Friday, December 08, 2006

Last of the Spectra

Sorry I missed yesterday's posting, but I managed to break some software on my computer and spent a lot of time getting it working again. I shouldn't have believed those who told me it was an "easy" update I was trying to do!

Anyway, let's spend a last day with some of the neater spectra I have. Remember, just click on the spectrum to get a larger version of the picture. As a reminder, here is a spectrum of the sun:


Remember that all those narrow lines are "fingerprints" of the elements in the sun, such as iron, magnesium, calcium, and hydrogen. Now look at this next spectrum of a star-like object called BL Lacertae:

It has almost no lines at all! It took a long time to figure out that what we astronomers were seeing was a black hole. We are looking right down close to its event horizon, where things are so hot, individual elements don't show up in visible light! For another strange spectrum, look at this spectrum of the star T Tauri:

Here, if you look closely, you can see a spectrum that looks almost like that of the sun, but there are bright lines on top of it! Those bright lines have the fingerprint of hydrogen. What we see here is a very young star about the size of the sun, with some hydrogen gas still falling onto the star.

Lastly, let's look at my favorite of all of these, the spectrum of the Crab Nebula:

This spectrum is different from the rest. In the other spectra, I smeared out the light from the star up and down so you could see the lines better. Here, I am showing the spectrum "as is." The straight vertical lines here are the streetlights of San Jose, CA. (I took those out of the other images). The horizontal lines are spectra of stars that happened to be in the way. But the crooked, knotted vertical lines are the Crab Nebula itself. Most of the light here comes from hydrogen and oxygen. The crookedness is caused by the gas of the nebula moving very fast -- almost 1000 miles each second! The light is Doppler shifted, meaning its color changes depending on how fast it is moving. So, we astronomers can use this spectrum to figure out how fast different parts of the nebula are moving. The gas here is moving so fast because the Crab Nebula is what remains of a star that exploded nearly 1000 years ago!

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