Back in the 1930s, when physicist and engineer Karl Jansky aimed his radio antenna at the heart of our galaxy, he picked up a steady stream of radio waves. Upon digging into the data, scientists figured out that these radio waves were coming from something way farther out in space than the sun. Surprisingly, though, they packed a similar punch in terms of energy to the waves we get from the sun.
Armed with this intel, they started thinking there had to be some serious cosmic muscle flexing in the middle of our galaxy. Eventually, astronomers figured out that the culprit behind those puzzling radio waves was none other than a supermassive black hole, over a million times heavier than our sun. We’ve given it the name Sagittarius A*, or just Sgr A*, and it’s basically the heavyweight champ, acting as the gravitational boss for the entire Milky Way.
Since those early sightings, astronomers have gotten the lowdown on Sgr A*. Because they can keep an eye on it, this black hole gives us the best shot at cracking a fascinating question: Can stars actually pop up around black holes?
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Are stars taking birth near a black hole?
Sgr A* is encircled by a bunch of molecular clouds — those misty spaces in the cosmos where a star might just decide to show up. But astronomers have had this hunch that the closeness of these clouds to the black hole might mess with any budding stellar baby factories. They reckon the intense tidal and electromagnetic forces in the vicinity could throw a wrench into the usual process of gas pockets coming together to birth stars.
“The combination of a low density medium and strong tidal forces by the [supermassive black hole] make it difficult for stars to form in the ‘standard’ way, that is from the collapse of dense gas clouds. They would be torn apart before being able to collapse,” astrophysicist Rosalba Perna from Stony Brook University in New York told Space.com.
But newer observations suggest something interesting: it’s possible that stars are actually being born much closer to Sgr A* than we initially thought. Astronomers have been keeping an eye on stars around Sgr A* for a while now. They’ve been brushing off their existence, thinking maybe these stars initially formed in far-off clusters and just migrated over to the black hole neighborhood.
The hitch with this idea, though, is that many of these recently spotted stars seem too fresh on the cosmic scene to have formed somewhere far away and then cruised through space to reach Sgr A*. Florian Peißker, a postdoc at the University of Cologne’s Institute of Astrophysics, headed up a group of astronomers who pinpointed the young star X3a.
“It turns out that there is a region at a distance of a few light years from the black hole which fulfills the conditions for star formation. This region, a ring of gas and dust, is sufficiently cold and shielded against destructive radiation,” Peißker explained in a statement.
A disk of gas and dust has a part to play
Around Sgr A*, and actually around other supermassive black holes too, there’s this disk made up of gas and dust. It’s basically getting sucked in toward the black hole because of its crazy strong gravitational pull. The specific disk hanging around Sgr A* stretches out from 5 to 30 light-years away from the black hole’s event horizon.
The crew thinks that X3a might have come to life in a gassy wraparound out in the outer ring of the accretion disk circling Sgr A*. Those gas clouds could grow big enough to crumple in on themselves and birth protostars. Scientists have also thrown around a few other ideas about why there are stars chilling so close to Sgr A*.
“The presence of young stars around black holes has made astrophysicists broaden their view of star formation, and various theories have been developed to explain them, such as formation in a disk resulting from the disruption of a molecular cloud, formation in a distant cluster followed by inward migration and shock compression triggered by a tidal disruption event,” says Perna.
Not too long ago, Perna wrote a paper proposing that when tidal disruption events (TDEs) happen near black holes, it might set the stage for star formation. TDEs are basically situations where gravitational chaos kicks in within the accretion disk of a black hole, like when a star takes a nosedive toward it.
A black hole’s age plays a major factor
In these TDEs, things go down in the accretion disk of a black hole in a way that leads to super-packed gas and dust, creating the perfect setup for dense clumps to collapse and give birth to new stars.
According to Perna, how stars form near black holes probably depends on what stage the black hole is at in its life cycle. When a black hole is in its “active” mode, typically in the early stages when the surrounding galaxy is all chaos, it’s wrapped in a big accretion disk of gas and dust.
This swirling disk is like prime real estate for making stars because it gathers up a bunch of super-dense stuff. But nowadays, with the Milky Way getting on in years, everything’s calmed down, and the process of stars popping up around Sgr A* has probably eased up compared to what it was way back when.
Even though black holes still fall into the “cosmic mystery” box, astronomers are figuring out more about how these enigmatic entities play a role in creating new stars and shaping the destiny of their home galaxies.
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Can black holes be used as batteries
Because black holes have this crazy strong gravitational pull where nothing escapes, folks are wondering if we could somehow harness their massive power for energy. In a recent study, scientists tossed out two ideas on how we might tap into black holes for energy down the road. They cooked up methods that bank on the spin and gravitational traits of these cosmic behemoths.
“We know that we can extract energies from black holes, and we also know that we can inject energy into them, which almost sounds like a battery,” lead author Zhan Feng Mai, a postdoctoral researcher at the Kavli Institute for Astronomy and Astrophysics at Peking University, told Live Science.
Picture this: scientists decide to “charge up” the black hole by firing in super-sized electrically charged particles. The study, dropped on Nov. 29 in the journal Physical Review D, says that these charges would keep rolling in until the black hole forms an electric field, basically batting away any more charges they attempt to shoot in. Once the electric push-back outstripped the gravitational pull of the black hole, scientists would give it the nod as “fully charged.” Following Einstein’s theory of general relativity, where mass equals energy, the black hole’s energy would come from both the zapped electrical charges and the mass of those charges.
The scientists did the math and worked out that the recharging gig would be about 25% effective. In other words, these black hole batteries could turn roughly a quarter of the mass they take in into practical energy, forming an electric field. According to the team’s number-crunching, this efficiency would be around 250 times higher than that of an atomic bomb. To extract the energy, the researchers would employ a technique called superradiance. This hinges on the notion that the gravitational field of a spinning black hole essentially drags space-time around its rotation.
Imagine this rotating zone around the black hole like a cosmic whirlpool: gravitational or electromagnetic waves coming in would get pulled along for the ride. But here’s the twist — if they haven’t hit the black hole’s point of no return (the event horizon) yet (the spot where even light can’t break free), some of these waves might bounce back with more energy than they bargained for, say the researchers. This little bounce-back move would essentially convert the black hole’s rotational energy, linked to its mass, into the waves that got pushed away.
