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Flux decreases with distance according to an inverse-square law, so the apparent magnitude of a star depends on both its absolute brightness and its distance (and any extinction). For example, a star at one distance will have the same apparent magnitude as a star four times as bright at twice that distance.
Early photometric measurements (made, for example, by using a light to project an artificial “star” into a telescope's field of view and adjusting it to match real stars in brightness) demonstrated that first magnitude stars are about 100 times brighter than sixth magnitude stars.
This equation and the usual value of a = 3.5 only applies to main-sequence stars with masses 2M ⊙ < M < 55M ⊙ and does not apply to red giants or white dwarfs. As a star approaches the Eddington luminosity then a = 1. In summary, the relations for stars with different ranges of mass are, to a good approximation, as the following: [2] [4] [5]
A star like Deneb, for example, has a luminosity around 200,000 L ⊙, a spectral type of A2, and an effective temperature around 8,500 K, meaning it has a radius around 203 R ☉ (1.41 × 10 11 m). For comparison, the red supergiant Betelgeuse has a luminosity around 100,000 L ⊙ , a spectral type of M2, and a temperature around 3,500 K ...
For example, the giant elliptical galaxy M87 has an absolute magnitude of −22 (i.e. as bright as about 60,000 stars of magnitude −10). Some active galactic nuclei ( quasars like CTA-102 ) can reach absolute magnitudes in excess of −32, making them the most luminous persistent objects in the observable universe, although these objects can ...
In astronomy, a phase curve describes the brightness of a reflecting body as a function of its phase angle (the arc subtended by the observer and the Sun as measured at the body). The brightness usually refers the object's absolute magnitude, which, in turn, is its apparent magnitude at a distance of one astronomical unit from the Earth and Sun.
Luminosity distance D L is defined in terms of the relationship between the absolute magnitude M and apparent magnitude m of an astronomical object. = which gives: = + where D L is measured in parsecs.
Other factors that might affect the maximum luminosity of a star include: Porosity. A problem with steady winds driven by broad-spectrum radiation is that both the radiative flux and gravitational acceleration scale with r −2. The ratio between these factors is constant, and in a super-Eddington star, the whole envelope would become ...