Stardate 01:04:98

Why are the stars different colors?

At first glance, the stars may appear to all be white, but if you look at them from dark skies for any amount of time, you will see they are a diverse variety of colors,red, yellow, white, and blue.

Why should this be? Why should the stars come in so many colors? Why not just white?

It turns out the answer is because they are hot. All matter emits radiation, peaking at a wavelength (which can sometimes be seen as a color) which is dependent solely on the temperature of the body. This is known as Wein's law (pronounced Veen's law).

Most objects we encounter in everyday life are somewhere around room temperature. Give or take a few thousand degrees. This effect, known as blackbody radiation, happens to all objects, regardless of it's temperature. Objects which have a surface temperature just a few degrees above absolute zero (which is -273 c.) radiate most of their radiation in the form of radio waves. Hotter than that, and the object glows in microwaves.

After that, it radiates largely in the form of infrared waves. Hotter still, and the object glows red, then orange, yellow, green, then blue. This is the reason metal glows when it is hot. When the temperature is hot enough to have the atoms or molecules moving at wildly different speeds at the surface, all at around room temperature, the object will be seen to glow white hot, as white light is made from a composite of all the visible colors.

When an object reaches a temperature where it becomes too hot to radiate black body radiation in the visible wavelengths, it will shine in ultraviolet wavelengths. Hotter still, x-rays, and finally gamma rays.

Why does this happen? Because as an object grows hotter, there is, logically, more energy being released. As the energy level of radiation is increased, the wavelength becomes shorter and shorter. When the human eye perceives different wavelengths of visible light, we interpret it as different colors. This is why a rainbow always appears with the same colors in the same order. Red, orange, yellow, green, blue, indigo, and violet. You will never see a rainbow with any other colors, or with the colors in any other order.

The coolest stars are around 3,000 degrees c. They are the red stars. Stars like the sun are next, glowing at about 6,000 degrees c., and are, of course, orange/yellow (although as we discussed last week, the Sun is greener than you think).

10,000 degrees c. will buy you a green star, and the going price on a blue star is around 25,000 degrees c. There are cool stars which radiate in the infrared, and will last dozens of billions of years. There are stars which expend their energy in the ultraviolet, and will last only a fraction of the lifetime of the Sun. Many of these stars cannot be seen with the naked human eye.

The next time you look at the stars from a dark sky, and marvel at their colors, remember; You haven't seen anything yet.

Clear skies, and good viewing.

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