'Dark stars': Dark matter may form exploding stars, and observing the damage could help reveal what it's made of

Dark matter is a ghostly substance that astronomers have failed to detect for decades, yet which we know has an enormous influence on normal matter in the universe, such as stars and galaxies. Through the massive gravitational pull it exerts on galaxies, it spins them up, gives them an extra push along their orbits, or even rips them apart.

Like a cosmic carnival mirror, it also bends the light from distant objects to create distorted or multiple images, a process which is called gravitational lensing.

And recent research suggests it may create even more drama than this, by producing stars that explode.

For all the havoc it plays with galaxies, not much is known about whether dark matter can interact with itself, other than through gravity. If it experiences other forces, they must be very weak, otherwise they would have been measured.

A possible candidate for a dark matter particle, made up of a hypothetical class of weakly interacting massive particles (or WIMPs), has been studied intensely, so far with no observational evidence.

Recently, other types of particles, also weakly interacting but extremely light, have become the focus of attention. These particles, called axions, were first proposed in late 1970s to solve a quantum problem, but they may also fit the bill for dark matter.

Unlike WIMPs, which cannot "stick" together to form small objects, axions can do so. Because they are so light, a huge number of axions would have to account for all the dark matter, which means they would have to be crammed together. But because they are a type of subatomic particle known as a boson, they don't mind.

In fact, calculations show axions could be packed so closely that they start behaving strangely—collectively acting like a wave—according to the rules of quantum mechanics, the theory which governs the microworld of atoms and particles. This state is called a Bose-Einstein condensate, and it may, unexpectedly, allow axions to form "stars" of their own.

This would happen when the wave moves on its own, forming what physicists call a "soliton", which is a localized lump of energy that can move without being distorted or dispersed. This is often seen on Earth in vortexes and whirlpools, or the bubble rings that dolphins enjoy underwater.

The new study provides calculations which show that such solitons would end up growing in size, becoming a star, similar in size to, or larger than, a normal star. But finally, they become unstable and explode.

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