By Don Lincoln, Fermilab
The Sun, indeed any star, isn’t a single temperature. The temperature inside of the Sun is about 15 million degrees Kelvin or Celsius or about 27 million degrees Fahrenheit. That’s much hotter than the surface of the Sun. All of those different environments together emit blackbody radiation at different wavelengths. How?
Blackbody Radiation
Light particles emitted at the center of the Sun don’t just stream outward to the surface. The inside of the Sun is so hot that atoms don’t exist, and the result is that the light particles bounce around. It can take a million years for a photon of light to claw its way out of the center of the Sun to the surface. The upper layers of the Sun that are just below the surface, are about 2 million degrees Celsius or 3.5 million degrees Fahrenheit. The actual surface of the Sun is about 5,800 degrees Celsius, but above the surface of the Sun is the corona, which is, rather surprisingly, hotter than the surface. The corona can be as hot as the center of the Sun.
All of those varied environs emit blackbody radiation at diverse wavelengths and what we see is the mix of them all. The dominant wavelengths come from the surface of the Sun, but all of them contribute. Then there is the fact that there are atoms of elements on the outside of the Sun that can absorb light at certain wavelengths. What we, thus, find is that the spectrum of any star is a mix of blackbody radiation of many temperatures, each contributing to a different degree, intermixed with a series of black lines that indicate the absorption of light by certain elements.
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Determining the Temperature of a Star
This mix is hard to model well, but it’s really a bit of a blessing. We can use the blackbody spectrum to determine the temperature of a star and we can use the location of dark lines in the spectrum to determine the chemical makeup of the star’s surface. The presence or absence of different elements tells us a great deal about the way stars grow and evolve over time.
However, there are a few more complicating factors that one should be aware of. One is that the light we see coming from a distant star isn’t necessarily the color that the star emitted. This is because of a familiar effect that we experience here on Earth. Some days, as the Sun is rising or setting, it’s much different color than it is at noon. A noontime Sun is white or yellowish, while a sunset or sunrise can be blood red. The color change is due to the Earth’s atmosphere preferentially scattering blue light. The same thing can happen in space. There is gas and dust between the Earth and a distant star. Thus, the light we see here on Earth from a distant star is typically redder than it was when it is emitted.
Forces that Drive Stellar Evolution
The bottom line is that to understand the nature of distant stars, we need to know a lot of things and get that information essentially solely from the light we receive. The elemental composition and the temperature are two crucial features of any star. Of course, that glow comes from somewhere. And that’s another story.
Stars are born. They grow into a productive middle age, followed by a crankier retirement, metaphorically yelling at the kids to get off the lawn. Then, depending on their size, they die either a spectacular death or more gently, fading peacefully into that long night. What drives that cycle?
Understanding How Sun Makes Its Energy
It all starts out with nuclear fusion, which is a form of atomic energy which occurs when two atoms merge together to form a heavier atom. The idea that the sun makes its energy by fusing hydrogen together into helium is actually a relatively new idea, only 100 years old. And the achievement was all the more remarkable given that at the time fusion hadn’t been discovered.
It was in 1920 that British physicist, Sir Arthur Eddington, turned his attention to the problem. He was instrumental in validating Albert Einstein’s theory of gravity and is partially responsible for catapulting Einstein into the pantheon of physics superstars. But in 1920, he was thinking about how stars gave off their energy.
To conclude, at the time, physicists really had no idea, except that they were pretty sure that earlier ideas involving gas condensing and heating due to increased pressure couldn’t do the trick. After all, if that were the mechanism, stars would collapse from interstellar clouds, live and die in only 20 million years or so.
Common Questions about the Sun and Blackbody Radiation
The spectrum of any star is a mix of blackbody radiation of many temperatures, each contributing to a different degree, intermixed with a series of black lines that indicate the absorption of light by certain elements.
We can use the blackbody spectrum to determine the temperature of a star and we can use the location of dark lines in the spectrum to determine the chemical makeup of the star’s surface. The presence or absence of different elements tells us a great deal about the way stars grow and evolve over time.
British physicist, Sir Arthur Eddington, was instrumental in validating Albert Einstein’s theory of gravity and is partially responsible for catapulting Einstein into the pantheon of physics superstars.
