How to Measure Luminosity

Luminosity is a measure of an object's brightness. It is defined as the amount of energy that an object releases in a given period of time and can be expressed with units of power such as watts. In practice, astronomers usually prefer to compare a star's luminosity to that of our sun which produces approximately 3.9 x 10^26 watts.

Instructions

1. Measure the apparent luminosity of a star. Apparent luminosity only considers visible light. When referring to apparent luminosity, always specify this.

2. Observe the bolometric luminosity of a star. This is the total radiant energy of a star and is measured with a bolometer. A bolometer absorbs the radiant output of a star over a wide band and measures the heat received. Bolometric luminosity is assumed unless otherwise specified.

3. Record the luminosity in units of solar luminosities L. If an object radiates 10 times the energy of the sun is its luminosity is 10L.

4. Calculate the apparent luminosity when the apparent magnitude and distance are known. L = m x A. L is the apparent luminosity, m is the apparent magnitude and A is the area of an illuminated surface. For point sources of light, A = 4 x pi x r^2. Note that r is the distance to the star. Therefore, we have L = 4 x pi x m x r^2.

5. Approximate the luminosity with the equation L = 4 x pi x C x R^2 x T^4 where C is the Stefan-Boltzman constant 5.67 x 10^-8. R is the radius of the star and T is its temperature.

Resources

• Learn more about measuring luminosity.