ONLY a handful of constellations are easily recognisable from just about anyone who looks up at the night sky: Orion, with its 3 stars lined almost perfectly in a straight line marking its Belt, The Pleiades (or The Seven Sisters) as a tiny cluster above the head of Taurus, and Cassiopeia with its unmistakable ‘W’ shape.
By Seanie Morris, Midlands Astronomy Club
Apart from these, the most recognisable asterism in the night sky must be The Plough (or the Big Dipper for North American observers). Part of the larger constellation of Ursa Major, or the Great Bear, it has been recognised by man since the earliest writings found on Earth. It is circumpolar, meaning it is visible all year round, changing position through the seasons as it rotates around the Pole Star, Polaris.
From the diagram attached you can see the 7 main stars of the Plough and their names: Alkaid, Mizar (with its double companion, Alcor), Alioth, Megrez, Phecda, Merak and Dubhe. It is good to know these when pointing out the routes to other stars and astronomical targets in the sky. The names come from Greek and Arabian lore. Stories about its existence originate with the Greeks, Chinese and Native Americans. You can find many versions of each on an internet search but the most common is that of Zeus lusting after Callisto, a young female nymph. His jealous wife, Hera, turned Callisto into a bear and as punishment placed her among the stars as a warning to other would-be lusts of her husband.
The Plough itself is a relatively close group of stars, themselves lying not far from Earth. The closest star, Mirak, lies 78 light years away while the furthest, Dubhe is only 124 light years away. The double pairing of Alcor and Mizar are in fact a binary star system, separated by only 3 light years.
For centuries the Plough has been used as a guide to finding other celestial delights and targets and here we look at some of those. The following ‘How to find’ targets each have objects easily found with a decent pair of 10 X 50 binoculars or larger and even small amateur telescopes.
How to find: Polaris
Follow a straight line from Merak through Dubhe heading upwards and the next bright star you come to is Polaris. Often called the North Star and the Pole Star it is also wrongly named as the brightest star in the sky – this would be Sirius in Canis Major. From an observer’s perspective in the northern hemisphere it appears that Earth rotates on its axis pointing to Polaris. This is indeed the case but it is only coincidence that a bright star lies at almost the exact point of 90 degrees elevation in relation to the equator. When you look at Polaris you are facing north. All other objects rotate around this point in the sky.
Polaris is the brightest star in the constellation of Ursa Minor. It lies 432 light years away and would be 7 times the size of the Sun, making is a Supergiant, F-type star. It is in fact a multiple system with its companion, Polaris B, visible only in telescopes. A much fainter companion, Polaris Ab also exists thus making Polaris a tertiary system.
Sky Atlas Tip: Ursa Minor is not very rich in deep sky objects but it is handy to know it in terms of finding North.
How to find: Arcturus
Follow the curve of the Plough’s handle from Megrez to Alkaid in an arc and the first bright star you come to is orange Arcturus. Arcturus is the brightest star in the constellation Bootes, the brightest in the northern hemisphere and the 4th brightest star in the sky. It shines as a bright, yellow star and has been confused by inexperienced observers as Mars. It is a K-type variable star showing signs of subtle variations in its brightness over long periods of time. This would not be indicative of a potential supernova (explosive) candidate but rather a star that will blow off its outer layers when it gets older, or go nova, instead.
Relative to our Sun, Arcturus moves quite fast through space at 122km/sec towards the Sun. It will never collide with the Sun but will makes its closest approach in around 4,100 years time. It is only slightly larger than our Sun and lies around 36.7 light years away.
Sky Atlas Tip: Below our imaginary arc drawn to Arcturus lies an area of sky rich in Messier and NGC objects. Most of these are distant galaxies. This area of sky lies between Coma Berenices and Leo and is often referred to as the Coma Cluster. You will be spoiled for choice here.
How to find: Spica
Continue the arced line mentioned above past Arcturus heading downwards and the next bright star you find is Spica, brightest star of Virgo and 15th brightest star overall. It is a blue giant that would consume 10 Suns and lies 260 light years away. It is actually a binary star with a companion so close it orbits Spica every 4 days. Located along the ecliptic it is sometimes eclipsed by the Moon.
Star Atlas Tip: Virgo is a constellation rich in deep sky objects, most of them distant galaxies but there is no shortage of globular star clusters either. This plethora of objects known as the Virgo
Cluster stretches in to the Coma Cluster.
How to find: Gemini
Follow a straight line from Megrez through Merak right across the sky and you will come to the next bright star of Pollux. To the right is his brother Castor: they are the heads of the twins of Gemini. One myth from Greek mythology portrays that when Castor died because he was mortal, Pollux begged his father Zeus to give Castor immortality. He did so by uniting them together in the heavens.
Sky Atlas Tip: Gemini is rich in star clusters and some nebulae including the Eskimo Nebula and the Cone Nebula. Some deep sky galaxies are also found within its boundaries.
How to find: Cassiopeia
Follow a line from Mizar through Polaris and you will come to the constellation of Cassiopeia. One of the most easily recognised constellations by simply looking like a large ‘W’ or ‘M’ depending on the time of year. Cassiopeia lies along the Cygnus arm of the Milky Way.
Sky Atlas Tip: Rich in star fields and clusters, the most famous being the Double Cluster shared with Perseus, there is ample opportunity to scan this region of the sky for targets.
Any good book store will be able to help you get star atlases of any kind to help you know your way around the sky. You can also download many free software programmes from the internet. One of the most recommended is Stellarium, which can be downloaded from www.stellarium.org. This programme, featured regularly at astronomy club meetings provides many options for display and animation of the night sky on your computer, enabling to jump forward and back in time also.
To find out more information about what you can see in the night sky you can visit www.midlandsastronomy.com or www.irishastronomy.org – alternatively, you can come to any of the monthly public lecture nights (first Tuesday each month) hosted by the Midlands Astronomy Club.
Seanie Morris is the Secretary of the Midlands Astronomy Club (MAC) having been a member since 1990 when it used to be known as the Tullamore Astronomical Society (TAS). Seanie’s favourite astronomical interests include meteor watching, deep sky telescope objects and the Moon. See more of what MAC is about at www.midlandsastronomy.com