Wednesday, June 30, 2010

Stars in our eyes

The hot, dense surfaces of stars emit light in every part of the visible spectrum, from deep red to violet. The part of the spectrum that is the brightest depends on the star's temperature. Cooler stars appear red (their temperature is the same as the red burner on your stove). The hottest stars appear bluish. Our Sun, with a middling surface temperature of about 6000 degrees C, is brightest in the middle part of the spectrum and appears yellow to the eye.

But all the colors are there, as you see when sunlight is refracted through raindrops and spread out into a rainbow. I love this pic of the solar spectrum, a recent APOD (click to enlarge). A modern spectrograph has spread sunlight out into one long solar "rainbow". Cut the image into 50 thin strips along the horizontal lines and stick them together end to end. That's the "rainbow," stretching across the room and then some.

But what are all the dark gaps in the spectrum, those myriad of "lines", some fat, some thin, some dark, some barely visible? These represent absorption of light by gases in the cooler, less dense, atmosphere of the Sun.

And here's the marvelous thing. Every element has its own absorption spectrum, as electrons are bumped up from one energy level to another. Every element has its own absorption "fingerprint." In this solar spectrum all the "fingerprints" are layered on top of one another, like the figerprints on a gun that dozens of people have handled. In spite of the jumble, the very same elements of which the Earth is made are identifiable in the Sun's atmosphere.

Without ever leaving Earth, we know that the entire universe is made of the same 92 naturally-occuring elements as the world I see outside my window. This beautiful solar spectrum is like looking at the shelves in a chemstry lab store room, with each element neatly labeled: hydrogen, helium, lithium, berylium, boron, carbon, nitrogen, oxygen, etc.

Beautiful and wonderful.