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
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!)
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.
Author: Nigel Bannister
Updated by: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001