Look out into the night sky and enjoy the majesty of the heavens.
What you see are countless individual dots of starry beauty.
But did you know some stars come in pairs?
Welcome to the world of Binary stars, little families of stars.
So hop on board your starship and tool about the galaxy to learn more.
Check out the big dipper and behold its beauty.
Now look closely to the star where the handle bends.
No, really close.
If you look close enough to the real constellation (not the photo, sadly) you'll see that it is, in fact, two stars that are very close together. (For a time, being able to see them was considered a test of good eyesight. However, even a person with poor vision can see them in the right conditions.) These stars are named Alcor and Mizar.
It turns out that these stars are bound together by gravity and move in orbits together.
These are called binary stars.
But their motion is much different than one usually expects. Our usual picture is of a fixed sun with planets going about it.
But this is actually a special case, a case where tiny planets surround a giant sun. But with binary stars things are different – you have two big stars (suns) pulling at each other in a massive tug of war.
Because of this, neither star can simply go around the other. Instead, they both are in orbit around a common point called the Barycenter (the center of mass in proper terms).
Depiction of a pair of binary stars and their paths. Both are in orbit around a fixed point called the Barycenter. (the tiny asterisk symbol)
It is a strange setup that makes one wonder what life would be like living on a planet in such a system.
Our neighbors, and beyond
So what are some examples of Binary star systems? Here are a few of the more well-known ones:
Sirius – also known as the Dogstar. It is one of the star systems closest to the Earth, 8.7 light-years away. (It takes light from this star 8.7 years or about 104 months to reach us.) The two stars orbit each other at about the same distance between the Sun and Uranus, taking about 50 years to do so.
Spica – the most noticeable star in the constellation Virgo. This pair is 260 light-years from the Earth, and the stars are so close that they orbit each other in a speedy four days.
Polaris – the North star. This system is likely to be over 400 light-years away (we don't have a precise distance yet). But there is a bit more to this system, as we shall see.
Rigel – the bright blue star that marks the foot of the constellation Orion. This system is out there, 860 light-years away. Like Polaris, Rigel also has something more to it.
Our nearest star system, Alpha Centauri, is 4.3 light-years away. It takes light about 52 months to reach us. It's a binary system – plus some.
Cheaper by the dozen
Alpha Centauri and Polaris do, in reality, have three stars in their systems. In Alpha Centauri's case, the first two stars (Alpha Centauri A and Alpha Centauri B) orbit close to each other while the third (Alpha Centauri C or Proxima Centauri) orbits at a distance.
These are examples of a larger group of binary systems called a multiple star system. Here three or more stars move together on what can be an intricate orbital dance. Rigel is an example with four stars in it.
And it can get a bit crazier.
Let's return to the pair that started it all, Alcor and Mizar. Later it was observed that each are, in fact, binary stars themselves with their own, separate star companion.
Ah, but hold on!
Both of the stars in the Mizar have a companion of their own. This means that, in all, there are a grand total of six stars doing an elaborate orbital waltz.
But even in this craziness, there is order. These stars don't just swarm around in random, wild ways. For example, in the Rigel system, we have two stars orbiting each other. A third star orbits this pair, and the fourth orbits this threesome. Each one paired off with its partner, no mayhem.
There is a great lesson here about the world around us, from the stars down to the ordinary. While life can sometimes seem random, and with rhyme or rhythm, the world around us surrounds us with examples of order behind it.
There is comfort in this.
On the web
Binary star orbit simulation test
This is a short video of a simulation of a pair of binary stars in orbit about each other. I like this as it gives a real feel of how binary stars move.
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