What is Luminosity?

Updated : 05/04/2025

Luminosity is the total amount of energy that a star or related object gives off during a period of time, whether it be second, minute, hour.... and more. Usually, the time frame used is seconds. It refers to the energy that is given in all directions not just the energy in the path of the observer.

Luminosity is usually expressed compared to the luminosity of the Sun. The Sun has a luminosity of 1. Sirius has a calculated luminosity of 25.84 times that of our Sun. The Sun radiates roughly 3.846 x 10²⁶ watts Britannica. To get the absolute amount, multiple the two figures together.

Dim stars such as Brown Dwarf Stars and Red Dwarf Star will have a lower luminosity than our Sun. When a star is less luminous, it will usually be given as a decimal. For example, Luyten's Star has a luminosity of 0.01.

A Supernova explosion can generate a luminosity of many millions times that of our Sun. Some stars are also many times more luminous than our star. The most luminous star in the Milky Way Galaxy is Eta Carinae. The most luminous that we know about is R136a1 which is located in the Large Magellanic Cloud, a satellite galaxy of our galaxy.

Galaxy Luminosity

Its not just stars that are luminous, a Galaxy can be luminous as well. The Ursa Minor dwarf galaxy has a luminosity of 100,000 whereas a larger galaxy or one that contains a quasar can be more than 2 trillion times luminous than our Sun.

Luminosity Factors

Major determining factors of a stars luminosity is its size and its temperature. A large hypergiant red star such as UY Scuti can have as much luminosity as that of a smaller bluer star. A blue star is a hotter star than a red star. A large blue star can be more luminous than UY Scuti. It is determined by both factors.

If you had two stars that were the same size but different colour and temperature, the bluer hotter star would be the more luminous. A larger red star could be more luminous than a smaller blue star but the red star would have to be larger to compensate for the lack of temperature.

If you had two stars that were of the same temperature, the larger of the two stars would therefore be crowned the more luminous of the two stars.

Absolute Magnitude

When we look up at a star in the sky, its brightness is recorded as apparent magnitude. The lower the number, the brighter the star. The Sun has an apparent magnitude of -26.74. The Absolute Magnitude of a star is the magnitude it is at a distance of 10 parsecs.

A star that is 10 parsecs will have the same absolute and apparent magnitude. The absolute magnitude of a star is a measure of a star's luminosity. Astronomy Notes

The lower the absolute magnitude is, the brighter, the more luminous a star is. To calculate the absolute magnitude of a star, the formula that I use is :-

Round (lApparentMagnitude.Value + 5.0 - (5.0 * Log10(lDistanceParsecs.Value>, 2)

Calculating Luminosity

A simple way of calculating luminosity is to use the following formula taken from S.D.S.S. :-

L = 4 π R²s T⁴

Where :-

L = Luminosity
R = Radius in Metres
s = Stefan-Boltzmann Constant (5.67 x 10^-8 Wm^-2K^-4)
T = Surface Temperature of the Star

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