Page 72 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

DOUBLE STARS

THE SIGHT OF TWO STARS PARTNERED TOGETHER CAN BE TRULY
STUNNING, ESPECIALLY WHEN THEY HAVE VIBRANT COLOURS. BUT
DON’T CONFUSE YOUR BINARIES WITH YOUR OPTICAL DOUBLES…

After the invention of the being found grew, it became in the night sky looks the same sky there is no way of telling
telescope in the early 17th necessary to divide the distance away from us. whether you’re looking at an
century, the true nature of the category up further, in order Then there are the double optical double or a binary. Only
night sky became apparent for to clarify exactly what sort of stars that are linked by gravity. with the careful study of the
the fi rst time. What had been double star it was. If you see one of these you’re movements in a double star can
mere fuzzy blobs as seen by the To understand the fi rst looking at a binary star. It’s no we gauge whether the stars are
unaided eye now had form, and category, optical doubles, coincidence that the stars of a gravitationally bound to each
suddenly a whole new world imagine the true 3D nature double appear to be in the same other or not.
of nebulae, galaxies and star of space with stars sprinkled place: they are both the same
clusters could be observed. all over the place. From our distance from us, and they INTERACTING STARS
When those fi rst telescopes viewpoint, one star may appear orbit around each other. It’s If you’re looking up at a binary
were trained on the stars, very close to another star, but estimated by some scientists star system, it’s fascinating to
an interesting discovery was this is only because the two that perhaps half of the stars know what could be happening
made: that not all the stars stars happen to lie in the same in our Galaxy may be binaries, with the stars themselves.
we see as single points of light direction from us in space; in although binaries account This is because sometimes the
with our eyes are, in fact, alone. fact, these stars are not linked for only fi ve per cent of stars stars in a binary system can
Some were revealed to be two in any way. One of them could observed so far. interact – especially when one
stars or maybe even more. be much further away from us So how do you know which of the stars is more massive
Double stars and multiple star than the other, but stargazing- is which? Well, unless your than the other. In this case, gas
systems were discovered. As wise, we have no way of magazine or star atlas tells can be pulled off the smaller
the number of double stars knowing, because everything you, simply by gazing at the companion, which can lead

JOHN CHUMACK/SCIENCE PHOTO LIBRARY, PETE LAWRENCE X 4, WILL GATER, PAUL WHITFIELD TOP 5 DOUBLES TO OBSERVE

1. ALBIREO 2. ALMACH 3. THE DOUBLE 4. MIZAR
Constellation: Cygnus Constellation: Andromeda DOUBLE AND ALCOR
Albireo is a lovely golden The third brightest star in Constellation: Lyra Constellation: Ursa Major
and blue double that’s a Andromeda is Almach. It’s To the naked eye, Epsilon Zeta and 80 Ursae Majoris are
binary star system. The golden made up of a brighter yellow Lyrae’s two yellow stars have a an optical double. The ability to
component is mag. +3.1, while star of mag. +2.3, close to a similar brightness of mag. +5.5. see the two white stars, mags.
the blue member is mag. mag. +5.1 greenish companion. However, with a scope you’ll +2.2 and +4.0, with the naked
+5.1. You’ll need a scope to To resolve them you’ll need to see that each part in fact has its eye was a traditional test of
see the pair. use a telescope. own binary companion. how good your eyesight was.
72 skyatnightmagazine.com 2012