Double and multiple stars

The Sun is a star on its own. A great many stars are not. They come in pairs, each one going round the other, held together by gravity. A pair like this is called a double star, or a binary.

Some families are bigger. There are stars in threes and fours, and astronomers have found as many as seven in a single system.

The nearest stars to the Sun are one of these families. Alpha Centauri looks like one bright star, but it is really two stars close together, with a third, small and faint, circling them a long way out. That third one is Proxima, the nearest star of all.

Sirius, the brightest star in the night sky, is a pair as well. Its partner is a white dwarf, the small, dense leftover of a star that has already died. The two go round each other once every 50 years.

Planets can live in these families too. One, called Kepler-16b, goes round two stars at once. From there you would see two suns in the sky.

A binary star is two stars in orbit round each other. Neither one stands still in the middle. Both go round the balance point between them, the barycentre, and the heavier star makes the smaller circle.

How common they are. A careful survey of 454 stars like the Sun, all close enough to study well, found these shares:

Kind of system	Share
One star alone	56%
Two stars	33%
Three stars	8%
Four or more	3%

So a little under half of these systems hold more than one star. Count stars and not systems and the picture shifts. Out of 100 such systems, the 56 single ones hold 56 stars, and the other 44 hold at least 102 between them. That sum is ours, from the survey’s figures: about two in every three stars like the Sun have a partner.

Three ways to find a pair.

• See both. If the two are far enough apart, a telescope shows them as two points, and over the years one can be watched going round the other.
• Watch the light dip. If the orbit happens to lie edge on to us, each star passes in front of the other in turn, and the light of the pair drops for a while each time. The size and length of the dips give the sizes of the stars.
• Watch the colour shift. A star swinging towards us has its light shifted very slightly towards blue, and towards red as it swings away. A star whose light rocks back and forth like this has a partner, even when no telescope can split the two.

Weighing stars. A star on its own gives no direct sign of how heavy it is. A pair does. The wider the orbit and the shorter the time taken to go round it, the stronger the pull that must be holding the two together, and so the more mass there must be. Measure the size of the orbit and the time for one turn, and the two masses added together follow.

Alpha Centauri A and B show how it goes. They take 79.91 years to go round each other. Worked out from the measured orbit and distance, they are about 24 times as far apart, on average, as the Earth is from the Sun, which would put one of them between Uranus and Neptune if the other sat where the Sun is. An orbit of that size and that length needs about twice the Sun’s mass to hold it. Watching how far each star moves then shares the total out: 1.13 Suns for A and 0.97 for B.

Orbits like this are the direct way to weigh a star.

Sirius and its dead partner. Sirius A, the bright one, has 2.06 times the Sun’s mass. Sirius B goes round it every 50.13 years. It is a white dwarf, the nearest one to us, with about as much mass as the Sun (1.02 times). The heavier a star is, the faster it burns out, so B is thought to have started out as the heavier of the two. It has finished its life already while A shines on.

A wide third star. Proxima belongs to Alpha Centauri, but only just. It is now about 13,000 times the Earth’s distance from the Sun away from the bright pair, more than 400 times as far as Neptune is from the Sun. One turn takes it about 550,000 years.

When two stars are very close. If one star of a close pair dies and leaves a white dwarf, a neutron star or a black hole, the strong gravity of that small, dense body can pull gas off its partner. The gas is heated to more than a million °C as it falls and glows in X-rays. Pairs like this are called X-ray binaries. And where both stars have ended as neutron stars, the two can spiral together and collide. Such collisions make some of the heavy elements, gold and platinum among them.

Planets with two suns. Kepler-16b is a planet about the size and mass of Saturn. It goes round two small stars together, once every 229 days, while the two stars circle each other every 41 days. Planets are also found going round just one star of a pair, with the other star far off. The survey above found planets about as often round stars with distant partners as round stars alone.

The record so far. As many as seven stars have been seen in one system. One with six, called TYC 7037-89-1, is built from three pairs. Two of the pairs go round each other about every four years, and the third goes round those four about every 2,000 years.

What the 3D view shows. In the view of the Sun’s neighbours, Alpha Centauri and Sirius are each drawn where the catalogue puts them. At that scale, where the whole picture is light years across, the two stars of each pair sit far too close together to tell apart. Proxima is the exception: it stands a little way off from Alpha Centauri, as it really does.