An Etymological Dictionary of Astronomy and Astrophysics
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A star's → intrinsic brightness, i.e. the total amount of energy radiated by the star per second. → Luminosity is often expressed in units of watts or erg/sec. The Sun's absolute luminosity is 3.86 × 10³³ erg/sec.

→ absolute; → luminosity.

Discrepant luminosity classes derived for the same → Am star when different criteria are used. Lore specifically, a luminosity criterion may indicate a → giant star, wheras another criterion indicates a → supergiant.

→ anomalous; → luminosity; → effect.

The total rate of energy output of an object integrated over all wavelengths.

→ bolometric; → luminosity.

A characteristic luminosity around which the → luminosity function of a sample of galaxies changes to a steeper slope or exponentially declines.

→ break; → luminosity.

A form of → Hertzsprung-Russell diagram in which the luminosity is the vertical axis and the → color index the horizontal axis.

→ color; → luminosity, → diagram.

Same as → Eddington limit.

→ Eddington limit; → luminosity.

The total number of → Lyman continuum photons emitted by an → H II region. It is usually derived using → radio continuum observations which are less affected by → interstellar extinction. The measured value is often a lower limit because of photon leakage from the H II region and absorption. See also → density-bounded H II region.

→ H II; → region; → luminosity.

The energy per second emitted by an astronomical object.

→ intrinsic; → luminosity.

The → total → brightness of a star or other astronomical object. It is expressed in watts and represents the total amount of → energy that the object radiates each → second over all wavelength regions of the → electromagnetic spectrum. Because this quantity is independent of distance, it is an → intrinsic brightness.
See also:
→ absolute luminosity, → anomalous luminosity effect, → bolometric luminosity, → color-luminosity diagram, → Eddington luminosity, → H II region luminosity, → intrinsic luminosity, → luminosity class, → luminosity distance, → luminosity function, → luminosity problem, → luminosity-size relation, → mass-luminosity ratio, → mass-luminosity relation, → peak luminosity, → period-luminosity relation, → solar luminosity, → stellar luminosity, → wind luminosity.

Verbal noun of → luminous.

A classification of stellar spectra according to luminosity for a given → spectral type. The luminosity class is an indication of a star's → surface gravity. It is shown by a Roman numeral as follows: I (→ supergiants), II (bright → giants), III (normal giants), IV (→ subgiants), and V (→ dwarf stars, or → main-sequence stars). Luminosity classes VI (→ subdwarfs) and VII (→ white dwarfs) are rarely used. Subclasses a, b, and c are especially used for supergiants, while the most luminous → hypergiants are assigned luminosity class Ia-0.

→ luminosity; → class.

1) Distance derived by comparison of → observed and → intrinsic luminosities. If an object has a known luminosity L, and the observed flux is S, the luminosity distance is defined by D_L = (L/4πS)^1/2.

2) In cosmology, the → expansion of the Universe results in a diminution of the photon flux and the above equation fails. The reason is that for a homogeneous and isotropic Universe (→ Robertson-Walker metric), the luminosity decreases by a factor (1 + z)⁴. Therefore, the luminosity distance is related to the → angular diameter distance (D_A) by: D_L = (1 + z)².D_A.

→ luminosity; → distance.

Number → distribution of → stars or galaxies (→ galaxy) with respect to their → absolute magnitudes. The luminosity function shows the → number of stars of a given intrinsic luminosity (or the number of galaxies per integrated magnitude band) in a given → volume of space.

→ luminosity; → function.

Low-mass → protostars are about an order of magnitude less luminous than expected. Two possible solutions are that → low-mass stars form slowly, and/or protostellar → accretion is episodic. The latter accounts for less than half the missing luminosity. The solution to this problem relates directly to the fundamental question of the time required to form a low-mass star (McKee & Offner, 2010, astro-ph/1010.4307).

→ luminosity; → problem.

The relation between the stellar luminosity of a galaxy and its physical size. More at → mass-size relation.

→ luminosity; → size; → relation.

The ratio of the mass of a system, expressed in solar masses, to its visual luminosity, expressed in solar luminosities. The Milky Way Galaxy has a mass-luminosity ratio in its inner regions of about 10, whereas a rich cluster of galaxies such as the Coma Cluster has a mass-luminosity ratio of about 200, indicating the presence of a considerable amount of dark matter.

→ mass; → luminosity; → ratio.

A relationship between luminosity and mass for stars that are on the main sequence, specifying how bright a star of a given mass will be. Averaged over the whole main sequence, it has been found that L = M^3.5, where both L and M are in solar units. This means, for example, that if the mass is doubled, the luminosity increases more than 10-fold.

→ mass; → luminosity; → relation.

The → bolometric luminosity of a → supernova corresponding to the highest brightness in its → light curve. The peak luminosity occurs after the → supernova explosion; it is directly linked to the amount of radioactive ⁵⁶Ni produced in the explosion and can be used to test various explosion models. Following → Arnett's rule, one can derive the ⁵⁶Ni mass from the peak luminosity of a → Type Ia supernova.

→ peak; → luminosity.

A → correlation between the periods and luminosities of → Cepheid variable stars: Cepheids with longer periods are intrinsically more luminous than those with shorter periods. The relation was discovered by Henrietta Leavitt in 1912 when studying Cepheids in the → Small Magellanic Cloud. Once the period of a Cepheid variable is determined from observations, the period-luminosity relation can be used to derive its luminosity. Since luminosity is a function of → distance, the distance can then be calculated with the luminosity. The period-luminosity relation is an invaluable tool for the measurements of distances out to the nearest galaxies and thus for studying the structure of our own Galaxy and of the Universe.

→ period; → luminosity; → relation.

The total → radiant energy, in all wavelengths, emitted by the Sun in all directions. It is 3.828 × 10²⁶ W or 3.828 × 10³³ erg sec^-1 (International Astronomical Union, Resolution B3, 14 August 2015, Honolulu, USA). This is the luminosity unit conventionally used to give the luminosities of stars. See also: → solar constant. When the Earth first formed, 4.56 billion years ago, the Sun radiated 30% less energy than it does today, thus giving rise to the so-called → faint early Sun paradox. Ever since then, its power has increased by 7% every billion years (I. Ribas, 2009, arXiv:0911.4872).

→ solar; → luminosity.

The total amount of energy emitted by a star per unit time. According to the → Stefan-Boltzmann law, the stellar luminosity is given by: L_* = 4πR_*²σT_eff⁴, where R_* is radius, σ is the → Stefan-Boltzmann constant, and T_eff is → effective temperature. A star's luminosity depends, therefore, on two factors, its size and its surface temperature. Stellar luminosity is measured either in ergs per second or in units of → solar luminosity or in → absolute magnitude. See also → luminosity class.

→ stellar; → luminosity.