Canis Major star map
Copyright: Melbourne Planetarium
Highlights of the tour:
- emission nebulae
- globular clusters
- open clusters
- a Cepheid variable star
- a binary star system
Of all the constellations in the Zodiac - the region where we find the Sun, Moon and planets - Sagittarius is the furthest south. This means it is highest in our part of the world, passing overhead in our evening skies during late winter. These stars are supposed to represent a Centaur, a mythical creature half-human and half-horse, holding a drawn bow and arrow which he is pointing at the scorpion in the neighbouring constellation of Scorpius. This is somewhat difficult to see in these stars. More obvious is the shape of the Teapot, made by some of the brighter stars.
When we look at this constellation, we are looking towards the centre of our Milky Way galaxy, so this region of the sky is filled with impressive star-fields. The whole area is worth admiring with binoculars.
Stop 1
The star at the top of the lid of the Teapot is Lambda Sagittarii, while the fainter star just above it is Mu Sagittarii. The area around these stars has a number of nebulae and clusters. Here Mu is at the top of your field of view and Lambda is at the left. At the lower right is M8, the Lagoon Nebula. This is one of the brightest emission nebulae, visible to the naked eye on a dark night and easily found with binoculars. It appears as a glowing white patch with dark lines running through it; a telescope is needed to bring out the red colour. A bright open cluster, NGC 6530, is visible inside the nebula. Above M8 is M20, the Trifid Nebula. This nebula is also an emission nebula, but is smaller and fainter than its companion. It is still visible to binoculars, but is not so easy to see. Less then a degree north-west is M21, a bright open cluster containing about 50 stars in a compact region 12' across.
Stop 2
Now put Mu at the bottom right of your field of view. Above it is M17, the Omega Nebula, This is another emission nebula, and also very bright. Just below and to the left is M25, a large open cluster which is over a degree across and contains 80 stars. It is bright enough to be seen with the naked eye from dark skies. One of the stars in this cluster is UÊSagittarii, a Cepheid variable whose brightness varies regularly over a period of 6.745 days. Since this cluster contains many stars of similar brightness, its variations are easy to monitor.
Stop 3
At the edge of the Teapot is Zeta Sagittarii. Just below this star is the globular cluster M54. It is quite small, so difficult to resolve into individual stars.
Stop 4
To the east of Zeta is another globular cluster, called M55. This is a large, bright cluster whose stars are packed very loosely. This is one of the best globular clusters for southern observers.
Stop 5
The two main stars you see here are Alpha and Beta Sagittarii. Unusually, they are not the brightest stars in the constellation. The lower of the two, Beta, is slightly brighter and is a binary star, with the two component stars easily separable with binoculars.