SI Units
L = 4πR2σT4 = 4π(x10 m)2σ( K)4 = x10 J/s
Solar Units
L = R2T4 = ( R)2( T)4 = L

Temperature

## Learning Objectives

After reading the background information and working through all exercises in the simulator a student should:

• know the definition of luminosity.
• understand that luminosity depends on the star's radius and surface temperature.
• recognize the vast range of values for luminosity.
• know that the sun's luminosity is near the middle of possible stellar luminosity values.

## Background Information

Luminosity is defined as the total energy a star releases each second. It is normally given in units of joules per second or watts.

The luminosity of a star depends on both its surface temperature and its size. Luminosity is the product of 1) the Stefan-Boltzmann Law describing how much energy is radiated by each square meter of a star in 1 second and 2) the star's surface area.

The luminosity of the sun is 3.8 x 1026 J/s. Other stars have a very wide range of luminosities: there are large hot stars that are 100,000 times as luminous as the sun and there are small, cool stars 100,000 times less luminous than the sun.

## Exercises

1. Set the simulator to the preset Antares. This star has a very large luminosity. Explain what makes this star so luminous.
2. Set the simulator to the preset Alkaid. This star has a large luminosity. Explain what makes this star so luminous.
3. Set the simulator to the preset Barnard�s Star. This star has a very small luminosity. Explain what makes this star�s luminosity so small.
4. Antares represents one of the most luminous stars. Barnard�s Star represents one of the least luminous stars. After looking at these two stars, how would you describe the luminosity of the sun?
5. The star Beta Aquarii has a surface temperature very close to that of the sun and a luminosity 2200 times that of the sun. What can you conclude about its size? Use the simulator to check your reasoning.
6. Estimate the luminosity of a hypothetical star having the same size as the sun but having a surface temperature of 12000 K. Check your answer in the simulator.