A black hole is an area in space so compact, with a gravitational pull so strong that light (with a speed of 300,000 km/s) isn't fast enough to escape if it ventures too close.
Black holes are produced by the death of large stars:
During the stars life there is a tug-of-war between the gravity pulling the stars mass inward and the pressure internally pulling the matter out.
At the end of the stars life, it runs out of internal nuclear fuel and so the gravitational pressure externally causes the star to collapse.
When large stars collapse (10 - 20 times the size of our Sun), scientists say that the core compacts into a point with virtually zero volume but with infinite density hence forming a black hole.
Neutron stars are formed if the collapse of the large star is halted.
It is only when an object ventures too close to the black hole is it "vacuumed up".
The Event Horizon is the distance from the black hole at which the velocity required by an object to escape the black hole is just equal to the speed of light.
Objects at a distance, outside the event horizon, which were orbitting the star prior to its death, continue to experience the same gravitational pull after its death.
This is because the mass of the star doesn't change - only its size changes and hence its density.
Black holes are invisible because light cannot escape the event horizon:
Black holes are known to exist because of their effects on surrounding stars and gas.
When the black hole is created, and the stars and gas swirl around it, friction is produced, heat eminates and X-rays are emitted.
This radiation emission allows scientists to measure the matters' heat and speed --> inferring the presences of the black hole.
The Accretion Disk is the hot matter swirling around the black hole within the event horizon.
There are three types of black holes:
Stellar
Produced by the death of large stars as described previously.
Supermassive
Its mass is equivalent to billions of suns
Located in the centre of galaxies where stars and dust are tightly packed.
When a black hole forms in galaxy centres, it will become more and more massive as its gravitational pull within the event horizon captures surrounding matter and adds their mass to its own.
Some Supermassive black holes are distinctive as they have two jets of hot gas perpendicular to the accretion disk, thought to be caused by the interaction of gas particles with the strong magnetic field of the supermassive black holes --> there is observational evidence of this with the Hubble Space Telescope in the galaxy M84.
Miniature
No observational evidence for the presence of miniature black holes.
Might have been formed shortly after the Universe was created 15 billion years ago.
It is hypothesised that the rapid expansion of matter during the "Big Bang" might have compressed slower moving matter enough to make them contract into black holes.
The Hubble Space Telescape can search for black holes using an instrument called the Space Telescope Imaging Spectrograph (STIS)
The Spectrograph uses defraction of incoming light to split light into its rainbow pattern.
