Simply astronomical – the Square Kilometre Array

Glossary

black hole. An astronomical object with a gravitational field that is so strong nothing can escape from it (including light). Because light cannot be emitted or reflected by black holes, they are invisible. Most black holes are thought to form from the collapse of massive stars. Nuclear fusion in stars normally causes an outward pressure from the heat produced. When the fuel for fusion runs out in massive stars, the force of gravity causes the star to collapse to form a black hole. Supermassive black holes are believed to lie at the centre of large galaxies and to have formed from matter in the early universe.

dark energy. A form of energy believed to make up 73 per cent of the universe. Dark energy has been proposed to account for the accelerating expansion of the universe. It is unknown whether it is constant throughout the universe or whether it varies in space and time. The properties of dark energy are investigated by observing its effects on the universe.

dark matter. Matter that is not visible but makes up around 22 per cent of the universe (normal matter only makes up 5 per cent). The observable matter in clusters of galaxies is not enough to create a gravitational field that would hold them together. Hence it is believed unseen dark matter accounts for the remaining mass.

ion. A positively or negatively charged atom or group of atoms.

neutron star. A middle-sized star that has used up its nuclear fuel so no longer has an energy source. This causes the star to undergo gravitational collapse. Neutron stars have an extremely high density; a teaspoon of their matter would weigh several million tonnes. Pulsars are believed to be neutron stars.

pulsar. A star that emits radiation at regular intervals. Believed to be neutron stars, pulsars emit radio signals as they rotate at very high speeds.

quasar. An abbreviation for quasi-stellar due to the resemblance of quasars to stars. Quasars are extremely distant, bright objects from the early universe. They are thought to be the cores of distant galaxies.

sensitivity. The ability of an object to detect weak signals eg, the sensitivity of a radio telescope is its ability to detect weak radio signals.

spectroscopy. The technique of detecting and analysing the spectrum of an object to get information on its chemical and physical nature (eg, temperature, motion). Using a spectroscope the radiation or light from an object is dispersed into its different colours or wavelengths (like a rainbow). The position of emission and absorption lines in the spectrum provides information on what chemicals are present. For example, emission at a wavelength of 21 centimetres corresponds to hydrogen. Large telescopes have spectroscopes to measure the properties of astronomical objects.

spectrum. Plural spectra. The distribution of electromagnetic radiation when it is dispersed (eg, the dispersal of visible light into a rainbow). Astronomers gain different information about astronomical objects by examining their spectra from different parts of the electromagnetic spectrum (eg, visible light, radio waves, X-rays).

wavelength. The distance between two adjacent wave crests. Visible light and X-rays are both electromagnetic waves and differ from each other only in the length of the wave. The wavelength of visible light ranges from 400 to 700 nanometres while the wavelength of X-rays ranges from about 0.01 to 10 nanometres. The relatively long wavelength of visible light sets the limit of how small an image it can produce. For more information see Electromagnetic radiation (Back to basics, Australian Academy of Science).


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Posted April 2009, edited September 2012.