What Is Luminosity?

Trumpler 14 star cluster star luminosities
This composite image of Trumpler 14 shows stars of similar brightnesses, against a backdrop of smaller, cooler, dimmer ones with different luminosities. NASA, ESA, and J. Maíz Apellániz (Institute of Astrophysics of Andalusia, Spain)

How bright is a star? A planet? A galaxy? When astronomers want to answer those questions, they express the brightnesses using the term "luminosity". It describes the brightness of an object in space. Stars and galaxies give off various forms of light.  What kind of light they emit or radiate tells how energetic they are. If the object is a planet it doesn't emit light; it reflects it. However, astronomers also use the term "luminosity" to discuss planetary brightnesses. The greater the greater the luminosity of an object, the brighter it appears. An object can be very luminous in visible light, x-rays, ultraviolet, infrared, microwave, radio, and gamma-radiation. It often depends on the intensity of the light being given off, which is a function of how energetic the object is.

Stellar Luminosity

Most people can get a very 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. Distance also affects the apparent brightness of an object. A distant, but very energetic star 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. The effective temperature is expressed in degrees Kelvin, so the Sun is 5777 kelvins. A quasar (a distant, hyper-energetic object in the center of a massive galaxy) could be as much as 10 trillion degrees Kelvin. Each of their effective temperatures results in a different brightness for the object. The quasar, however, is very far away, and so appears dim. 

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 regardless of where it lies in the universe. 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

Another way to understand and measure an object's brightness is through its 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. Essentially, a second-magnitude object is about two and a half times brighter than a third-magnitude one, and two and a half times dimmer than a first-magnitude object. The lower the number, the brighter the magnitude.The Sun, for example, is magnitude  -26.7.  The star Sirius is magnitude -1.46. It's 70 times more luminous than the Sun, but it lies 8.6 light-years away and is slightly dimmed by distance. 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 we 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 the infrared and visible light. The newborns themselves are quite bright in the ultraviolet and visible light. 

Edited and revised by Carolyn Collins Petersen