Double Stars - Mizar and Alcor

At the time of writing, there’s a bright moon in the sky. As a rule of thumb, if the moon's up then it's the only thing worth looking at. But I’m not looking the moon this month, I’m saving that up. Instead we’re going to take a closer look at a star we used last week in our search for Polaris.

Naked Eye

Take a second look at the Plough. The middle star of the “Handle” is the one we’re interested in.

It doesn’t take a dark moonless night to see that we’re not looking at one star here, we’re actually looking at two. Mizar and Alcor are, perhaps, the most famous double star in the sky. In reality they’re quite far apart and don’t orbit each other. In fact all but two stars in the Plough are moving in the same direction in space, suggesting that they are all related. This is unusual for a constellation, but the Plough isn’t a constellation, it’s an asterism, a grouping within a constellation. If you look around the area of the Plough, most nights you’ll be able to pick out the rest of Ursa Major, the Great Bear.

Unusually it pretty closely resembles what it’s supposed to depict – you can see the legs, the head, the snout and… the tail. Yes, the Great Bear has a great big long tail, which doesn’t make a lot of sense, but nearby in the sky is a constellation called Lynx that most certainly doesn’t look in any way, shape or form, cat-like. So a long-tailed bear is pretty close in astronomical terms.

Binoculars

Through binoculars there are, quite clearly, two stars. The ancients used it as a test of eye-sight. Through the bins you may be able to see a third star in between the two. It was named Sidus Ludoviciana after a German nobleman and was originally thought to be a planet. In the end it turned out to be a star in the same line of sight, but much further away. To see a proper binary system here, you need a telescope.

Telescope

With the greater magnification that a ‘scope brings, you can finally reveal the secrets of Mizar. In fact this bright star is a double with the two stars very close together (14 arc seconds). Each component of Mizar is itself double making the star a quadruple system. Because of the proximity of each pair, the only way to tell they’re double is by looking closely at the light produced by the stars – the science of spectroscopy. It transpires that double stars are quite common in the universe and our own sun is unusual for being on its own.

Next time it’s going to get even more exciting because we’re going to be making babies. Baby stars that is…

Pole Position - Polaris

Finding your way around the stars can be quite a bewildering prospect. To make things easier the stars visible in the night sky are divided up into patterns – the constellations. It’s not the only way of navigating around, but recognising constellations does make finding things easier. This week we’re going to use one of the most famous constellations in the sky to find the most important star of all to humans – Polaris.

Actually, I say most important, really there is one other star that is far more important than all of them. Our own sun is a star, and it’s the only star we can see close enough to work out what stars really are.

 

Credit: NASA/JPL

The sun is, in effect, a giant nuclear explosion in space. It’s so enormous that it has about 10 million years worth of its primary fuel inside it. Inside stars, atoms of hydrogen are pushed together at enormous temperature and pressure so that they fuse. This nuclear fusion process generates heat and light, and as a by-product helium.

Not all stars are the same – the sun is classed as a dwarf star, pretty small in the grand scheme of things. There are much larger stars out there and also much smaller – many of these we may meet over the coming months. They also come in a variety of colours – looking around now you might spot bright red Betelgeuse, yellow Capella or blue-white Rigel.

Stars are grouped by us into constellations. These groupings aren’t physically associated with each other or even close to each other in space – often they’re separated by many thousands of light years. But they’re a useful way of dividing up the night sky and learning them is both fun and useful. There are a few that are little more than a couple of faint stars but many are very bright and recognisable.

This week we’re going to use one of the most recognisable patterns – the Plough (Actually an asterism, a group of stars within a constellation) – to help us find the North Pole, handily marked by Polaris, the pole star.

Naked Eye

Finding Polaris is one of the most useful astronomical skills you can learn. The star is at the North Celestial Pole so being able to find it allows you to find out which way is north.

To do it you first have to locate the Plough – this is a group of seven stars that forms most of the constellation of Ursa Major. Once you’ve found the Plough, look for the two stars at the front of the shape. If you take a line through these two, from bottom to top, and keep following it, they point to Polaris.

 

Finding Polaris

Polaris is the brightest star in Ursa Minor, the little bear. It’s a faint constellation of seven stars – the number of other stars visible can give an indication of how dark the night is.

Binoculars and Telescope

There’s not a whole lot to see of Polaris through binoculars, and the situation is the same through a 3’’ telescope. However at high magnification Polaris yields a surprise. Next to it, you may be able to make out a faint star. This is because Polaris isn’t one star, it’s two. Double stars are actually fairly common, although Polaris is maybe not an ideal example: the faint companion star is drowned out by the glare from Polaris. We’ll look at a better example of one in a bit more depth next week.

n.b. I had originally envisaged using sketches to illustrate this blog. Unfortunately my scanner is not yet working, so we'll have to stick with these simple paint images for now.