A beast is hiding in the center of the Milky Way and it is barely moving.
This supermassive black hole, Sagittarius A * (SgrA *), has a mass 4.15 million times that of our sun. It was first revealed to scientists as a mysterious source of radio waves from the center of the galaxy in 1931; but it wasn’t until 2002 that researchers confirmed that radio waves came from something as massive and compact as a black hole – a feat that brought them 2020. Nobel Prize in Physics. Just days before the team learned about their Nobel on October 6, another group learned something new about the black hole: It spins slower than a supermassive black hole, moving less than (probably far less) than 1
Black holes, despite their formidable power, are extremely simple objects. All the distinctive features of the matter that forms and nourishes them are lost in their infinitesimal features. So each black hole in the galaxy can be described by only three numbers: mass, rotation, and charge.
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Once researchers find a black hole in space, measuring mass is pretty easy – just check how hard its mass is pulling nearby objects. To obtain the mass of SgrA *, scientists have just observed its effect on the “S-stars”, a collection of the innermost stars of the Milky Way, which accelerate to incredible speeds as they fight around the black hole in narrow orbits. And researchers suggest that, like most large objects in space, black holes do not have strong electromagnetic charges.
(Planet Earth, for example, has some positively charged particles and some negatively charged particles, but they cancel each other out across the planet. Other planets and known stars work the same way. Researchers suggest that black holes have a neutral charge.)
This leaves rotation as the remaining measurable characteristic of SgrA *, and researchers now believe they have evidence that the supermass is an unusually slow spinner.
Spin is important for two main reasons.
First, like Previously, Live Science reported, the horizon of the events of the black hole – the shady region in which even the light can not escape the monster gravity —— grows with increasing mass, reaching farther and farther from the singularity of the black hole. But as the black hole spins faster and faster, the event horizon shrinks. Very fast rotating black holes should have smaller event horizons than slower moving black holes with the same mass.
Second, rotation is thought to play a role in the two hot jets of matter that are sometimes launched into space at incredible speeds from the axis of rotation of a black hole. Most galaxies the size of the Milky Way have supermassive black holes at their center, and often these galaxies have huge jets visible exploding from their nuclei.
But the Milky Way has no visible jets. This in itself suggests that SgrA * probably does not rotate very fast. Researchers believe that the rapidly rotating black hole moves the disk with accumulating matter, rotating just beyond its event horizon, accelerating some of this matter to explode like a jet. What scientists know about silent SgrA * already suggests that it either has a small accretion disk, or barely rotates, or both.
In a new article, a team of researchers tried to measure the rotation of SgrA *. Once again, they relied on visible S-stars to learn what their large dark satellite was doing.
Right now, the researchers found, S-stars orbit SgrA * in two orbital planes. If you had to draw their orbits around the black hole and see the system from the side, they would form an X. SgrA * should rotate at less than 10% of the speed of light, they found, because any faster movement already would kill S-stars from their X-shaped orbital planes.
This is because these orbits, the researchers write, are probably as old as the S-stars themselves. The stars still follow the orbits with which they were born. If SgrA * rotated very fast, this would not be the case.
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When heavy objects rotate very fast in space, this rotation affects everything in orbit around them. Over time, this massive object pulls the orbits of these smaller objects, causing them to align more and more with their own direction of rotation of the rotating object. The slower the rotation, the weaker the effect and the longer it takes for these objects to line up in orbit around their strong leader.
The stars are old enough that the rotation would have to pull them noticeably if it was very strong. Because their orbits are as virgin as the day they were born, S-stars strongly recommend an upper speed limit for SgrA * of only one-tenth the speed of light. And it can spin much slower.
This result would also explain why SgrA * does not appear to have visible jets, they write. The first close-up image of the SgrA * shadow, which is expected to come in the near future from another research team, should help confirm this, they write.
The report was published on October 1 in Astrophysical Journal Letters.
Originally published in Live Science.