Well, That was Fun.

By Julianne Dalcanton | January 29, 2007 6:14 pm

I’ve not yet actually posted about what I’m working on. I was saving up for a nice juicy post, oozing information and insight from every line. However, I’m compelled to post sooner than planned because my primary research tool may have just gone belly-up.

So, what I had been working on was an enormous Hubble Space Telescope (HST) project to map millions of stars in nearby galaxies. As Steinn nicely outlined, HST has become primarily an instrument for imaging. It has exquisite spatial resolution, which allows one to distinguish between photons emitted from closely adjacent locations on the sky. From the surface of the Earth, with the turbulent atmosphere in between, telescopes would see a blurry mush of stuff (not to get too technical on you). However, when viewed with HST, the blurry mush would resolve into individual features, much like you’d experience if you cleaned off a layer of Vaseline from your glasses. Now, this ability to zoom in on tiny astronomical features is useful for both imaging and spectroscopy, but, like an episode of the Sopranos, HST’s spectrographs have kept getting bumped off, leaving imaging (i.e. “taking pretty pictures”) as HSTs primary capability.

So what kind of science can you do with “taking pretty pictures”? One thing you can do is to measure the color and brightnesses of individual stars. If you change the age, mass, or metal content of the star, its internal structure changes as well, leading to measurable changes in its color and luminosity. As a result, when you make a plot of color vs brightness (where redder is to the left, and brighter is upwards), you see revealing patterns that tell you a tremendous amount about what kind of stars there are, how old the stars are, and what the stars are made of:

You can see how the stars are not distributed randomly across the plot, and instead tend to cluster in well-defined sequences. It turns out that stars with different ages, masses, and metal contents fall into different sequences, so with data like those above, we can try to piece together the history of the galaxy on a star-by-star basis — stellar archeology if you will.

The project I was running was designed to extract this information for all the galaxies in a many cubic megaparsec volume of the local Universe. We were gathering hundreds of images like:

which, when zoomed in, look like this:

See all those individual stars, with different colors and brightnesses? Neat, huh? Anyways, the project would have measured the properties of more than ten million stars. I’d been planning this for a couple of years, and it had been underway since September. We have about half of the data in hand, but now the camera we were using, the Advanced Camera for Surveys (ACS), is probably down for the count. What this leaves me with is a half-finished data set, and an uncertain future for me, my students, and my postdoc who moved his pregnant wife across the country to work on this. There’s plenty to do with what we have in hand, but it’s not going to be exactly what we’d planned. I have no regrets, because one of my principle motivations for designing this project was that we’d be embarrassed not to have these data if Hubble fell into the ocean. It’s better that we have some of it than none of it.

Still, it’s not the best birthday present I’ve ever received.


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