When you look at a clear night sky you can see stars, planets and the Moon. But did you know that all of the stars you can see in the sky belong to a galaxy?
A galaxy is a group of stars that are held close together. Galaxies come in different sizes. Some have millions of stars, others have billions.
Galaxies also appear in various shapes. Some look round like or ball; others appear oval like an egg. These are known as elliptical galaxies.
Some galaxies appear to be shaped like a fried egg and are called spiral galaxies. A few galaxies have no shape, so astronomers call them irregular.
Home sweet home
The Milky Way is a spiral galaxy and has nearly 100 billion stars. It is also one of the largest galaxies in the Universe, measuring 1 000 000 000 000 000 000 kilometres from one side to the other.
Although we can only see the closest stars in our own galaxy as separate stars, those further away look like a milky river that stretches across the night sky. This is how the Milky Way got its name.
Our Sun is almost halfway from the middle of the Milky Way. It takes nearly 250 million years to complete one orbit around the galaxy’s centre.
Our largest neighbour
One of our closest and largest ‘galactic’ neighbours is the Andromeda galaxy. It is very similar is shape to our own galaxy, but has nearly 10 times as many stars.
In the year 964, Persian astronomer Abd Al-Rahman Al Sufi about Andromeda, saying that he saw a strange small cloud in the night sky. It wasn’t until 1925, when American astronomer Edwin Hubble looked closely at Andromeda that we realised it was a galaxy and not part of the Milky Way.
Astronomers have recently discovered that Andromeda is heading towards the Milky Way and the two galaxies may collide in three billion years time.
Orbiting the Milky Way are a number of smaller galaxies. Two irregular shaped galaxies that are easy to see, are the Large Magellanic Cloud and the Small Magellanic Cloud. These ‘mini-galaxies’ are named after the Portuguese explorer Ferdinand Magellan.
In 1998, astronomers using CSIRO’s Parkes radio telescope discovered that our galaxy is ‘sucking’ hydrogen gas from the two Magellanic clouds and slowly pulling them into us.
The monster in the middle
For many years astronomers have believes black holes exist, but have never seen them. Although are ‘invisible’ it is possible to see their effects. Black holes have strong bursts of x-rays coming from their centre.
Recently astronomers have found x-ray bursts coming from black holes in the middle of most galaxies – including our own. Thankfully we’re a long way from them.
Where to see our neighbouring galaxies?
The Magellanic Clouds are easy to find during the spring months – especially if you are a long way from the city. Look in the southern sky to the left of the Southern Cross.
The Andromeda galaxy is best seen during August and September. Look just above the northern horizon to the left of Pleiades and Taurus. You will need binoculars or a telescope to see its spiral shape.