- 3487 - SAGITTARIUS A* - our galaxy’s blackhole? As you read this you are 26,000 lightyears from the blackhole at the center of our galaxy. The radius is 50,000 lightyears, making the diameter 100,000 lightyears. You are halfway to the edge. But, that edge is surrounding by a massive amount of Dark Matter that we don’t understand. Whatever it is it is holding the galaxy from flying apart holding the stars at a near constant orbital velocity.
--------------------- 3487 - SAGITTARIUS A* - our galaxy’s blackhole?
- Astronomers are studying the blackhole at the center. They are measuring its size by the speed and orbits of stars circling close to the center.
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- Recently discovered is a star that has shifted its orbit around the galaxy’s central blackhole. The result of the math matches a prediction from Einstein’s framework of gravity and gives us a new window on the way blackholes warp the spacetime around themselves.
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- This star is one of dozens that huddle close around the Milky Way’s supermassive blackhole, named Sagittarius A*. But at 26,000 light-years away and veiled by giant clouds of dusty gas, these bright stars are invisible to all but the most powerful infrared telescopes on Earth.
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- Astronomers have been assailing the blackhole’s fortress with a series of increasingly advanced techniques and equipment. The stars close to the blackhole zoom through a gravitational environment like nothing we can probe in the solar system.
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- One star in particular is a key informant: a bright, massive B-type star called “S2”. This star comes closer to the blackhole than the rest, some 120 astronomical units, or four times Neptune’s distance from the Sun.
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- It comes so close that it actually dips into the well the blackhole makes in the fabric of spacetime. Its 16-year, elongated orbit enables astronomers to test two predictions of Einstein’s general theory of relativity (GR): gravitational redshift and orbital precession.
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- Using observations covering S2’s most recent pass by the blackhole in 2018, both teams successfully detected gravitational redshift: the star’s light temporarily reddened as it dove through the gravitational well.
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- Now, the MPI team says it has measured orbital precession. In General Relativity, gravity behaves differently very close to a massive object than it does in Newtonian gravity. The massive object warps spacetime in a way that Newtonian gravity doesn’t account for.
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- When a star moves through this warp, its path shifts slightly, changing its incoming angle a tiny amount. That moves the location of its closest pass by the blackhole. The effect of moving the “periapse” also shift the orbit. Over time, it will draw a rosette around the central object, instead of a single, closed ellipse.
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- We see this orbital precession in the solar system. Mercury precesses around the Sun, largely due to the other planets’ influence. But it took Relativity to explain a puzzling “extra” amount of precession that Newtonian gravity couldn’t account for. Providing a solution to the innermost planet’s motion helped convince both Einstein and other scientists that his framework of gravity was correct.
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- To see this precession, the MPI team combined more than 300 image and spectral measurements, spread over 19 years and done with a variety of instruments. Taken together, these observations trace S2 as it flew through more than a full circuit around Sgr A*. During its closet approach, the star hurtled past the blackhole at some 7,700 km/s, or more than 2% the speed of light.
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- By comparing S2’s movement to the coordinate system built of the galactic center, the researchers detected a precession of 12 arcminutes (0.2 degree). This number matches the prediction from General Relativity, exactly.
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- One notable implication from the MPI team’s measurements: There cannot be a second, massive blackhole closely orbiting Sgr A*. Once Sgr A*’s mass is accounted for, S2’s motion precludes anything bigger than 1,000 solar masses from hiding inside its orbit.
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March 3, 2022 SAGITTARIUS A* - our galaxy’s blackhole? 3487
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