The four images of a quasar formed by the galaxy G2237+0305 acting as a gravitational lens. The resulting multiple image is in the form of a ring. See also: Einstein ring.

An alternative term for gravitational redshift.

An X-ray astronomy satellite launched by the USA on 13 November 1978. Officially designated HEAO-2 2nd High Energy Astrophysical Observatory, it was informally renamed by the scientific staff working on it in celebration of the centenary of the birth of Albert Einstein. It was the first X-ray observatory to be able to detect faint sources and to make images of complex or extended sources. Many important and significant observations were made until the supply of gas needed to control the telescope’s position was exhausted in April 1981. See also: X-ray astronomy.

A perfect circular image of a distant point source formed when a point mass along the line of sight acts as a gravitational lens. This idealized case of lensing was described by Einstein and has been observed for the imaging of radio emission from the quasar MG1654+1348 by an intervening galaxy.

The simplest of the modern cosmological models, in which the universe has zero pressure and zero curvature i.e. flat geometry, is of infinite extent, expands without limit and endures for all time. Proposed in 1932, it is a special case, with zero curvature, of the more general Friedmann universe.

Material that has been excavated during an impact or thrown out by volcanic activity and thus redistributed. Ejecta typically forms a circular blanket of shattered rock fragments and solidified gas and liquid droplets around the impact or volcanic centre. Some impact ejecta can escape from the planet or satellite altogether.

A small satellite of Jupiter number VII, discovered by C. Perrine in 1905. It is only about 80 kilometres 50 miles across and belongs to a group of four satellites whose closely spaced orbits all lie between 11.1 and 11.7 million kilometres from Jupiter. The others are Leda, Himalia and Lysithea.

One of the brighter stars in the Pleiades, also known as 17 Tauri.

A form of energy that propagates through a vacuum at a speed c of 3 × 10 to the power of 8 metres per second. The name reflects the nature of the radiation, which consists of linked and rapidly varying electric and magnetic fields. Its character varies according to wavelength λ. Radio waves have the longest wavelengths, ranging from several metres to millimetres. The shortest radio waves are usually termed microwaves. These merge into the infrared, which ranges down to just under a micrometre. Visible light is a narrow band of wavelengths between about 700 and 400 nanometres nm. Ultraviolet goes down to about 10 nm, then X-rays to 0.1 nm. The shortest waves of all are gamma-rays. The full electromagnetic spectrum is the whole range of radiation types, from the shortest wavelengths to the longest. The Ã¥ngström is also used as a unit of length for the measurement of wavelength. 1 Ã… = 0.1 nm. Electromagnetic radiation, in common with any wave, also has an associated frequency ν. The link between frequency and wavelength is ν = c/λ. Thus, as wavelength decreases, frequency increases. The energy E associated with electromagnetic radiation increases in direct proportion to frequency, according to the relation E = hν, where h is Planck’s constant. The energy is quantized in units of this size, which are termed photons. Electromagnetic radiation and its detection are crucial in the study of astronomy, which depends almost entirely on the receipt and analysis of such radiation from distant objects. Optical and radio astronomy can be undertaken from the ground because these wavelength bands pass relatively unobstructed through the atmosphere. Astronomical observations in other wavebands are largely carried out from orbiting spacecraft, though some are possible from high mountain sites and aircraft.

electromagnetic radiation.

The number of electrons per unit volume of space. In interstellar space the mean value is about 30,000 per cubic metre. In astrophysics the electron density is important in calculations concerned with the emission and propagation of electromagnetic radiation.

For a population of electrons, the temperature required to account for their observed energy, if that energy is assumed to be entirely thermal.

A unit of energy, principally used in atomic and molecular physics. It is defined as the energy acquired by an electron when it is accelerated through a potential difference of 1 volt in a vacuum. 1 eV = 1.602 × 10-19 joules. Electron volts are convenient units for measuring the energies of particles and electromagnetic radiation. The energies of X-rays are expressed in thousands of electron volts keV. The wavelength associated with 1 keV is 0.124 nanometres. Millions MeV and thousands of millions GeV of electron volts are also used as units for the highly energetic atomic particles. An electron with a kinetic energy of a few MeV is travelling at almost the velocity of light.

One of the basic materials of the universe, each of which is characterized by having a different number of protons in the atomic nucleus the atomic number. Isotopes of the same element differ in the number of neutrons in the nucleus. There are 90 elements occurring naturally on Earth. Some, uranium for example, have no stable isotopes. Other radioactive elements can be created artificially. See also:

orbital elements. Those most commonly used for the orbits of planets and comets around the Sun are semimajor axis, perihelion distance, eccentricity, inclination, argument of perihelion, longitude of the ascending node and period .