Exploring Hidden Milky Way
August 1, 2013

Exploring The Hidden Milky Way

John P. Millis, PhD for redOrbit.com - Your Universe Online

Understanding how the Milky Way formed is one of the central scientific questions in astronomy. However, given the scale and complexity of the galaxy, there is a lot to consider.

One of the ways researchers have found to tell the story of our galaxy's history is to create a detailed stellar census, analyzing spectra of large numbers of stars. "A star's spectrum is a powerful tool for learning about the star - it tells us key details about the star's temperature and size, and what elements are in its atmosphere," said Jon Holtzman of New Mexico State University. "It's one of the best tools we have for learning about stars, like getting someone's fingerprints instead of just knowing their height and weight."

This too, however, can be an enormously challenging task. Because of the large amount of dust in the galactic neighborhood, many wavelengths of light are incapable of penetrating the disk. The problem is especially significant when attempting to study the region around the galactic center.

To combat this problem, astronomers with the Sloan Digital Sky Survey III (SDSS-III) created the Apache Point Observatory Galactic Evolution Experiment (APOGEE) to monitor stellar spectra of thousands of stars over a broad region of the galaxy. Now, the team has released the latest set of high-resolution spectra - known as Data Release 10 - covering some 60,000 stars.

"This is the most comprehensive collection of infrared stellar spectra ever made," said Steven Majewski of the University of Virginia, the lead scientist for the APOGEE project. "Sixty thousand stars is almost ten times more high-resolution infrared stellar spectra than have ever been measured before, by all the world's telescopes. Selected from all the different parts of our galaxy, from the nearly-empty outskirts to the dust-enshrouded center, these spectra are allowing us to peel back the curtain on the hidden Milky Way."

Taking A New Approach

Because observing tens of thousands of stars can be a daunting and time consuming task, the APOGEE instrument observes up to 300 different stars at the same time. Light from each star passes through a large aluminum plate drilled with holes, which line up with each star. Then fiber-optic cables channel the light from each hole into the APOGEE spectrograph. The photons are then separated by wavelength using a prism-like grating.

"The grating is the first and largest of its kind deployed in an astronomy instrument," said John Wilson of the University of Virginia, who led APOGEE's instrument design team. "That technology is critical to APOGEE's success."

This data will allow scientists to chart the age and characteristics of the stars across the galaxy, and compare them to each other. "By finding which parts of the Galaxy contain older versus newer stars, and by putting this together with how the stars are moving, we can write a detailed history of how the Galaxy formed, and how it evolved into what we see today," said Peter Frinchaboy, professor of physics and astronomy at Texas Christian University and coordinator of the APOGEE observations.

Astronomical Multi-Tasking

While the APOGEE project is targeting these stars to better understand the origins and evolution of the Milky Way galaxy, the data also provides useful information about the stars themselves. Specifically, astronomers are interested in how the spectra of these stars vary. As a result, the team has discovered some unusual types of stars that rapidly change over time.

Additionally, the Data Release 10 also contains another 685,000 spectra from the SDSS-III Oscillation Spectroscopic Survey (BOSS), though these data are of whole galaxies known as quasars at the distant edge of the known Universe. The goal of the BOSS project is to map the mysterious influence of dark energy - the force that appears to be driving the observed acceleration of the Universe.