Orbiting very close to our Galaxy's central black hole

[lpetrich]

In the center of our Galaxy is a black hole, an object so compact that it has an (approximately) spherical region of no return around it. That black hole is Sagittarius A* - Wikipedia identified as one from how fast its nearby stars move around it - much faster than what one would expect from the amount of stars nearby.

Its mass is huge, about 4.3 million times that of the Sun, and its Schwarzschild black-hole radius is 12.7 million kilometers or 0.0847 astronomical units (AU: mean distance of the Earth from the Sun). That radius of the Sun is 3 kilometers and of the Earth is 9 millimeters. That was determined by watching stars near it, and a very close one was recently discovered:

S301 - Wikipedia and Fastest known star could reveal the spin of our galaxy’s giant black hole | Science | AAAS and Discovery of a star sensitive to the spin of Sagittarius A* | Nature

S301 orbits Sgr A* with a period of 8.68 years, and its distance goes between 12 and 1400 AU. At its closest distance, it travels at 25,000 km/s or 8% the speed of light, though it spends only about a week near its closest distance.

The previously known closest star was S2 (star) - Wikipedia or S0-2. It has a period of 16 years and its minimum and maximum distances are 120 AU and 1900 AU. Its greatest orbit speed is only 7,700 km/s or 2.6% of the speed of light.

S301 is getting some publicity because it orbits close enough to Sgr A* for the gravity of that black hole's spin to cause measurable orbit precession if that spin is large enough. In general relativity, not only mass but also spin makes gravity, in the form of dragging space-time: frame dragging. This dragging made by the Earth's spin caused some very tiny but still measurable precession of the spins of some rotating high-precision spheres of the Gravity Probe B satellite. This dragging also causes orbits to precess, but that is a very tiny effect unless one is close to a rapidly-spinning black hole or neutron star, as S301 is.

Precession is cumulative, so in coming years, it should become more evident, and we might then get some results on the spin of our Galaxy's central black hole.

 

[lpetrich]

Let's see what Sagittarius A* would look like if one was near it. I will take the Earth's distance from the Sun for definiteness. To stay in that orbit, one would have to move at 20% of the speed of light and have a "year" of 11 hours.

What would the black hole would look like? With essentially nothing coming out, completely black. How big would it be in the sky? One has to look at what happens to the light of surrounding stars as that light passes near the BH. It would get deflected more and more the closer and closer it passes the BH. A star directly behind the BH would have light go around it on all sides, making an "Einstein ring" around the BH. Further inward, and one sees an Einstein ring from a star directly behind, then one from a star in front, then behind, ... the angular distance between them exponentially declining, giving a limiting size of 2.6 of the Schwarzschild radius's size that one observes.

That first Einstein ring, the outermost one, one would observe around *any* massive object if one was far enough away, while the next ones require the lack of obstruction of a BH.

Let us follow the image of a star as a BH moves past it. It will get more and more deflected and brighter and brighter, until it becomes an Einstein ring. Then it will shrink back down, inside the ring distance, on the opposite side of the BH. It will then shrink and become faint. When the star is on the opposite side from the BH, it will make another Einstein ring, faint and close to the limit. It will then repeat that, making closer and closer and fainter and fainter Einstein rings.

 

[Loren Pechtel]

Curious, looking the other way. What are conditions like on a planet that is far enough out to have only completed 6,000 revolutions in the age of the universe? And would it be bound tightly enough not to be ripped off? (Obviously it would be in a perilous place as the S-stars would cross it's orbit.)