat and light. Stars appear as tiny twinkling points because they are extremely far from us, and their light is bent a little by the moving air of the atmosphere before it reaches our eyes. Our nearest star is the Sun. Other stars look small only because of distance; many are much larger than the Sun.

The Sun and the Solar System

The Sun, the eight planets with their satellites, and smaller bodies such as asteroids, comets and meteors together form the solar system. The Sun lies at the centre, and everything else moves around it under its pull of gravity.

The Sun is an average-sized star, but it is the largest member of our solar system and holds almost all of its mass. It is about 150 million km from the Earth. Its heat and light make life on Earth possible: they warm the planet, drive the weather and let green plants make food.

The Planets

Planets are celestial bodies that do not have their own heat and light; they shine because they reflect sunlight. They are roughly round (spherical). Each one travels around the Sun along a fixed path called an orbit, and the journey is called revolution. Most planets revolve from west to east. While revolving, every planet also spins on its own axis, which is called rotation.

The eight planets in order from the Sun are:

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune

The first four are the inner, rocky planets; the last four are the outer, giant planets. Pluto was once counted as a planet but is now classed as a dwarf planet.

Other Members of the Solar System

1. Satellites are bodies that revolve around planets; the Moon is the Earth's only natural satellite.
2. Asteroids are small rocky bodies, found mostly in a belt between Mars and Jupiter.
3. Comets are icy bodies that grow a glowing tail when they come near the Sun.
4. Meteors are streaks of light made when small pieces of rock burn up in the Earth's atmosphere; the pieces that reach the ground are called meteorites.

Key Points to Remember

1. Stars make their own light; planets and moons shine by reflected light.
2. Revolution is motion around the Sun; rotation is spinning on an axis.
3. The Sun's light takes about 8 minutes to reach the Earth.
4. The Sun is a star and not a planet.

Worked Examples

Example 1. Why is the Sun the most important body for us?
It is the centre of the solar system and the main source of heat and light. Without it there would be no day and night warmth, no plant growth and no life on Earth.

Example 2. The Sun is about 150 million km from the Earth and light travels at about 300 000 km per second. How long does sunlight take to reach us?
Time = distance ÷ speed = 150 000 000 ÷ 300 000 = 500 seconds. That is about 8 minutes and 20 seconds.

Example 3. Sort these into stars, planets and others: Jupiter, Sirius, a comet, Neptune.
Sirius is a star. Jupiter and Neptune are planets. The comet is a small icy body of the solar system, neither a star nor a planet.

Revolution and Rotation

The Earth completes one rotation in about 24 hours, which gives us day and night. It completes one revolution around the Sun in about 365 and a quarter days, which is one year. The Moon revolves around the Earth in a little over 27 days.

Quick Check Questions