Equations

Let’s call B the Apparent Brightness received by an observer (light energy per unit of area per unit of time). Suppose “Observer 1” is 2 Astronomical Units (AU) away from a star, and “Person 2” is 10 Astronomical Units away. How does the Apparent Brightness of Person 1 compare to that of Person 2? (Solve by comparison.)

B= L/4pid². B=4pid²/4pid². Cancel out the 4 and pi. B=d²/d². Plug the distances in. 10²/2²=25.

If it asks for the temperature of a star, what equation do you use?

Wien’s law. The one with Ymax and T in it.

When a question asks for a star’s lifetime, what equation do you use?

Mass-luminosity equation.

If a star is half as massive as the sun (Msun/Mstar = 2), how many times longer will it live?

a x m^-3/am^-3. Divide the two suns together. They cancel. 2³=8.

Equation for energy flux

F = σT4. Cancel the first two and divide the two different temperatures.