Archive for the ‘Deep Space’ Category

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.

Aperture Synthesis

A technique used in radio astronomy where by an array of radio telescope dishes are used together, effectively giving the observer a much larger dish size e.g. the VLA in New Mexico. More recently, as technology has improved, it has been possible to link smaller optical telescopes on the same site to act as if they have a much larger resolving power, thus extending the range and usefulness of Earth based telescopes.

Binary Star

double star in which the components orbit one another. Some binary systems orbit so close to one-another the two stars are distorted by each other’s gravity.

If the two stars eclipse one-another when seen from the Earth, the light changes regularly and predictably over a period of time. This type of binary is an eclipsing binary and is a type of variable star.

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.

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.

Blue Shift

If an astronomical body is moving towards the observer, the light will seem to be shifted to the blue end of the spectrum. The faster the movement, the greater the blue shift. It occurs because the wavelength of light is slightly compressed by the Doppler effect as the body moves towards the observer. Blue shift is measured by looking at the key spectral lines. For an object moving towards the Solar System, they will appear closer to the blue end than normal. The faster the object is approaching, the greater the blue shift will be.

Bok Globules

The Dutch astronomer Bart Bok first drew attention to these small black objects. They appear in gaseous emission nebulae and are thought to be protostars that are still forming but have not yet become hot enough to shine.

Cosmic Rays

High speed particles that reach the Earth from outer space. Some low energy cosmic rays come from the Sun, but high energy cosmic rays are thought to originate from outside of the Solar System. These possibly originate from supernovae. The highest energy cosmic rays may originate in quasars.


The study of the origin of the Universe and its subsequent evolution.

When I was a lad, it was easy, you either supported Fred Hoyle and his Steady State theory or the ‘Big Bang’. The latter has survived the test of time and is now the best current model that we have. It is constantly in a state of evolution itself and has thrown up many new questions requiring answers such as the issue of Dark Matter.

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.

The Big Bang Theory

The theory that the universe came into existence at one set moment in time. It has gradually come to be accepted as being correct.

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