 One of the very important relationships between light and matter is called black body radiation. Turns out that the color of most matter at high temperatures depends completely and totally on the temperature. Nothing else really matters. Take a look at this iron rod as it heats up. See how it goes from red hot at the outer edges through yellow to white hot at the center. If we could get it hot enough, you'd see it turning blue. Here's why. As temperature increases and the electrons start moving more rapidly, two things happen. One, the object emits more radiation at all wavelengths. And two, the peak emission frequency shifts towards the shorter, higher energy blue wavelengths. As the heating starts, the radiation is all in the infrared range, so we can't see it. As the temperature approaches 2,000 degrees Celsius, we begin to see red. We've seen the red star Aldebaran. It's a good example of this. By 3,000 degrees, the red has morphed to orange. Arcturus is an example of this. By 4,000 degrees, it is quite yellow. Capella and our own sun are yellow. Around 6,000 degrees, it is turning white. The star Sirius A is an example of this. And by 10,000 degrees, it has a distinct bluish color. Spica is a good example of a blue star. So the bottom line is measure the color of a star by the frequency of the light it emits, and you've determined its temperature. It's that simple. Now that we know star temperatures via their color and luminosity via their parallax distance, we can build a diagram I mentioned in the introduction.