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A star’s luminosity is the amount of energy a star puts out per second. This is normally measured in the units joules per second or watts.
The radiant energy, or luminosity, a star emits manifest in two ways. The first is visible light. That is what can be seen by the human eye every time you look at the night sky. The other is radiation. This is energy not in the visible electromagnetic spectrum but still a major part of the total energy emitted by a star.
Astronomers measure the luminosity in two ways. One is apparent magnitude. This measures only the visible radiant energy a star emits. The second is bolometric magnitude. This is normally measured using an instrument called a Bolometer. This device measures the full spectrum of electromagnetic energy emitted by a star.
So we know how its is measured, but what are the units used to measure it? First of all we use what we know. The closest star that we can observe is the Sun. The Sun also has the added benefit of being a middle range type star so it serves as a perfect baseline for measuring the luminosity of other suns. The Sun’s luminosity is 3.839 x 1026 watts. Scientists use this measurement when measuring the luminosity of other stars. Essentially a star luminosity can be determined by comparing its energy output to our Sun. It is then given in an order of magnitude of how many times more or less energy it emits compared to our Sun.
Luminosities can be determined in other ways. One uses the inverse law. Basically it states that the further you get from a light source, the weaker it gets. All stars can also have their luminosities determined without distance and with two key characteristics. The first is their temperature and the second is their size. The higher the temperature of a star, the greater its luminosity. The same for its radius. A stars spectroscopic chemical make up can also be used to determine its luminosity.
Luminosity can tell us many things about stars. How hot they are, their size, or whether they might support planets with life. Other than sending a probe to a star, which we have not done yet, measuring the energy output is our best source of information. This along with the knowledge we have gained about the movement of celestial bodies gives us a great picture of the universe.
If you enjoyed this article there other articles about Astronomy you read on the Universe Today website. For example there are two great articles about solar luminosity and star luminosity. They give more information about luminosity and how it is determined in Astronomy.
The Australia telescope and outreach education site also has great information about luminosity and the formulas used to determine it. Another great site is nasa.gov. There is an interesting Question and Answer article about how chemical composition can affect a star’s luminosity.