ESO Astronomical Glossary - R


Radial velocity
Radial velocity is the velocity with which an object travels towards or away from an observer. It is measured by examining the Doppler shift of features in the spectrum of astronomical objects. Ultra-precise measurement of a star's radial velocity, such as carried out by the ESO instrument HARPS at La Silla, can reveal if the star has a companion; such stars will show slight and regular changes in radial velocity caused by the gravity of the companion 'tugging' on its host.

Radio astronomy
Radio astronomy is the branch of astronomy that studies radio waves from astronomical sources. Radio waves are detected using special receivers, or 'antennae'.

Radio waves
Radio waves are a type of electromagnetic radiation with the longest wavelengths (and lowest energies), ranging from 0.3 mm to several kilometers. They form a very broad category, which includes the important submillimetre (with a wavelength of 0.3 - 1 mm) and microwave regions (1 mm - several cm).

Red dwarf
A red dwarf is a small, cool, very faint, main sequencestar whose surface temperature is under about 3500 K. They generally have masses of less than one-third that of the Sun. In the neighbourhood of the Sun the majority of stars are red dwarfs.

Red giant
Red giant stars are stars that have entered the final stages of their evolution. After hydrogen in the stellar core runs out, a star swells in size and cools, giving it a characteristic red appearance at visible wavelengths.

See Doppler effect.

Reflection nebula
Reflection nebulae are clouds of gas and dust that are visible from Earth from their reflection of starlight. The gas in the cloud is not ionised, so does not produce its own light, but the starlight is scattered by dust, illuminating the cloud.