Glorious Galaxies

Galaxies - like most other things in astronomy - come in many shapes and sizes. Hubble (the astronomer rather than the telescope) grouped galaxies into three or four main types. Actually, astronomers like to endlessly classify things, so these classes also break into many sub-groups, but basically there are spiral, elliptical, lenticular and irregular galaxies.

The most familiar shape is probably that of the whirlpool and our own galaxy - the Milky Way - is a spiral galaxy with several spiral arms. How exactly these arms were discovered, considering that we are embedded in it so can't look back at it from afar, is a nice piece of detective work and I'll keep it for another post sometime. Spiral arms are interesting places; they are regions where gas and dust gets compressed and new stars form. The centres are exciting too as they are usually very dense and contain a supermassive black hole. Spirals are the poster-galaxies of the universe and they are always pretty whichever way you look at them.

NGC 908
Image of Starburst Galaxy NGC 908 obtained with FORS2 on the Very Large Telescope between 13 and 14 August 2000. CREDIT: ESO
The second class, and perhaps the most boring to look at, are the elliptical galaxies. As with spiral galaxies they consist of hundreds of billions of stars, but they resemble large, diffuse, football-shaped (in both the American football or the soccer sense) collections of stars. They don't tend to get as much coverage as the flashier galaxies that look like pinwheels, whirlpools or sunflowers.

Next are the lenticulars. These look a bit like a strange cross between an elliptical and a spiral galaxy as they have a disc of material but with a huge bulge of stars too. The Sombrero Galaxy is a brilliant example of a lenticular galaxy.

The last class are the irregular galaxies. This group basically covers all the galaxies which don't neatly fall into one of the other three. Two of our neighbours (the Large and Small Magellanic Clouds) are nice examples of irregular galaxies. These odd-shaped, relatively small galaxies are really interesting because they have either undergone or are in the process of being ripped up or smashed together by the tidal effects of the bigger galaxies. In the case of the LMC and the SMC, our galaxy is messing them up although not quite as badly as it has done to our closest satellite galaxy the Sagittarius dwarf galaxy.

The European Southern Observatory recently published a nice image of NGC 1427A (above) which is about 60 million light years away in the direction of the constellation Fornax.

NGC 1427A
Colour composite image of NGC 1427A, based on observations collected with FORS1 (2002/2003). CREDIT: ESO
This galaxy is moving at a heady 600 kilometres per second towards the centre of the Fornax group of galaxies. This high velocity causes the galaxy to get squashed at the front as it hits gas sitting between the galaxies and this encourages new stars to form. Any life developing on planets orbiting those stars would have a very different view to the one we have as they wouldn't have a milky band across the night sky. They would also have to contemplate their galaxy finally being ripped apart and its contents spread into the space between the other galaxies in the cluster. Mind you, we can't be too smug sat here in our pretty large spiral galaxy. After all, we are due for a close gravitational encounter with the Andromeda galaxy in only three billion years. We too could be thrown out.

Posted in astro blog by Stuart on Friday 28th Jul 2006 (10:03 CEST) | Permalink
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