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…