When telescopes were first trained on the stars, an interesting discovery was made: not all the stars we see as single points of light with our eyes are in fact alone. Some were revealed to be two stars or maybe even more. Double stars and multiple star systems were discovered.
As the number of double stars being found grew, it became necessary to divide the category up further to clarify exactly what sort of double star it was.
To understand the first category, optical doubles, imagine the true 3D nature of space with stars sprinkled all over the place.
Read more about stars:
From our viewpoint, one star may appear very close to another star, but this is only because the two stars happen to lie in the same direction from us in space. In fact, these stars are not linked in any way.
One of them could be much further away from us than the other, but stargazing-wise, we have no way of knowing, because everything in the night sky looks the same distance away from us.
Double star Albireo in Cygnus by Houssem Ksontini.
Then there are the double stars that are linked by gravity. If you see one of these you’re looking at a binary star.
It’s no coincidence that the stars of a double appear to be in the same place: they are both the same distance from us and orbit around each other.
It’s estimated that perhaps half of the stars in our Galaxy are binaries, although binaries account for only 5% of stars observed so far.
Unless your stargazing app or star atlas tells you so, simply by gazing at the sky there is no way of telling whether you’re looking at an optical double or a binary.
Only with the careful study of the movements in a double star can we gauge whether the stars are gravitationally bound to each other or not.
Two young stars in a binary system, known as binary proto-stars, as seen by the ALMA telescope, 4 October 2019. Credit: ALMA (ESO/NAOJ/NRAO), Alves et al.
If you’re looking up at a binary star system, it’s fascinating to know what could be happening with the stars themselves.
This is because sometimes the stars in a binary system can interact – especially when one of the stars is more massive than the other.
In this case gas can be pulled off the smaller companion, which can lead to tremendously destructive stellar explosions called supernovae.
Of course, you won’t see any of this going on when you look through a telescope, but double stars are still amazing to aim at.
Some doubles show startling colour differences between the two stars, for example a shimmering yellow next to a vivid blue, while with other double stars, the two will be more or less the same brightness, yet sit startlingly close together.
For more on this, read our guide to star colours.
A small telescope should enable you to pick out some beautiful double stars. Credit: Pete Lawrence
Seeing double stars through a telescope
You can use double stars to test your telescope’s optics. How well you can split the stars depends on the quality of your optics, as well as the size of your telescope’s aperture, or front lens.
If you have a small, good quality telescope, say 4 inches in diameter, you should be able to see doubles up to 1.15 arcseconds apart (if seeing conditions are perfect). Our top five doubles should all be within your reach.
To split double stars closer than this, you need a bigger telescope. To find out the closest double stars a telescope will theoretically split, you just divide 4.6 by the diameter of the telescope’s front lens in inches.
It’s theoretical because if the atmosphere is fairly turbulent then you won’t be able to see the components of
a really close double star as well.
If you can spot our top five favourite doubles, you’ll soon be hooked on these jewels of the night sky.
5 double stars to spot in the night sky
Albireo is a lovely golden and blue double that’s a binary star system. The golden component is mag. +3.1, while the blue member is mag. +5.1. You’ll need a scope to see the pair.
The third brightest star in Andromeda is Almach. It has a brighter yellow star of mag. +2.3 close to a mag. +5.1 greenish companion. To resolve them you’ll need to use a telescope.
The Double Double
Epsilon Lyrae is a naked eye double star where both yellowish stars have a similar brightness of around mag. +5.5. However, with a scope you’ll see that both parts have their own binary star.
Mizar and Alcor
Constellation Ursa Major
Zeta and 80 Ursae Majoris are in fact an optical double. The ability to see the two white stars, mags. +2.2 and +4.0 respectively, with the naked eye is a traditional test of good eyesight.
Phaeo and Phaesyla
This orange and white optical double is easily visible to the naked eye, with mags +3.8 and +3.4 respectively. Also called Theta Tauri, it is part of the Hyades star cluster.
This guide originally appeared in the January 2009 issue of BBC Sky at Night Magazine.