Evidence for black hole
Dust disk around a black hole
This Hubble Space Telescope image contains three main features. The outer white area is the core or centre of the galaxy NGC4261.
Inside the core there is a brown spiral-shaped disk. It weighs on hundred thousand times as much as our sun.
Because it is rotating we can measure the radii and speed of its constituents, and hence weigh the object at its centre. This object is about as large as our solar system, but weighs 1,200,000,000 times as much as our sun.
This means that gravity is about one million times as strong as on the sun. Almost certainly this object is a black hole.
Black hole in M87
M87 is an active galaxy, one in which we see interesting objects. Near its core (or centre) there is a spiral-shpaed disc of hot gas. The first picture places it in context. The second superposes spectra from opposite sides. This allows us to determine the speed of rotation of the disk and its size. From this we can weigh the size of the invisible object at the centre. Although the object is no bigger than our solar system it weighs three billion times as much as the sun. This means that gravity is so strong that light cannot escape. We have a black hole.
In the first figure, there is a diagonal line. This is believed to be the passage out of those fortunate particles which escape along the axis of rotation and avoid being swallowed by the black hole.