Several different photometric systems exist. These are not generally luminosities in the strict sense of an absolute measure of radiated power, but absolute magnitudes defined for a given filter in a photometric system. The term luminosity is also used in relation to particular passbands such as a visual luminosity of K-band luminosity. Bolometric luminosities can also be calculated using a bolometric correction to a luminosity in a particular passband. In some cases, the process of estimation is extreme, with luminosities being calculated when less than 1% of the energy output is observed, for example with a hot Wolf-Rayet star observed only in the infrared. In practice bolometric magnitudes are measured by taking measurements at certain wavelengths and constructing a model of the total spectrum that is most likely to match those measurements. While bolometers do exist, they cannot be used to measure even the apparent brightness of a star because they are insufficiently sensitive across the electromagnetic spectrum and because most wavelengths do not reach the surface of the Earth. The IAU has defined a nominal solar luminosity of 3.828 ×10 26 W to promote publication of consistent and comparable values in units of the solar luminosity. A star also radiates neutrinos, which carry off some energy (about 2% in the case of the Sun), contributing to the star's total luminosity. ![]() A bolometer is the instrument used to measure radiant energy over a wide band by absorption and measurement of heating. When not qualified, the term "luminosity" means bolometric luminosity, which is measured either in the SI units, watts, or in terms of solar luminosities ( L ☉). The distance determined by luminosity measures can be somewhat ambiguous, and is thus sometimes called the luminosity distance. Apparent magnitude is a logarithmic measure of apparent brightness. Apparent brightness depends on both the luminosity of the object and the distance between the object and observer, and also on any absorption of light along the path from object to observer. In contrast, the term brightness in astronomy is generally used to refer to an object's apparent brightness: that is, how bright an object appears to an observer. Luminosity can also be given in terms of the astronomical magnitude system: the absolute bolometric magnitude ( M bol) of an object is a logarithmic measure of its total energy emission rate, while absolute magnitude is a logarithmic measure of the luminosity within some specific wavelength range or filter band. In astronomy, values for luminosity are often given in the terms of the luminosity of the Sun, L ⊙. ![]() In SI units, luminosity is measured in joules per second, or watts. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a star, galaxy, or other astronomical objects. Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. ![]() A star with four times the radiative power of the Sun has a luminosity of 4 L ⊙. In astronomy, this amount is equal to one solar luminosity, represented by the symbol L ⊙. The Sun has an intrinsic luminosity of 3.83 ×10 26 watts. For other uses, see Luminosity (disambiguation).
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