Posts Tagged ‘black hole’

Event Horizon

The event horizon is the boundary that marks the point of no return of a black hole.

Once light (or matter) passes this, the gravitational force is so strong that the escape velocity would be greater than the speed of light.

Black Hole

I guess no-one will probably see this page …

A localised region of space from which not even light can escape, due to a super massive star collapsing in on itself. This is because the gravitational field of a black hole is so strong that the escape velocity is greater than the speed of light. Astronomers believe that the location of large black holes can be observed because of radiation emitted from the accretion disc as matter is pulled into the black hole. The event horizon marks the outer limits of these objects.

BL Lacertae object

Named after the first discovered member of this type of object. It appeared to be a variable star at first but it was realised that they were much more luminous than stars. They are less luminous than quasars and one idea is that a BL Lacertae object is a quasar seen from a narrow angle.

Accretion Disc

An accretion disc is a disc of material formed under the influence of gravity. It forms from smaller particles which are drawn towards the central body by the influence of Gravity.

The central body is generally a star although it can also be the nucleus of a galaxy or a black hole.

Also the rotating disc of material formed around a Black Hole.

The force of gravity is also reponsible for the emission of electromagnetic radiation from the disc. Gravity compresses the particles of the disc, heating them and giving rise to the radiation. The radiation wavelength depends on the strength of the gravitational force. Accretion discs around a black hole for instance can emit X-rays as the gravitational forces are immense. Accretion discs around forming or newly formed stars will only emit infrared as the gravitational forces are less, leading to a lower level of energy and therfore longer wavelengths of radiation. The Hubble Space Telescope is alleged to have seen and measured an accretion disc around a black hole using gravitational lensing to work out the colour profile.

The accretion discs around black holes and quasars are probably the most efficient way of generating energy from matter known, with about 10% matter to energy conversion. It is thought that accretion discs could be the source of the Gamma Ray Bursts seen from time to time in the universe. The Eddington Luminosity of Eddington Limit defines the point at which the outflow of energy from a star exactly balances the inward pull of gravity i.e. it is stable. Super eddington accretion discs are thought to be the source of the gamma ray bursters. Turbulence can cause material to fall inwards which is then converted into high energy radiation. These are very short lived events

Accretion discs round young stars supply the material for planets to form.

Accretion discs are also found in binary star systems. It happens when the two stars are of unequal ages and sizes and close together – the younger one will have lived out its life and become a white dwarf or even neutron star, then, when the older one reaches the end of its main sequence life, it expands. It it expands sufficiently, the outer envelope of gases can be more strongly attracted by the gravity of the smaller star and forms an accretion disc around the other.

The physics of an accretion disc is way beyond what we intend to explain here, however, if you want a detailed explanation, click here.

Switch to our mobile site