Black Holes ain't so black

Black holes are among the strangest things in the universe. They are massive objects – collections of mass – with gravity so strong that nothing can escape, not even light. The most common types of black holes are the stellar-mass and supermassive black holes. Stellar-mass black holes are created when massive stars explode, leaving behind a black hole with the mass of just a few suns. Supermassive black holes exist in the hearts of galaxies and usually contain the mass equivalent of millions of suns.

Famous Black Holes

Cygnus X-1: a stellar-mass black hole and x-ray source that lies some 6,500 light-years away. It is a binary system that contains a blue supergiant variable star and the x-ray source thought to be the black hole.

Sagittarius A*: the supermassive black hole at the heart of the Milky Way Galaxy. It lies in the direction of the constellation Sagittarius. This black hole contains the mass of about 4 million suns.

M87: this elliptical galaxy has a 3.5 billion solar-mass black hole at its heart. The black hole is surrounded by a disk of superheated material and has a jet of superheated material streaming away from the black hole that extends across 5,000 light-years from the galaxy’s core.

Centaurus A: this galaxy, which lies in the direction of the constellation Centaurus, is a giant spiral galaxy with an incredibly active nucleus. It contains a 55 million solar-mass black hole at its heart, with two jets of material that stream away from the galaxy at about half the speed of light across a million light-years of space.

Facts about Black Holes

• The massive gravitational influence of a black hole distorts space and time in the near neighbourhood. The closer you get to a black hole, the slower time runs. Material that gets too close to a black hole gets sucked in and can never escape.
• Material spirals in to a black hole through an accretion disk — a disk of gas, dust, stars and planets that fall into orbit the black hole.
• The “point of no return” around a black hole is called the “event horizon”. This is the region where the gravity of the black hole overcomes the momentum of material spinning around it in the accretion disk. Once something crosses the event horizon, it is lost to the pull of the black hole.
• Black holes were first proposed to exist in the 18th century, but remained a mathematical curiosity until the first candidate black hole was found in 1964. It was called Cygnus X-1, an x-ray source in the constellation Cygnus.
• Black holes do not emit radiation on their own. They are detected by the radiation given off as material is heated in the accretion disk, and also by the black hole’s gravitational effect on other nearby objects (or light passing by).