The amount of power that a star produces in light is related to the temperature of its surface and the area of the star. The bigger a star is, the more surface it has. When these relationships are combined, two stars at the same temperature can be vastly different in brightness because of their sizes.

The Luminosity of a star depends on BOTH its temperature and its radius (surface area): L is proportional to R2 T4. A hotter star is more luminous than a cooler one of the same radius. A bigger star is more luminous than a smaller one of the same temperature.

The hottest stars are the blue stars. A star appears blue once its surface temperature gets above 10,000 Kelvin, or so, a star will appear blue to our eyes. So the hottest stars in the Universe are going to be a blue star, and we know they’re going to be massive.

For stars of the same luminosity, size decreases as temperature increases. Star x has the same temperature as star y. Star x is more luminous. Therefore star x has the larger surface area.

On the main sequence, star sizes and colors are directly related. Larger stars are hotter and more massive than smaller stars. Stars are very big in comparison with the Earth, but they are also very far away.

Stars have different colors, which are indicators of temperature. The hottest stars tend to appear blue or blue-white, whereas the coolest stars are red.

Red stars are cooler. They are red because they emit red light, which is lower in energy than blue light. Energy in turn, is proportional to temperature, which means that higher energy corresponds to higher temperatures.

Stars die because they exhaust their nuclear fuel. The events at the end of a star’s life depend on its mass. Really massive stars use up their hydrogen fuel quickly, but are hot enough to fuse heavier elements such as helium and carbon.