Let’s take a closer look at one of the most easily recognisable patterns of stars in the northern hemisphere.
The Big Dipper goes by several different names, including the ‘Plough’ and the ‘Saucepan’. It is, though, very recognisable with its bowl-shaped pattern of four stars connected to a ‘handle’ of three more. This is a group of stars which has been recognised from time immemorial and by nearly all cultures around the world.
It is not a constellation in its own right, but just an easy-to-spot pattern of stars which form part of the larger constellation of Ursa Major, or the Great Bear. Patterns of stars like this that are only a part of one or more constellations are known as ‘asterisms’. It’s a really useful asterism for several reasons, one of the most important is that it can help us find Polaris or the ‘pole star’, which in turn helps us understand where true north lays, so it is of great benefit to navigators to know how to use the Big Dipper to aid finding this. The two end stars, opposite the handle, are called the ‘pointers’, because if you draw an imaginary line through these stars heading out of the ‘bowl’ the next bright star you will arrive at will be Polaris. By sheer chance Polaris sits almost over the north celestial pole.
If you drop an imaginary line directly from this point to the horizon, you will know the position of true north.
For anyone living north of the latitude of southern Spain, the Big Dipper is circumpolar. This means that from these latitudes it never appears to set or disappear below the horizon.
It rotates around the north celestial pole as do all the other stars and constellations, but because it resides near the pole it can always be found in the night sky.
The star in the handle which lays higher than the others is interesting; as if you look closely you’ll see it is two stars. This is a naked eye double star and unusually for double stars they both have names. These are Mizar and Alcor. If you have good eyesight you should be able to make out both stars. What is particularly interesting about this binary or double star is that each component is also a multiple star system. Mizar itself has four stars in orbit around their common centre of gravity, so all in all, this double star is in fact a sextuplet system, although most of these stars cannot be resolved using even the largest Earth-based telescope. We know of their existence from spectroscopy, whereby the light from the star is split by a prism into its constituent colours.
All the seven stars in the Big Dipper have names. The two stars of the pointers are called Merak (the lower of the two) and the other, at the top of the ‘bowl’, is called Dubhe. This is the brightest star in the group. Ursa Major plays host to several amazing deep sky objects including several galaxies.
You can use the stars of the Big Dipper to find a couple of them. If you draw an imaginary line from the bottom left star in the bowl through the top right one (Dubhe) and keep going for roughly the same distance again, you will come across a beautiful pair of galaxies known as M81 and M82. If you form an equilateral triangle with the two end stars of the handle, Mizar/ Alcor and Alkaid as the base, at the other point of the triangle you will find the galaxy M101.
The Big Dipper is also a great signpost to other constellations. If you use the two stars in the bowl nearest the handle, Megrez and Phad, as pointers but head away from the pole star, you can find the bright star Regulus in the Leo constellation. Again if you use the handle as a signpost, the next bright star you’ll come to is Arcturus in the constellation of Bootes.
So you can see what an amazing and useful group of stars the Big Dipper really is.