Re: How do we really know how hot the surface of the sun is?

This is a really good question Marilyn. The answer is a great example of how science really works. Since we cannot directly take the temperature of the sun, we have to use a more indirect way. We need to compare information from different places to form a conclusion. How can we get information from the sun? The answer must be hidden in the sunlight that reaches earth each day. What information can be hidden in sunlight? I am sure that you have seen a rainbow. The rainbow is a continuous spectrum. If we were to use a special machine called a spectroscope, we would see that the spectra (rainbow) from the sun is actually made of many bright and dark lines. That is the first piece of information that we need.

If you have seen fireworks, our next clue is there. What makes the colorful patterns in the sky? When things burn they give off light. The color of that light acts like a fingerprint for that material. Each material has a very special fingerprint. We can use that fingerprint to tell us a lot about the stuff. This can be easy at times. For example, look at the streetlamps in your town. If they look yellowish, they are probably sodium lamps because sodium gives off a yellow light. If they look bluish, they are probably mercury lamps because mercury gives off a bluish light.

In the laboratory, we can examine these fingerprints very closely. Each element on the Periodic Table has a well known fingerprint. Each bright line is caused when the atom absorbs and releases exactly right amount of energy. Using the spectroscope we can measure the wavelength of the light given off. When we know the wavelength we can calculate the energy. We can then determine the temperature that the material needs to produce that light.

So, I think that we are ready to answer your question now. By closely observing the spectrum of the sun, we can figure out the temperature that the surface must be in order for the spectra to look that way. We also determine the temperatures of all stars this way. Hotter stars have a different spectra than cooler stars. Hotter stars are blue and cooler stars are red.

I hope that this answers your question. If I can be of more help, just ask.