The Centre of our Galaxy
In 1783 the geologist John Michell wrote to Henry Cavendish describing what we think of nowadays as a black hole: "...all light emitted from such a body would be made to return towards it by its own proper gravity." Basically, a black hole is an object that has so much mass that even light (which is the fastest thing in the Universe) cannot escape its grasp. To describe these strange objects we invited Professor Merrifield from Nottingham University to guide us safely around the event horizon.
All bodies have an associated escape velocity, which is the speed that an object, such as a rocket, needs to attain in order to break free from a gravitational field. To leave Earth an astronaut´s spacecraft would have to reach speeds of roughly 11 kilometres per second or 40 thousand kilometres an hour. This is reduced for the Moon as it has less mass and therefore a lower escape velocity (approx. 2.4 km per second) but increases for the more massive Sun (about 42 km per second).
In theory a black hole can have any mass, from the microscopic to billions of times the mass of our Sun. The latter type of black hole is termed `supermassive´ and is thought to exist in the centre of most galaxies, including our own Milky Way. Travelling down the mass scale then comes intermediate black holes with a mass of `only´ thousands of solar masses, followed by stellar-mass black holes which have 3-15 times the mass of our Sun. Micro black holes have yet to be observed and are proposed to have formed at the birth of the Universe spawned by the incredible densities and pressures at the time or in high-energy particle accelerators such as the Large Hadron Collider.
To find black holes astronomers look out for various phenomena. One likely signpost is the presence of a large spinning disc of dust and gas that emits high-energy X-rays; known as an accretion disk. Other indicators are jets of gas that are shot out at speeds approaching that of light from the axes of a galaxy i.e. at right angles to an accretion disc.
One well-studied object that has both an accretion disc and jets is "SS 433". This is thought to consist of a black hole and a blue-white Class A star in orbit around each other in the constellation of Aquila the Eagle. The black hole is cannibalizing the Class A star and, as the material spirals into it, the extreme heating results in the emission of intense X-rays and the formation of opposing jets of hot hydrogen along the axis of rotation zipping along at 26% of the speed of light.
It is very fortunate that we do not live too close to the centre of our galaxy where a supermassive black hole is to be found in a region known as Sagittarius A* (pronounced "A-star"). It has been estimated from the motions of stars´ orbits that it has a mass of 3.7 million solar masses fitting in a radius of 6.25 light-hours and it also has the telltale accretion disc and relativistic jets.