A binary star is actually a pair of stars that orbit each other in their own solar system. With the naked eye they look like a single point of light, but using a powerful telescope you might be able to tell them apart.

Sirius, the brightest star in the night sky, was discovered to be a binary system this way. Most binary stars, however, can only be distinguished using careful analysis of data. Binary stars can orbit each other at a vast range of distances. On one extreme, a pair can be separated by a distance almost as large as our entire solar system.

The shape of their orbit all depends on their relative sizes. If the two stars have the same mass, they will orbit a point halfway between them. The more one outweighs the other, the closer the barycentre will get to the heavier star. In an extreme case, this can result in the smaller star orbiting the larger one like a planet.

Planets themselves also orbit binary stars, in two distinct manners. An S-type orbit is when a planet closely orbits one star, virtually ignoring the other. Alternatively, the planet can travel in a loop encircling both stars, known as a P-type or circumbinary orbit.

Binary stars are generally born as twins, although occasionally they can 'adopt' a partner. All stars are formed when gravity causes massive clouds of dust to collapse in on themselves. During this process, the cloud can separate into multiple clumps of matter, each of which becomes its own star.

These continue to orbit around each other, leading to the vast majority of binary systems. The remainder occur when two fully-formed stars drift close to each other. Having two stars so close together can lead to some strange effects, many of which astronomers are only observing now for the first time.

In 2016 a 'vampire' star was caught stealing a large amount of gas from its partner, an act that made it briefly shine over a thousand times brighter than usual. This event was recorded "by accident" by the Kepler Space Telescope during an unrelated study, so it took three years for the findings to be properly studied and announced in October last year.

More recently, the discovery of a teardrop-shaped binary star was reported in March. This star is pulled into its unusual shape by its twin's gravity. Other binary stars have been recorded that go through cycles of squashing and stretching as they orbit, but this is our first example of one being permanently deformed. They may seem outlandish to us, but binary stars are more the rule than the exception in the universe.