What is Astronomy ?

Astronomy is the scientific study of celestial objects.

Celestial objects are objects such as stars, planets, moons, asteroids, and galaxies.


A star is a sphere of gas that is hot and dense enough for nuclear fusion to happen in its core.


A planet is a large spherical object that orbits (travels around) a star.


A moon is a large object that orbits (travels around) a planet .

Asteroids & Comets

Asteroids & comets are chunks of rock and metals (Asteroids) or dust and water-ice (Comets) left-over from the creation of the solar system.


Galaxies are huge collections of billions of stars, dust and gas that are held together by gravity.

Distances In Space

Light travels at 300,000,000 ms-1 or 3x108 ms-1.

In most cases, this is so fast that can we assume that Light moves instantaneously.

Scientific Notation

The x10number is ‘how many places to the left (positive) or right (negative) have you moved the decimal point ?’.

For Example -

4500 m = 4.5x103 m (d.p. 3 to left)

660,000 m = 6.6x105 m (d.p. 5 to left)

0.002 m = 2x10-3 m (d.p. 3 to right)

0.000007 m = 7x10-6m (d.p. 6 to right)

Scientific Notation on a calculator

All scientific calculators will have either a button labelled ‘x10x’ or ‘exp’.

To enter the number 4x106, simply push ‘4’ then...

Then the ‘x10x’ button then...

Then the ‘6’, then...

Then the ‘=’, and the calculator will display the number 4,000,000 as you wanted!

Distances in Space

Space is so huge, however, that even Light takes a large amount of time to travel these distances.

This means that when we look into Space, we are actually looking back through time!

In our Solar System, measurements are usually made in Astronomical Units (A.U.).

1 A.U. = Distance from the Earth to the Sun (1.5x1011 m)

In our galaxy, measurements are usually made in Light-Years.

1 ly = Distance travelled by Light in one year ( 9.5x1015 m )

To distant galaxies, measurements are usually made in Megaparsecs.

1 Mpc = 3.26x106 Light-Years ( 3.1x1022 m )

Below shows some example conversions:-