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Name: Collin M.
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 4/26/2004


Question:
Why is the sun the color that it is? To clarify: I understand that the helium plasma of the sun emits light when electrons from the ionized helium give off energy in the form of photons when they return to a lower energy level, but I do not understand why it is the orangeish color that we see. I am confused because I thought the helium on the sun was not burning, yet the helium gives off the same spectrum as if it were burning? (Please correct any of this that is wrong, as well.)


Replies:
Collin-

It is just glowing all across the spectrum (white) because it is white-hot. There is relatively little spectral dependence from electron-orbital-energy-levels of the helium and (majority) hydrogen atoms. Almost any substance will do about the same color if it is that same temperature, about 5000 degrees C. It is called, strangely enough, "black-body" emission.

Spectral lines do occur in sunlight, but I think they are mostly narrow lines of dark in an otherwise glow-filled spectrum. They are caused by cooler gasses in the space around the sun, absorbing only their favorite parts of the sunlight as it passes through.

I am not sure what perceptual color to call the sun. When I glance up at it at noon on a hot clear day, to me it looks merely too white to keep looking. When I project it on a white paper using a pinhole in another paper, that looks white too, unless the sun is low in the sky and hence colored by passing through all the air and maybe even brown smog.

Black-body radiation is always broad-spectrum, but it has a color tendency which is exactly determined by its temperature. Middle temperatures (5500K) are white, lower temperatures are dimmer and yellow, orange, red, then finally dark, glowing in the infrared but not visibly. Higher temperatures are brighter and bluish-white, but it never gets totally blue. You may have seen the term "color-temperature" in photography or associated with light-bulbs.

Technically I think the sun's color-temperature is considered just about white. Interesting evolutionary question why our eyes' color-range is so well matched to the sun's peak wavelength range, whereas insects are more sensitive to the blue parts of sunlight.

When you are staring down into the flaming ball of the sun, at those wavelengths where atoms emit, the glow comes from close to the surface. If you dropped a dark rock into the plasma, you would loose sight of it quickly, if you were looking through a filter that only allows exactly those colors. At colors in between, the gas tries to be more transparent, but it does not matter. You can see a little farther down, but it is thousands of miles deep, so eventually there are enough photons emitted that it is just as bright at in-between colors as it is at the atomic-emission wavelengths. This is why the substance does not matter, for black-body glowing situations.

One reason emission occurs at wavelengths in-between atomic-emission lines, is that the gas is a plasma. There are freed electrons running around bumping into atoms, ions, and other free electrons. The act of an electron changing speeds can emit light, of blue-er wavelengths for larger speed changes. All velocities are represented in a thermal mix, so there are emissions at all different wavelengths.

On earth, hydrogen can combine with oxygen to make water molecules (aka burning) at 1000 degrees C, which makes heat, raising its temperature up to 2000 C, and it can glow from that. This provides nowhere near enough energy to power the sun for billions of years, and there is not much oxygen there, and it is too hot for any molecule to survive more than a microsecond anyway. The combustion product, water molecules, would immediately break up into hydrogen and oxygen atoms, more than taking back any heat produced. A single-molecule chemical reaction makes more or less 1 electron-volt of energy. A single-atom nuclear reaction releases more or less 1 million electron-volts of energy. The sun is a natural thermonuclear reactor. In the core of the sun, the temperature is millions of degrees, and hydrogen nuclei occasionally combine to make helium and other heavier nuclei. This releases much more heat, which then spends thousands of years migrating to the top of this 400,000 mile-deep pile of gas, where it can finally get out as 5000-degree sunlight. The surface of the sun is very much cooler than the core.

In the long run (another 4 billion years?), almost all the hydrogen will be "burned" into helium and other things, and then the sun will start to have hiccups as it shifts gears to burn helium into still heavier nuclei. Long enough that we do not need to worry about it.

Break it down into functional parts:

- what raw materials are present to make heat, and to glow? (mostly hydrogen, small but useful percentage of heavier things. )

- what is the condition there? (Temperature so high that all molecules are broken down to atoms, and most atoms broken into nuclei and free electrons, especially deeper down than the part we see. High pressure inside, too.)

- by what reaction does energy (heat) get released? (nuclear fusion reaction, like a big slow-motion H-bomb)

- by what mechanism does heat energy turn into light? (atom resonances, and free-electrons bumping into things)

- understand the system (The reaction rate is self-regulated: gravity compresses the core, increasing the reaction rate, until generated heat starts expanding the core, and a balance happens. The surface color temperature goes only as high as it needs, to emit all the generated heat into space. Lucky us, it all balances smoothly!)

Jim Swenson


The spectrum of light from the sun is closely described by the "black-body" spectrum characteristic of the temperature of the surface of the sun. Though the sun is certainly not black, the molecules at the surface of the sun have a velocity distribution characteristic of the temperature of the surface of the sun, which is about 6000 C.

There is also light characteristic of the elements making up the surface of the sun (helium was first discovered on the sun by it's spectrum), but most of the radiation is due to the radiation emitted when molecules collide at high speeds due to the high temperature.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University



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