Black Hole

Black Hole near a star cluster captured whit SpaceEngine.

A stellar Black Hole is a "Compact Star", it has a mass greater or equal to 2.5 M☉ (greater to 2.5 times the mass of the sun), and has a diameter of a few kilometers, therefore, is a extremely dense object,  this means it has an extremely strong gravitational field, so that everything that gets close enough to this is trapped by its field and is absorbed.

Black Holes have a very dark color, but they´re not completely black, this is due to emissions of Hawking radiation in their "Event Horizon" 

Event Horizon is the limit of the Black Hole, beyond that nothing can leave, don't even photons (Light).

This is the life cycle of different stars, the most massive are those that become  Black Holes

Types of Black Holes:

If they are classifieds according to their mass, exist three types of Black Holes:

1. Supermassive Black Hole: They have a mass of millions of solar masses and are at the center of all galaxies.
2. Stellar Black Hole: They are black holes that are part of the life cycle of the stars of a mass equal to 30-70 solar masses (After the supernova a big star loses some of its mass and the remainder is pressed into this type compact star).
3. Micro Black Holes: They are the smallest Black Holes, their diameter is up to 1 mm (millimeter), however they are also very dense.

The Black Hole at the center of the Galaxy

Milky Way is a spiral galaxy, it is formed by clouds of gas, cosmic dust, dark matter, energy, planets and stars all these structures are linked by the existence of gravity and they revolve around the center of the galaxy, also The Sun revolves around the center of the galaxy at a speed of 220 Km/s.

If the structures that exist in the galaxy revolve around galactic center then what about the center?

Catch of the accretion disk of Sagittarius A * taken from 

the space simulator spaceengine.

In the center is a supermassive black hole its name is Sagittarius A* (Sgr A*).
Sgr A* has a mass of 4.31*10 (Exponent 6) Solar masses, that is to say, 4 million times the mass of The Sun. it has never been seen (because it doesn't emit radiation in the form of light), but it exist by the mysterious movement of the nearest stars, and of all the Galaxy.

In the Center of the galaxy there are many stars, and emits large amounts of radiation.

The movement of the accretion disk of Sagittarius A * and its stars, along with the black hole recorded from spaceengine looks like this: