A few years ago, a detector on the International Space Station spotted something unexpected: atoms of anti-helium, the antimatter version of helium, flowing freely in space. Only this element wouldn’t make your voice squeakier, it would release a burst of gamma rays after coming in contact with helium.
Anti-helium has been created in the lab, but having a candidate detection in space (presented at a conference, but yet-to-be peer-reviewed) opens up an intriguing but remote possibility. There could be enough antimatter in the universe to not only clump up but also to make stars. Well, actually, antistars.
The idea is outlandish but intriguing. Everything we touch and see is made of matter, and it has been like this since almost the very beginning of the cosmos. We are not exactly sure why this is the case, but some researchers consider that maybe some antimatter survived and then collapsed into stars, those stars then producing and releasing antihelium that eventually might have been detected by the Alpha Magnetic Spectrometer 2.
So, apart from a possible detection of anti-helium, is there any other evidence of the existence of antistars? Not yet, but a paper in Physical Review D has put some constraints on how many of these antimatter objects might exist around the Milky Way.
Antimatter and matter don’t get along. When they encounter each other, they annihilate, turning into gamma-rays. So Dr Simon Dupourqué and colleagues thought to look at observations from NASA’s Fermi gamma-ray observatory. Going through ten years’ worth of data, they found 14 sources that are not associated with known objects. The sources had to match the profile that antimatter annihilation would produce.
This is obviously not a discovery of antistars, but more like an intriguing discussion of, if they exist, how many of them are out there and where we would find them. Surprisingly, if such a cosmic object were to exist it wouldn’t simply be destroyed by matter, as stars don’t tend to encounter enough mass in their lifetimes.
“The amount of accreted matter is very low, and the rate of accretion is linked to the mass of the star: if it accretes matter, then its mass will decrease, which will further decrease the accretion efficiency. So an antistar could not be destroyed only by accreting regular matter both in the galactic disk or in the halo,” Dr Dupourqué, from the Institut de Recherche en Astrophysique et Planétologie (IRAP), told IFLScience.
“However, there is no good explanation on why an antistar could be in the disk, while their presence in the halo could be explained under certain [scenarios] and assuming that they are very old objects.”
If – and that remains a big if – antistars are out there, this work estimates that there shouldn’t be more than one for every 400,000 regular matter stars.