Luminosity

In physics is meant by the brightness, L, the energy radiated per unit of time, i.e., over all areas of the electromagnetic spectrum radiated power summed.

• 2.3.1 The luminosity of the Sun

Determination

For a radiation source which emits its energy uniformly into a solid angle, the luminosity results to

Herein denotes the energy flux density ( energy per time and area) at a distance from the source.

The radiation source is isotropic, then the energy is distributed over the surface of a sphere having a radius and it is

Astronomy

Since large parts of astronomy with the observation or description of electromagnetic radiation (hence light ) deal, the notion of luminosity will often use. One speaks, for example, of the luminosity of stars, galaxies, quasars, etc.

The luminosity in the visible light is the most important parameter in the Hertzsprung -Russell diagram, which describes the stellar evolution. The term is in special research topics often restricted to a particular band ( a portion of the electromagnetic spectrum). Thus, it is customary to speak of the "X-ray luminosity " when the integrated over the X- band radiation power is meant.

Units

To specify the brightness, all units of the power may be used, in particular Watt ergs per second. In certain contexts, it may be convenient to use other units such as " solar luminosities ." What units ultimately are common, but is less a matter of practicality than the habit or historical legacies. For example, the luminosity earlier Hefner Candles ( Hk) is specified.

Stars

The luminance is usually a measure of the energy emission of a star in the form of electromagnetic radiation. In general, there is a star, although most of its energy in this way from, but not all his. Other possibilities include loss of energy as neutrino radiation or stellar wind.

The luminosity depends on the radius of the star and its effective temperature. The correlation with the effective temperature is by definition and the term of the temperature should not be taken too seriously here.

Luminosity classes

Although the effective temperature of two stars is similar, their luminosities can be significantly different from each other as their radii need not be equal. The determination of the effective temperature, which is reflected in the spectral type, therefore, not sufficient to classify the stars according to their luminosity. The luminosity class was introduced to meets this requirement. It is determined from the spectrum of the star, and is reflected by the width of spectral lines, which in turn is a measure of the star radius.

Stars are therefore classified according to their luminosity luminosity classes.

→ See also: Hertzsprung -Russell diagram, main sequence, giant star, mass-luminosity relation

Luminosity determination in astronomy

To determine the luminosity of astronomical objects such as planets, stars, galaxies, etc., the above equation is also used. Looking at very great distances, as in the case of distant quasars or clusters of galaxies, the equation must be expanded or interpreted differently, since relativistic effects become important.

Often the complication of the luminance determination is to determine the distance and the energy flow.

Distance determination is an important area in astronomy and often very expensive. The distance of nearby stars can be determined by measuring the parallax. A greater number of such parallax was measured in the Hipparcos mission for example. To determine the distance of distant objects such as star clusters, galaxies, quasars, etc. can have a whole range of methods are used. An important role is played by objects of known luminosity. They are called standard candles. If we measure the energy flow and assumes isotropy for example, one can use the formula for the luminosity to calculate the distance.

For flow measurement suitable detectors are necessary. CCD detectors in astronomy to be used in many places that have displaced photographic plates that were used previously. In general, however, only observed a part of the entire spectrum, so that the measurement of the total luminosity is limited.

The luminosity of the Sun

The sun is an isotropic radiator. At the distance R = 1 AU = 1.496 · 1011 m from the Sun's center meets every second an energy of 1367 J per m. The size of 1367 J / ( m² s ) is also called the solar constant and is equated with the flow of energy in the luminosity formula.

The energy of 1367 J can be distributed to well over 1 square meter of Earth's surface, because the surface of the earth, wherever the sun is not directly overhead, not perpendicular to the direction of incidence of the sunlight; the changing orientation of the Earth's surface to the sun is the reason for the seasons.

The total solar radiation is distributed over a spherical surface of size

Substituting this value and the solar constant in the formula for the luminosity of a, one obtains

This represents the average luminosity of the Sun