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Omega Centauri

Omega Centauri (NGC 5139) has got to be the best globular cluster in the sky by a big margin! Its easy to find near the Southern Cross, its easily visible to the naked eye and covers a large area (20 arc minutes for the central area and up to 65 arc minutes for outlying stars, which is bigger than the full Moon). In a 36cm (14 inch) reflector with a wide field eyepiece it shows thousands of tiny yellow and white stars.

Omega Centauri is a class VIII globular cluster, one of more than 100 associated with our galaxy. It is big as far as globulars go, about 5 million times the mass of the sun, which makes it 10 times the mass of many other big globulars. Its true width is about 150 light years and the density of stars in the core is about 80000 times higher than in the area of the Sun. Visual brightness is magnitude 3.68. Distance is about 15,000 light years. The exact location is right ascension 13h 26.8m, declination -47° 29m. Its a very old object, with an estimated age of 16 billion years.

Although it had been recorded as a star for thousands of years, it was Edmund Halley who recognised it as a cluster in 1677 (its hard to see how anyone would classify it as a star because it is a conspicuous large patch even without optical aid).

Above are some pictures of Omega Centauri to show what it looks like using amateur equipment. The picture on the left is a photo I took using a CCD camera attached to a 15 cm apochromat refractor. Next is a picture which shows what the object would look like in a medium sized amateur telescope (about 25 cm in diameter). Note the distinctive yellow color caused by the bright red old stars typical of globular clusters. On the right is the view through a small telescope where the stars aren't fully resolved (around 10 cm diameter).

This is how to find Omega Centauri. Find the two bright pointers to the Southern Cross, Alpha and Beta Centauri. Off at an angle of about 45° is Epsilon Centauri. Follow the line from Beta through Epsilon and continue the same distance again in a straight line. You should see a large fuzzy patch which is the globular you are looking for. If your sky is reasonably dark you will see it easily without any optical aid at all. When you can find it this way, try the same thing with binoculars - the globular will be very obvious. Finally try it with your telescope. Use low power because this thing is big!

The picture on the left shows a photo of Omega Centauri taken with a large, Earth-based telescope. The stars in the core appear white, but this is a side effect of the photographic process. In reality, all the bright stars in a typical globular are red giants and generally appear yellow when observed. The picture on the right is the core of Omega Cen taken with the Hubble Space Telescope. Even to cover this small part of the total object several smaller images had to be joined. The HST field is small and Omega Cen is very big. There are about 30,000 stars in this image and the true size of this area is about 10 light years across. A similar sized area in our part of our galaxy would contain only about 4 stars.