Diamond dust

The European Southern Observatory's Very Large Telescope (VLT) has released a great image of the central part of a globular cluster of stars named 47 Tucanae (47 Tuc to its friends) which can actually be seen by eye from the Southern hemisphere. It looks about the same diameter of the Moon (around half a degree) on the sky but in reality is about 120 light years across. Globular clusters look fantastic when seen through a small telescope (in any hemisphere) and I was lucky enough to see some when I visited Ian back in March. Globular clusters contains tens of thousands of stars and Ian put it brilliantly (MP3) when he said they looked like "diamond dust" through the telescope.

47 Tucanae
A 7 arc minute wide image of the central part of 47 Tucanae taken in 2001 with FORS1 on Kueyen, UT2 of the Very Large Telescope CREDIT: Kotak/Boffin/FORS/VLT
Each cluster is thought to have formed from an individual cloud of gas which is interesting because it means that all the stars in it formed at roughly the same time. As the stars are similar ages, this lets you directly compare the development of stars of different masses at one point in their lives. I guess this is sort of like looking in a high school year book; you get a single snapshot of the different kids lives with which you might guess that the kids that did well at maths and science may go on to get jobs in finance, engineering or research. The analogy shouldn't be taken too far because the future lives of stars are more predictable than the lives of the kids in the year book!

I have a particular fondness for 47 Tuc because I remember spending hours making measurements on photographic plates to work out the radius of the cluster. The radius for something with no distinct edge, such as a cluster, is something of a fuzzy concept so I was actually working out the tidal radius and the core radius. In this case, the tidal radius is the distance from the centre of the cluster where the gravitational effect of our galaxy dominates the gravity of the cluster. Any star further than this distance from the cluster will be pulled away towards the galaxy. The core radius is the distance at which the brightness of the cluster is half of the brightness at the centre. My measurements basically involved counting stars within millimetre squares at a whole range of distances from the cluster centre. Although this doesn't take much thought to do, it is very frustrating when you keep losing count, especially when you have counted 60 stars in a particular square and have to start again.

Posted in astro blog by Stuart on Friday 09th Jun 2006 (13:23 CEST) | Permalink
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