Just a short one this week, as I’m very busy at work. I’m also working on a couple of astronomy podcasts, the first of which will be online in a few weeks. I’ll let you know when they’re up.
Stephan's Quintet, by Hubble. Click to enlarge.
This beautiful group is a cluster of galaxies called Stephan’s Quintet. Its members are orbiting each other in a slow dance which, many millions of years in the future, will most likely end with a series of mergers to form a single super-galaxy. Two of them, near the centre, are already quite easy to mistake for a single object.
But one of the galaxies here is an odd one out, not really part of the cluster at all: in fact it’s about seven times closer to Earth than the others, at a mere 40 million light years instead of 290 million. Can you tell which one?
In fact it’s the bottom right galaxy, named NGC 7320. It’s pretty hard to see much difference in this photograph, though - and that underlies the importance of tools like redshift which let us measure the distance to objects, since plain observation is often not enough. And if a simple question like the distance to an object - and whether it belongs as a member of a certain group or not - is so hard, you can begin to appreciate that some of the other classifications astronomers attempt require very clever techniques indeed.
