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What is Luminosity? Definition, Equation and Factors

Sep 6, 2022

Astronomers use the word “luminosity” to represent the brightness of these objects while trying to provide answers to these puzzles. It explains an object’s brilliance in space. Light emanates from stars and galaxies in many different ways. The type of light they reflect or radiate reveals their level of energy. If the celestial object is a planet, it will reflect light rather than emit it. However, astronomers also refer to interplanetary brightnesses as “luminosity.”

The brightness of an object increases with its luminosity. In addition to visible light, an object can be extremely luminous in ultraviolet, x-rays, radio, infrared, microwave, and gamma rays. It frequently depends on the light’s intensity, which measures the object’s activity.


What is Luminosity?

Luminosity meaning: Luminosity describes the total energy produced by various solar bodies (stars, galaxies) per unit of time. It is essentially measured in watts or joules per second.

Luminosity = energy radiated or reflected per unit of time.


The absolute bolometric magnitude of a celestial object, often known as luminosity, is a measurement of the total energy emission and is used to express luminosity values. The bolometer can measure light energy using heating and absorption methods.

Luminosity Equation

Luminosity measures the energy an object emits, for instance, from the sun or galaxies. The star’s luminosity in the main sequence is proportional to its temperature; the hotter a star is, the better it illuminates. On the other hand, cooler stars radiate less energy and are more difficult to locate in the dark sky.


The Stefan-Boltzmann law provides us with the straight formula for star luminosity. According to this law, the light energy emitted by a dark body per unit of time is equal to:

P = σ AT4



σ = Stefan-Boltzmann constant (equal to 5.670367 × 10-8)


A = Object’s surface area (equal to 4πR2 for the spherical object)

T = object’s temperature (measured in Kelvin)


In most cases, researchers use an abbreviated version of this formula to calculate the luminosity of a star. We can relate any star to the sun’s luminosity as an alternative to computing an approximate energy value. The luminosity equation is then obtained by eliminating the constants:

L/L⨀ = (R/R⨀)2 (T/T⨀)4



L = luminosity of a star

R = radius of a star 

T = temperature of a star (calculated in kelvins)

L⨀ = luminosity of the sun, equal to 3.828 × 10²⁶ W

R⨀ = radius of the sun, equal to 695,700 km

T⨀ = temperature of the sun, equal to 5778 K

Interesting fact: 

We can frequently estimate the luminosity of stars in luminosity units. For example, we might write Lstar = 5.2 x Lsun for a specific star, suggesting that the star produces 5.2 times as much energy per second as the sun does.

Factors Affecting Object Luminosity

There are two important variables: the object’s effective temperature and size, and its radius, R, which affects the object’s luminosity.

Size and Effective Temperature: Astronomers examine a star’s size and effective temperature to calculate its brightness. A star’s luminosity and output of energy increase with its size.

But suppose two stars are of similar size but possess different temperatures. In that case, the star with a higher temperature will be more radiant (luminous) than the star with a lower temperature. The effective temperature is represented in degrees Kelvin, so the sun is 5777 kelvins. The temperature of a hyper-energetic, far-off stellar which may be at the core of a giant galaxy, may reach 10 trillion Kelvin. These stars may be brighter at one of their different effective temperatures. But since these stars are so far from earth, they appear dull.

In addition to these factors, the luminosity of a star can also be influenced by:

Distance: Most people can determine an object’s luminosity pretty well by looking at it. If the object appears bright, it has higher luminosity than if it’s dull. But that outward aspect can be misleading. An object’s apparent brightness is also influenced by its distance. We may perceive a distant, highly energetic star to be fainter than a nearby, lower-energy star. Another reason an item appears dark could be that the dust and gas absorb the illumination between them and the earth’s atmosphere.

Have You Heard?

According to measurements made on earth, the apparent brightness is the amount of energy emitted from the star per square metre every second. Watts per square metre (W/m2) are used as the units.

Magnitude: Understanding and calculating an object’s magnitude will help you determine how luminous it is. It’s helpful to know if you enjoy watching stars since it clarifies how watchers can discuss the luminosity of a star with one another. The magnitude value accounts for both the object’s distance and luminosity. Generally, a second-magnitude stellar is roughly 2.5 times brighter (luminous) than a third-magnitude one and 2.5 times dimmer (dull) than a first-magnitude one. The magnitude becomes brighter the lower the number.

For example, the sun has a magnitude of -26.7. Star Sirius has a magnitude of 1.46. Although it is 8.6 light-years away and 70 times as bright as the sun, the distance has caused it to lose some of its brightness. It’s essential to comprehend that while a dim star that is considerably closer can “see” brighter, a bright star that is far away can appear quite dim due to distance.

Informative Details:

Astronomers use advanced equipment, such as a bolometer, to precisely measure a celestial object’s luminance. They are mostly employed in radio frequencies in astronomy, particularly the submillimeter range. These instruments are typically cooled to one point above absolute zero to be the most accurate.

Solar Luminosity

Solar luminosity is the term used to describe how much energy the sun emits each second in all dimensions. Astronomers typically use solar luminosity as a unit of radiant flux (power released in the form of photons) to gauge the brightness of stars.

Astronomers typically use solar luminosity, abbreviated L, as a measure of radiant flux to compare the brightness of galaxies, stars, and other celestial bodies to that of the sun.

According to the International Astronomical Union, one nominal sun luminosity equals 3.828 1026 W. The solar neutrino luminosity, which would contribute to 0.023 L, is not included in this. The actual luminosity of the sun changes periodically because the sun is a highly fluctuating star. The major parameter is the eleven-year solar cycle (sunspot cycle), which results in a periodic variation of around ±0.1%. There have likely been other, smaller fluctuations during the past 200–300 years.

Moon Luminosity

Moonlight is mostly made up of sunlight (with a small amount of earthlight) reflected from the moon’s surface where the sunlight streams.

Moon luminosity energy that the moon emits every second in all directions. In the International System of Units, it is expressed in lux, illuminance unit, or luminous flux per square metre. In terms of lumens, it is one per square metre. 

Depending on the lunar phase, moonlight strength varies widely, but even a full Moon normally emits just 0.05–0.1 lux of illumination. When a full Moon near perihelion (a “supermoon”) is seen from the tropics at upper culmination, the illumination can approach 0.32 lux.


The quantity of light an object emits in a given amount of time is measured as luminosity. The sun emits 3.846 1026 watts of light per second (or 3.846 1033 ergs per second). Since luminance is an absolute measure of radiation power, its value is undisturbed by the viewer’s closeness to an object. One solar luminosity is equal to the brightness of the sun. Hence when astronomers talk about an object’s luminosity, they typically refer to it in terms of luminous solar flux. Millions of solar luminosities are emitted by the brightest stars.

Frequently Asked Questions

1. Explain “Luminosity meaning”

Ans. An object’s luminosity, the quantity of energy it emits over a predetermined period, is a measurement of its intrinsic luminosity. Since it measures the object’s power output, it can express it in units like Watts. However, astronomers frequently choose to express luminosities by relating them to the sun’s luminosity (about 3.9 1026 Watts). Instead of 3.9 1027 Watts, the luminosity of a star could be written as ten solar luminosity (10 L⊙).

2. What is apparent and absolute magnitude?

Ans. Apparent magnitude is the illumination of the celestial body as it appears in the sky when we view it, irrespective of how distant it is. The absolute magnitude measures the object’s intrinsic luminosity. Absolute magnitude doesn’t actually “concern” about distance; regardless of how far away the viewer is, the star or galaxies will still release an equal amount of radiation. Because of this, it is more useful to comprehend how bright, hot, and big an object is.

3. What is spectroscopy, and how do astronomers use it to investigate star luminosity?

Ans. Astronomers “fragment” the incoming light in different wavelengths using a spectrometer or spectroscope to investigate the various wavelengths of light from astronomical bodies. This technique, known as “spectroscopy,” provides excellent insight into the mechanisms that cause objects to illuminate.




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