In 1974 scientists discovered a very compact source of radio waves originating from a region in the Sagittarius constellation, 26,000 light-years from Earth. Dubbed Sagittarius A* (Sgr A*), the source is now known to be a supermassive black hole at the center of our galaxy weighing more than four million suns.
Though tranquil today, Sagittarius A* may have fed on a star or gas cloud a hundred times the mass of the sun as recently as 20,000 years ago. The meal produced x-ray jets blasting outward from the black hole’s poles, which are tilted 15 degrees from the plane of the galaxy.
A black hole’s spin can twist space, speeding or slowing matter orbiting nearby. The static limit is the orbit where objects traveling at light speed against the black hole’s spin seem to stand still.
The event horizon, extending some eight million miles around Sgr A*, is the boundary beyond which even light cannot escape the black hole’s gravity.
According to Einstein’s equations, at the center of a black hole a star’s entire mass has collapsed into an infinitely dense, dimensionless point called a singularity. Singularities likely don’t really exist but point to a mathematical hole in our understanding of gravity.
A whirling disk of superheated gas and dust likely spins at near light-speed around Sagittarius A*. The disk emits heat, radio noise, and x-ray flares but is placid compared with accretion disks in other galaxies.
Sources: Avery Broderick, Perimeter Institute for Theoretical Physics, University of Waterloo, Canada; UCLA Galactic Center Group