Once a black hole forms, its intense gravitational field produces a surface beyond which even light cannot escape, and it appears black to aliens.
All the details of the complex mixture of matter and energy of its past are lost, leaving it so simple that it can be completely described by just three parameters: mass, spin and electric charge.
Astronomers can measure the masses of black holes in a relatively simple way by observing how matter moves around them (including other black holes) under the influence of their gravitational fields.
Black hole charges are believed to be insignificant when positive and negative infallible charges are balanced in number. The spins of black holes are difficult to determine; they are usually determined by interpreting the X-ray emission from the hot inner edge of the accretion disk around the black hole. Spin is quantified as a number between zero and one, and black hole spins have been measured with results ranging from a few tenths to close to one.
The Milky Way galaxy is home to a supermassive black hole (SMBH) at its center, Sagittarius A, with around four million solar masses. At a distance of about twenty-seven thousand light years, it is by far the closest object to us, and although it is not as active or bright as other supermassive galactic nuclei, its Relative proximity offers astronomers a unique opportunity. to probe what’s going on near the “edge” of a massive black hole.
The SMBH galactic center is surrounded by a star cluster and faintly luminous material clusters, and in recent years, astronomers have been able to push general relativity tests to new limits by measuring and modeling the motions of these. clusters when they swing around the SMBH. . The black hole’s rotation, however, has not been consistently determined, but its value would help constrain the jet’s eventual activity patterns.
CfA astronomers Giacomo Fragione and Avi Loeb realized that the spatial distribution of a group of clustered objects, the so-called S stars, could be used to probe spin. There are currently around 40 known S stars that revolve around the SMBH in as little as 9.9 years, and recent analyzes claim that, collectively, they are found in two nearly lateral discs, with the stars in each disc rotating. around the black hole but the opposite. directions.
The two astronomers realized that this unusual geometry could allow an estimated measurement of the spin. One of the most curious and unintuitive predictions of relativity is that space is not only distorted by gravity from a massive body, it is also distorted (albeit to a lesser degree) by rotation. of a body.
This is the so-called “image sliding effect”, a minor and difficult to measure phenomenon (which has, however, been confirmed). The two astronomers show that in the case of SgrA, the frame slip will have an appreciable effect on the orbits of the S stars in these disks.
Assuming that the orbital planes of S stars are stable over time, they are able to show that the spin of SMBH in the Milky Way must be less than about 0.1.
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