What Can You See in the Night Sky This Week?

21 November 2011 by Ryan Houghton

All that glistens is not gold: sometimes it’s red, or blue, or both. This week, we take a look at some double stars in the night sky and learn the difference between an optical double and a true binary. You will need binoculars or a telescope, and this week’s sky map.

When we look around the night sky, we usually only see single stars. But did you know that around 50% of all stars are in binary systems? That’s when two stars are so close they orbit each other. Binaries come in a range of sizes: sometimes the stars are so far away, they take thousands of years to orbit each; other times, they take just hours!

It’s very difficult to see binaries by eye. If you use this week’s skymap (as always, set for 8pm on Wednesday night), you’ll see a star in Ursa Major (also known as The Plough) labelled Mizar. If you look carefully at this star, you might just convince yourself that there is another, fainter star next to it. Is this a binary then? Good question! That other faint star is called Alcor. Before 2009, Astronomers thought that Alcor and Mizar were not a true binary system but just an optical double: projection on the sky alone caused them to appear close together (while they are in fact a few light years apart and do not orbit one another). But recently some authors [1, 2] have questioned that old view.

However, if you focus your binoculars/telescope on Mizar itself, you’ll see that it is not a single star, but is in fact two stars (called Mizar A and Mizar B). This is a true binary! Mizar A and B orbit each other with a period of 5000 years and are currently separated by around 14” (read: 14 arc seconds).

Just to confuse you even further, we now know that Mizar A and Mizar B are each spectroscopic binaries in their own right: Mizar A is a group of two stars that orbit each other; Mizar B is a group of two stars that orbit each other, and each group orbits the other! And to cap it all, it’s even been found that Alcor is a true binary in its own right. Binary stars really are this common.

On the skymap, we’ve also labelled Albireo in Cygnus (the head of the swan). With your binoculars or telescope, you should see that Albireo is a double star, consisting of Albireo A (the brighter yellow/amber one), and Albireo B (the fainter blue/green one). The two are separated by 35” (read: 35 arc seconds) on the sky. We don’t know if these two are in a true binary, but if they are, then the orbital period will be very large indeed - we haven’t seen them move significantly yet! That said, Albireo A is actually two stars in a true binary system. As far as we know, Albireo B is a single Be star. We’ll hear more about Be stars later this week...

Try to notice the two very different colours of Albireo A and B. This is one of the best examples of different star colours. Another good one is the shoulders of Orion, now up in the late evening. The two shoulders are Betelgeuse (bright orange, on the left) and Bellatrix (blue, on the right). The colour of a star tells us about the star’s age, or evolutionary stage: click on the links for these stars and compare their masses, radii and distances. But that’s a different story...

Don’t forget that if you want to know about other objects up in the night sky this week, visit the Blackett Observatory’s What’s Up page.

Categories: what's up | night sky | sky map | binaries | stars | stargazing