Friday, July 2, 2010

National Geographic lied to me!

One of the great disappointments of my childhood was when I used to go to several Astronomy events at Columbia University, and we were invited to see a nebula through a telescope. I looked through the lens and saw a grayish blob! What an unbelievable buzzkill!

You see, I was, thanks to National Geographic’s multiple published pictorials from the Hubble Space Telescope, expecting something brilliant and beautifully colored. I expected something like the purple nebula where Captain Kirk had his final duel with the evil superman Khan. Or the pictures sold on mousepads or museum gift shop posters.

In truth, the brilliant colors of most nebulae are not visible to the normal human eye but are only apparent through astrophotography, a series of techniques to get information about stars that involves long-term exposure, where after 30 minutes or more of holding a single image on a plate, colors and even objects that are typically not visible become that way – astrophotography can usually be done with over the counter film, which makes it a great hobby (for people that aren’t as poor as me, that is). Images color-shift because some types of light wavelengths are more successfully absorbed by film, so it typically becomes necessary to take photographs at different wavelengths and combine them for a color-corrected image.

Actually, what “color” a nebula is typically tells us a lot about them. For instance, the majority of nebula are red, because of the presence of hydrogen, which as a result of ultraviolet light are stripped of electrons and produces the red color. Fun fact – 90% of the entire universe is Hydrogen, with the remaining 9% as Helium and less than 1% as heavier elements.

This goes back to an even more interesting field: emission spectroscopy. Using the light from distant stars, it is possible to determine exactly what it is they’re made of by the light that comes off when something is heated (the fact that things give off light when heated is a property called incandescence). Only photons with a certain energy type are emitted from atoms, giving us their emission spectrum. Therefore, by seeing what colors are absorbed by a specific atom, it is possible to determine what the composition of it is. In this way, the element of Helium was discovered on the sun before it was ever discovered on earth!

There was a great article in “Wired” about the recent photography of a rare blue nebula. Nowhere is it mentioned in the article, however, as to why blue nebulae are so much rarer, however: blue light scatters much more in the deep vacuum of space, so “blue” nebulae are only likely to be visible when something reflects light off of them, like for instance, the stars forming inside of them.

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