What Is Luminosity?

bright stars show different luminosities
The bright stars in the cluster Trumpler 14 illustrate the concept of luminosity for stars -- as a way of measuring their brightnesses. NASA

Luminosity is a term that astronomers use to describe the brightness of an object in space. When we see stars in the night sky, for example, we observe their radiation, in the form of light. The light they emit tells us something about how energetic they are. If the object is a planet, of course it doesn't emit light; it reflects it, but astronomers also use the term "luminosity" to discuss that brightness.

The greater the greater the luminosity, the brighter it appears. If it's a star or a galaxy, the luminosity also contains clues to how energetic it is.

Stellar Luminosity

You can get a general idea of an object's luminosity simply by looking at it. If it appears bright, it has a higher luminosity than if it's dim. However, that appearance can be deceptive. A distant, but very energetic object can appear dimmer to us than a lower-energy, but closer one.

Astronomers determine a star's luminosity by looking at its size and its effective temperature. They express a star's size in terms of solar units — that is, a star that is five solar radii has a radius five times that of the sun. The effective temperature is expressed in degrees kelvin. The luminosity that matters when it comes to understanding what's powering an object, from stars to quasars, is the intrinsic luminosity. That's a measure of the amount of energy it actually emits in all directions each second.

You might sometimes see the term "intrinsic luminosity" used to describe an object's brightness. It's a way of understanding the processes inside the object that help make it bright.

Another way to deduce a star's luminosity is to measure its apparent brightness (how it appears to the eye) and compare that to its distance.

Stars that are farther away appear dimmer than those closer to us, for example. However, an object might also be dim-looking because the light is being absorbed by gas and dust that lies between us. To get an accurate measure of the luminosity of a celestial object, astronomers use specialized instruments, such as a bolometer. In astronomy, they are used mainly in radio wavelengths — in particular, the submillimeter range. In most cases, these are specially cooled instruments to one degree above absolute zero to be their most sensitive.

Luminosity and Magnitude

You've probably heard about a star's magnitude. It's a useful thing to know if you're stargazing since it helps you understand how observers can refer to stars' brightnesses with respect to each other. The magnitude number takes into account an object's luminosity and its distance. You can read more here about magnitudes, but essentially, a second-magnitude object is about two and a half times brighter than a third-magnitude, and two and a half times dimmer than a first-magnitude object. The lower the number, the brighter the magnitude. However, it's important to understand that a very bright object at a great distance can appear very dim because of its distance, whereas a dim object that is much closer can "look" brighter.

Apparent magnitude is the brightness of an object as it appears in the sky as you observe it, regardless of how far away it is. The absolute magnitude is really a measure of the intrinsic brightness of an object. Absolute magnitude doesn't really "care" about distance; the star or galaxy will still emit that amount of energy no matter how far away the observer is. That makes it more useful to help understand how bright and hot and large an object really is. 

Spectral Luminosity

In most cases, luminosity is meant to relate how much energy is being emitted by an object in all the forms of light it radiates (visual, infrared, x-ray, etc.). Luminosity is the term that we apply to all wavelengths, regardless of where they lie on the electromagnetic spectrum. Astronomers study the different wavelengths of light from celestial objects by taking the incoming light and using a spectrometer or spectroscope to "break" the light into its component wavelengths.

This method is called "spectroscopy" and it gives great insight into the processes that make objects shine.

Each celestial object is bright in specific wavelengths of light; for example, neutron stars are typically very bright in the x-ray and radio bands (though not always; some are brightest in gamma-rays). These objects are said to have high x-ray and radio luminosities. They often have very low optical luminosities.

Stars radiate in very broad sets of wavelengths, from the visible to infrared and ultraviolet; some very energetic stars are also bright in radio and x-rays. The central black holes of galaxies lie in regions that give off tremendous amounts of x-rays, gamma-rays, and radio frequencies, but may look fairly dim in visible light. The heated clouds of gas and dust where stars are born can be very bright in infrared and visible light. The newborns themselves are quite bright in ultraviolet and visible light. 

Edited and revised by Carolyn Collins Petersen