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Written on: Friday November 2nd, 2007

A journal entry from: universe

A galaxy is a giant group of stars. Our Sun is one of billions of stars in a galaxy called the Milky Way, and the Milky Way is only one of billions of galaxies in the Universe. Galaxies are not usually found on their own; instead they are found in groups called ?clusters?, and those clusters also form bigger groups called ?superclusters?.

Galaxies do not look very bright in the night sky. Apart from the Milky Way and two other nearby galaxies called the Magellanic Clouds, only a few can be seen without a telescope. They usually look like faint "fuzzy stars" in the sky.

There are about twenty galaxies in our part of space, which we call the ?local group?. The biggest one of these is the Andromeda galaxy. It is about twice as big as our own galaxy. This is a picture of Andromeda.

The picture also has two bright patches, one above and one below. These are smaller galaxies.

There are three main types of galaxy. These are called ellipticals, spirals and irregulars.


  • Galaxies started to form when the Universe was about a billion years old and was filled with gas.

    Astronomers think that the galaxy formed out of a large ball cold gas in space.

    The cloud began to fall in on itself and started to form stars around the edges. These early stars are called population 2 stars.

    The cloud continued to fall in, with more and more stars being made. These later stars are called population 1 stars.

    The type of galaxy that is formed depends on how fast the cloud spins. If it spins quickly it forms a spiral galaxy, and if it spins slowly it makes an elliptical galaxy.

    Irregular galaxies aren?t made in the same way as the others. They are created when other types of galaxies hit each other.

Most of the bright galaxies near to the Milky Way are called spiral galaxies. Spiral galaxies have a bright bulge in the middle, called the ?nucleus?, and bright arms that spiral out from this. They can have anything between 10,000,000,000 and 400,000,000,000 stars in them, and they are so big that light would take from 16,300 to 163,000 years to travel across them. As well as stars, they have a lot of gas and dust in them. The Milky Way is a big spiral galaxy.

Types of Spiral Galaxy

Spiral galaxies are named depending on how they look. All spiral galaxies are labelled with the letter "S", followed by another letter, either 0, a, b or c. This depends on their spiral arms and their centres. How the galaxy looks in the sky depends on the direction we see it from. A galaxy seen sideways on will look very different to one seen face on. This can affect how it is named.

The picture on the left (SEDS archive) is of the galaxy M60, which is a type S0 galaxy. It has a lens shape because it is seen side-on. S0 galaxies often look like ellipticals, but they can be told apart when they are seen from the side.

The next type of spiral galaxy is the Sa type. They have very tightly wound spiral arms, and large central bulges. A good example of an Sa galaxy is Messier 65 (NGC3623), shown here thanks to AAO. It is hard to see the spiral arms because the galaxy is seen from the side, but it is easy to see the difference between this and the S0 galaxies pictured above.

The galaxy shown on the right is Messier 77 (NGC 1068), and is pictured here from the SEDS archive. This is a member of the next class of spiral, the Sb. The bright centre contains young stars, while the areas further away from the middle have older stars. The spiral arms are less tightly wound than for the Sa galaxies and the middle is smaller. Most spiral galaxies are Sb types.

Finally we come to the Sc class of spirals. These galaxies, like NGC 2997 (right, from AAO) have very open, "untidy" spiral arms and small centres. They have more gas and dust than any of the other spiral galaxy types. If you look along the spiral arms, you can see small red patches. These are glowing clouds of gas and dust where stars are being formed.


Rings and Bars

M83, Barred Spiral Galaxy There are another type of spiral galaxies called ?barred spirals?. These have a bar of stars stretching through the middle of the galaxy, and the arms start to spiral from the end of this bar. These galaxies are given the letters SB instead of S, and astronomers think that the Milky Way is one of these. Barred spirals are labelled in the same way as normal spirals, depending on how their arms and centres look. A good example of a barred spiral is M 83(right, taken by AAO). This galaxy is one of our nearest neighbours in space, lying at a distance of around 12 million light years (this means the light we see from the galaxy tonight started out on its journey to us over 12 million years ago!)

NGC 2523, a ringed barred spiral As well as bars, some galaxies have rings around their centres. One of these galaxies is NGC 2523 (left, from the Digital Sky Survey). Here we can see the ring around the middle, and the bar that passes through the centre, touching the ring on opposite sides. Spiral arms begin at the point where the ring and bar meet.

What are the spiral arms?

The spiral arms are the areas where stars are formed. Here we find the hottest, youngest and brightest stars, and this is why we can see the arms so clearly. Along with fully formed stars, we find hot glowing clouds of gas and dust called ?nebulae? where stars are born.