Highlands Astronomical Society

By Antony McEwan - Highlands Astronomical Society

Are you (Heaven forbid) dog-tired of looking at the same old mainstream constellations every clear winter night? Are you straining at the leash for a bit of a change? Fed up with Orion, bored with Cassiopeia, finished with Gemini? Are you being hounded for something fresh to show your observing companions?

Well, in the same southerly part of the sky as those big, brash constellations are two smaller ones – one much more so. If you have been looking at Orion at all lately you will undoubtedly have noticed a very bright star below and to the left of the Hunter. It may have appeared to scintillate and shimmer, changing colour in front of your eyes. This star is the brightest in our northern skies. Indeed, it is the brightest star that can be seen in the sky from anywhere on Earth. It is known as Sirius, or colloquially as the “Dog Star”.

Sirius shines at magnitude –1.46 and has quite often been mistaken for a UFO, a planet, a searchlight, or any number of other things that may conceivably (or even inconceivably) be in the southern part of the sky, just above the horizon, on a winter night. So why “Dog Star”? Well, the constellation that Sirius resides in is Canis Major, or the great (or big) dog. There are several myths that have been linked with Canis Major, but the most commonly used one associates the constellation to Orion, the Hunter. In this mythology the dog is one of Orion’s hunting dos, and may be either helping the Hunter fight the bull, Taurus, or hunt down a hare, Lepus.

Whatever the story behind it, the constellation is teasingly interesting to observe. I say teasingly because although it does actually look slightly like a dog, (to us it seems to be on its hind legs with Sirius perhaps being a gemstone in the dog’s collar) it stays very close to the horizon at all times. It almost appears to be gazing adoringly up at Orion rather than actually helping him in his pursuits. Part of the challenge of observing such lesser-known constellations is locating the shapes, marked by quite faint stars, that appear to be much more obvious in the star-charts you may look at before going outside.

Sirius itself, of course, stands out like a beacon in the low southern sky. If you look at it through a telescope it can be quite mesmerising. The star becomes a writhing shimmering gemstone, constantly changing colour as you watch. This is because we are looking at Sirius through a thicker layer of atmosphere than if we were viewing it high overhead. The shimmering and scintillation of the view is caused by turbulence in the multiple layers of atmosphere, which then distort the light passing through them from the star. It can be very pretty but it can also be quite frustrating if you are trying to telescopically resolve two objects that appear very close together, like a double star for example…

Which, funnily enough, Sirius is! But don’t get excited, you won’t be able to see its companion through an amateur telescope. The companion (Sirius B) is a white dwarf – a very faint, dense object that once would have been like the vast majority of stars in our galaxy, shining bright and fierce. White dwarfs are basically dying stars that are very near their end, having used up all their fuel and shrunk down to a point where they are super-dense and appear extremely faint. They would be quite bright if close to us but appear faint because they are very small, being only roughly Earth-sized.

For binocular or small telescope users, Canis Major has a very nice little open cluster that is worth a look. Scan down from Sirius about four degrees, and you should come across it. Although not very big, M41 has a smattering of about a hundred stars of differing brightness and colour (hence different spectral types) with some strong red stars being apparent.

Now, if Canis Major is the big dog, where is the little dog? Look at the top of Orion and note the positions of the stars marking his shoulders, then draw a horizontal line out to the left of the constellation. Continue until you connect with a bright white star that makes an almost equilateral triangle with Sirius and Betelgeuse. This triangle is known as the Winter Triangle, and the third star is Procyon. This is the main star in the constellation of Canis Minor – the little (or lesser) dog.

Canis Major has the brightest star in the sky, and Canis Minor has… well, not much really. The constellation itself only consists of two stars, and since there is only one way of connecting two stars (a straight line), nobody is ever going to accuse the constellation of being a particularly accurate representation of a dog. That said, Procyon itself is the seventh brightest star in the sky and is nicely placed to make up the Winter Triangle asterism previously mentioned. Handy that. Strangely though, there is something else that connects the two celestial dogs. Both constellations have principal stars that have companions. Yes, Procyon also has a companion, and this really takes the dog-biscuit: it’s a white dwarf too!

So, cosmologically both constellations have very interesting main stars which both have white dwarf companions, and they and their companions are eliciting a great deal of study and interest in the scientific community. Observationally, they have very few interesting objects in their environs for amateur astronomers to hunt down, apart from Big Dog’s open cluster, M41. Nevertheless, they remain very noticeable in the night sky and are excellent markers for finding your way to other surrounding objects or constellations.

However, mythologically, if I were Orion I’d have chosen a couple more useful hunting dogs than one that just follows me around gazing up at me all the time, showing off its shiny collar, and one that is trying to be as inconspicuous as possible. I bet he’s dog-tired of them now…