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Question #88563 posted on 12/13/2016 11:16 a.m.
Q:

Dear 100 Hour Board,

Why is the sun a blackbody and doesn't just emit hydrogen spectral lines?

-Dallin

A:

Dear Dallin,

Well, this question reached 100 hours and no one has tackled it yet, so I'll tell you the little bit I know.

As you said, we characterize the sun as a blackbody. A blackbody is a physical body that is a perfect absorber and emitter of radiation. And the radiation that a blackbody emits is known as blackbody radiation, and blackbody radiation is solely dependent on temperature, not the chemical makeup of the blackbody. The sun emits blackbody radiation that peaks in the central, yellow-green region, and also UV radiation, which is why we wear sunscreen. However, the sun is not a perfect blackbody. Nothing is ideal in science, and the sun is no exception. It's not a perfect absorber of radiation, and it's not a perfect emitter, but it's close. As to why it is? I'm not completely sure.

The sun is almost a perfect absorber of radiation because the solar atmosphere and gases around the sun essentially scatter all radiation until it is absorbed by the sun. I'm sure there are other reasons, but that's part of it. It's also near to a perfect emitter, but not quite. Here's a comparison of the radiation of a perfect blackbody, and the sun.

lol sun.gif(source)

As to the spectral lines, I'd refer you to the Fraunhofer lines, which are a set of spectral lines from the optical spectrum of the sun. The dark lines are caused by different elements, and as you said, more than just hydrogen spectral lines are seen. Studies of these lines have shown us that there are many different elements in the sun, and there are specifically atoms of many elements that exist as gases in the sun's atmosphere. This seems fairly logical to me, as we're never truly dealing with things that are purely made of, say, hydrogen. Also the sun is constantly fusing atoms together in it's core. We know that though the sun is about 76 percent hydrogen and 26 percent helium, it also contains trace amounts of other elements, like iron, carbon, oxygen, etc.

Anyway, I hope that helps a bit. Feel free to clarify in the corrections, physics people. I'm just a poor little biochemistry major.

Keep it real,
Sherpa Dave