APOGEE: Mapping the Galaxy from Inside
The American Astronomical Society held their biannual meeting in Austin, Texas, earlier this month, producing a flurry of astronomically-themed press releases. I realize that is ages ago in “internet time,” but I think my brain and work schedule are only just now recovering from attending. One of the coolest projects to be introduced is APOGEE, a survey aimed at characterizing 100,000 stars in our Milky Way Galaxy.
APOGEE stands for Apache Point Observatory Galactic Evolution Experiment, and it is probably one of the least tortured acronyms in all of astronomy. This project will survey red giant stars across the entire sky in order to measure the shape, motions, and chemistry of our Galaxy.
The APOGEE team designed an infrared camera, part of which was built at the University of Virginia, for the 2.5-meter telescope in Apache Point, New Mexico, that was originally used for the Sloan Digital Sky Survey.
WATCH VIDEO: THE MILKY WAY’S BLACK HOLE
You might think that, since we’re IN it, we would know more about our own Galaxy than we do about other galaxies. However, it’s hard to get a complete picture of the Galaxy from inside of it. Think about it: we’ve never seen it from outside of it, and probably won’t for a very long time!
Because our sun is embedded in the disk of the Milky Way, we’re still not sure how many spiral arms it has, how big the arms are, or whether it obviously has a central “bar.” We can only approximate our location on the famous, though anachronistic, Hubble fork diagram. We literally can’t see the forest for the trees.
With 100 billion stars estimated to be in our galaxy, how do you choose which stars to sample?
The APOGEE team are surveying red giant stars since they are large, bright, and easy to see across the Galaxy. Plus, by looking at infrared light, this telescope can peer through the dust clouds that block visible light in our Galaxy’s disk.
The camera splits the light of each star into a “rainbow” or spectrum. The spectrum of the star provides the team with a fingerprint of sorts, with absorption lines corresponding to the various molecules and atoms in the stellar atmospheres. This tells us something about the star’s origin and age, as many of the stars of our Galaxy came from smaller satellite galaxies that were “eaten up” and torn apart by the Milky Way’s gravity.
The absorption lines may also appear to shift as the stars move towards or away from us. This Doppler shift will help astronomers map the motions of the whole Galaxy and determine more about the structure. So really, APOGEE is a window into the Galaxy’s chemical history and kinematic present.
Also impressive is the speed at which the survey takes place. Instead of taking spectra one at a time, the telescope observes a whole field of stars at once and takes many spectra at once using optical fibers placed around the field of view. The image above shows the size of the field of view (in an image from NASA’s WISE) in the white circle which spans the width of six full moons, or three of your fingers held out at arm’s length. The green circles indicate the location of open star clusters, and the red circles show the location of each star that will get its own optical fiber, and thus its own spectrum.
It’s been really cool for me to see this project grow up during my time at UVa, and my officemate, Gail Zasowski actually does a lot of the target star selection for the project! You can see her explain a bit more about APOGEE and the amazing start that it has had since first light.
Image credit: P. Frinchaboy (Texas Christian University), J. Holtzman (New Mexico State University), M. Skrutskie (University of Virginia), G. Zasowski (University of Virginia), NASA, JPL-Caltech and the WISE Team.