DARK SKIES: Thousands of star clusters scattered in our sky
OUT of the billions of stars, nebulae and solar systems that make up our Milky Way galaxy, two objects amaze the human eye in a telescope: open star clusters and globular star clusters.
The first type is made up of thousands of young stars with lots of space between them. Like a black cloth sprinkled with a few bits of glitter, these OCs come in all shapes and forms. Some are dubbed the "jewel box”, "butterfly”, "wild duck”, a "poodle with two tails” and the "seven sisters”.
This is how they appeared to the early telescope observers who drew and described them, since Galileo pointed his little optical tube to the night skies in 1610.
Out of the 250 OCs that we know of, the average distance is 7000 light years, which is quite close by astronomical means - one light year is equal to 9.4trillion kilometres.
While some of these OCs can be glimpsed with the eye on dark, moonless nights, binoculars show more but to really see these pretty OCs in their full bloom, you need a telescope at 50X to 100X.
Some OCs have numerous coloured stars, with the most famous of all, the "jewel box” open cluster located under Mimosa (Beta Crux), the second brightest star in the Southern Cross. Within are three coloured stars: yellow, green and red. This is why it's a favourite to all stargazers with a telescope, best seen from March to September, peaking in June.
It got its name when astronomer John Herschel - the son of famous English astronomer Sir William Herschel - first observed the cluster with a large telescope at Cape Town, South Africa, in the early 1800s, when he set about observing, recording and sketching the southern night skies.
Globular clusters, on the other hand, are a lot fainter to the eye. They not only contain millions of stars but all of them are packed into one gigantic cluster, six to 10 times further than open clusters.
Stars in GCs have little colour to the human eye but are easily detected with spectrographs, as they can determine their age, based on their average distance of 25,000 light years.
From the spectra data collected over many decades, it's now a forgone conclusion that all GC stars were the first stars formed in our galaxy, 14.7 billion years ago, while our solar system is only 4.5 billion years old.
In large telescopes like we have at the Observatory, globular clusters are a wow factor as they appear like a kaleidoscope of stars. Like a handful of glittering small diamonds but in binoculars, they appear as little fuzzy blobs or smudges.
The largest of all is Omega Centauri, best seen in autumn and winter.
At a distance of 13,000 light years, this globular cluster contains about eight million stars, while its smaller cousin, 47 Tucana at 48,000 light years' distance, is best seen in the spring or summer night skies.