How Stars Work by Dr. Frank Bash Even though stars are very large and very hot, they are relatively simple. There are some complications when they form and some complications when they die. For long periods of a star's life, between its birth and death, it changes very slowly. The changes are so slow that astronomers who study stars consider a star's mass, size, temperature, and luminosity to be constant.
Stars come in a variety of sizes and colors, but they all shine because they are hot.
The temperature of a star's surface indicates how much energy each square foot radiates. The energy radiated by each square foot multiplied by the number of square feet on the star's surface gives the star's luminosity. The star's luminosity measures how much energy it radiates per second, just like the number stamped on the top of a light bulb gives the amount of energy the light bulb radiates into space per second. Luminosity is not the same as brightness. A 100-watt light bulb has a constant luminosity but can appear less bright the farther away it is.