Rare Supernovae Could Be Spewing Antimatter Across the Milky Way

A type of supernova detected in other galaxies—but as of yet, rarely in our own—could explain why the Milky Way contains so much antimatter.

Antimatter is an umbrella term for a family of antiparticles; for every “regular” particle, there’s a corresponding antiparticle that has the opposite electrical charge but the same mass. When a particle merges with its antiparticle, the two annihilate each other and release a spurt of energy.

An artist’s impression of two white dwarf stars destined to merge and create a Type Ia supernova

Here’s Charles Choi, reporting for Scientific American:

More than 40 years ago, scientists first detected that the kind of gamma-rays that are given off when positrons are annihilated were being emitted from all around the galaxy. Their findings suggested that 10^43 positrons—that’s a 1 with 43 zeroes behind it—were being annihilated in the Milky Way every second. Oddly, most of these positrons were detected in the galaxy’s central bulge rather than its outer disk, even though the bulge hosts less than half of the Milky Way’s mass.

These positrons could have been emitted from radioactive material synthesized by stars. However, for decades, researchers have been unable to pinpoint a type of star that could generate such vast amounts of antimatter. This led to suggestions that many positrons could originate from exotic sources, such as the supermassive black hole thought to exist at the center of the galaxy, or from dark matter particles annihilating one another.

But astronomers hadn’t yet consider supernovae, since most classes of supernova didn’t seem to fit the bill—at least, the kind that scientists could see. A more unusual type of supernova, known as SN 1991bg-like, had largely been overlooked.

This breed of supernova, which results from colliding low-mass white dwarf stars—is difficult to detect because it’s very dim and relatively uncommon. These supernovae generate a radioactive isotope known as titanium-44, which gives off the positrons that cosmologists are seeing in such large quantities throughout the Milky Way’s central bulge.

Roland Crocker, the study’s lead author and a particle astrophysicist at the Australian National University in Canberra, said that this phenomenon could account for most of the positrons scientists observe in the Milky Way. Also, it makes sense that these supernovae—which are more frequent in regions home to older stars—are supplying them, since the galaxy’s central bulge has a greater proportion of older stars than the outer disk.