There are massive black holes and there are supermassive black holes. There are even ultramassive black holes.
And yet, we so rarely ponder the little ones. It's not as if a black hole that isn't, say 40 billion times as massive as our sun — like the ultramassive Holm15A* — doesn't have its own strange and spellbinding properties.
But only recently have scientists begun looking for black holes on a much smaller scale. And surprise, surprise, it didn't take long to find one.
In fact, the latest black hole, discovered by researchers at the Ohio State University, may be the smallest one detected yet.
Although, theoretically, a black hole could be microscopic in size, the black hole this team discovered is far from pocket-sized.
Publishing the results this week in the journal Science, researchers note the black hole is roughly 3.3 times as massive as our own sun — and inhabits a binary system on the fringe of our Milky Way galaxy, about 10,000 light-years away.
"It's always interesting in astronomy when you look in a new way, and you find a new type of thing," lead author Todd Thompson, an astronomy professor at Ohio State, tells Vice. "It makes you think that all your ways of looking before were biased."
Indeed, previous methods for hunting black holes may have been strongly slanted toward the heavier contenders. So far, those we've been able to detect are, on average, between five and 15 solar masses. But that's not necessarily the average size for a black hole — just the size that we've found. That's for the simple reason that when it comes to these matter-hoovering bodies, bigger is easier to find.
Supermassive black holes, like the one at the heart of our galaxy, make for disruptive neighbors — hoovering up all surrounding matter, including errant stars, with gleeful abandon. It's not hard for earthling astronomers to spot a black hole's culinary ravages — or rather the crumbs left around its mouth in the form of a radiant accretion disk.
Small black holes, on the other hand, aren't nearly as obvious, munching quietly in their corner of the cosmos and producing much less X-ray radiation for scientists to zero in on. As a result, when known black holes are tallied, the heavyweights are disproportionately represented.
But smaller rifts may be able to teach us a lot more about our universe.
"People are trying to understand supernova explosions, how supermassive black stars explode, how the elements were formed in supermassive stars," Thompson explains in a news release. "So if we could reveal a new population of black holes, it would tell us more about which stars explode, which don't, which form black holes, which form neutron stars. It opens up a new area of study."
The new discovery fills a longstanding gap on the scale of time- and space-bending anomalies. On one end, there were massive (and even more massive) black holes. On the other end were neutron stars — the cores of giant stars that collapsed on themselves. Neutron stars eventually grow into black holes, but they typically begin their existence at around 2.5 solar masses.
But the spectrum was notably blank in the middle. Where were all the small-ish black holes?
To find them, Thompson and his team relied on data from the Apache Point Observatory Galactic Evolution Experiment, or APOGEE. That installation, based in New Mexico, records light from more than 100,000 stars in our galaxy.
Researchers used APOGEE data to determine if light shifts from one star in a binary system indicated the presence of an otherwise invisible companion — a decidedly darker companion.
Under that scrutiny, the smallest known black hole made itself known, and the wealth of knowledge it contains will likely lead scientists to cast an even wider net for more of its black hole brethren.
"What we've done here is come up with a new way to search for black holes, but we've also potentially identified one of the first of a new class of low-mass black holes that astronomers hadn't previously known about." Thompson explains. "The masses of things tell us about their formation and evolution and they tell us about their nature."