The constellation Sagittarius is high overhead this month. Sagittarius represents a centaur, a half-horse, half-human creature and an archer, equipped with a bow and arrow in ancient representations. Modern imaginations seem to be less than those of the past and it is hard to see Sagittarius’s stars as showing a centaur: to most it looks like a teapot.
On this month’s sky map, Sagittarius is split between the north and south segments. Therefore, you can have a look at the July star map to get an understanding of how the constellation appears before searching for it overhead in the real October sky.
The left four stars are the teapot’s handle, the three stars in a triangle at the top are its lid and on the right is the spout.
Though we cannot tell from light-polluted cities, Sagittarius lies in the densest part of the Milky Way. As such, it has numerous nebulae and star clusters of interest. One of these is the Lagoon Nebula, which is a large cloud of gas and dust illuminated by nearby young stars. It is about 5,000 light years from us. Some of the light is absorbed by dust, giving the nebula the characteristic appearance that gives it its name.
The real distinction of Sagittarius is that it contains the centre of our galaxy, a 4 million solar mass black hole that astronomers call Sagittarius A*. We know the mass of the black hole because in Nobel Prize-winning research, astronomers carefully plotted stars moving around it. Knowing the period of the stars and their distance from the black hole, they could then calculate its mass.
Sagittarius A* is also a bright source of radio waves. Since the source passes overhead for those of us in the Southern Hemisphere, Australian astronomers have a great opportunity to study it.
