The University of Arizona

This is Your Galaxy: Exploring the Hidden Milky Way

By University Communications, July 31, 2013

New data released by the Sloan Digital Sky Survey's APOGEE project, which uses technology UA scientists helped develop, enables astronomers to better understand how the Milky Way formed.

An infrared map of the whole Galaxy showing the plane and bulge of the Galaxy full of stars and dust. APOGEE uses new IR instrumentation to study stars within the disk and is less affected by the extinction from interstellar dust.
An infrared map of the whole Galaxy showing the plane and bulge of the Galaxy full of stars and dust. APOGEE uses new IR instrumentation to study stars within the disk and is less affected by the extinction from interstellar dust.
A photo of four SDSS-III scientists working on the APOGEE spectrograph. (Photo: Dan Long/Apache Point Observatory
A photo of four SDSS-III scientists working on the APOGEE spectrograph. (Photo: Dan Long/Apache Point Observatory
Astronomers with the Sloan Digital Sky Survey III, or SDSS-III, have released a new online public dataset featuring 60,000 stars that are helping to tell the story of how our Milky Way galaxy formed.
 
The highlight of Data Release 10 is a new set of high resolution stellar spectra – measurements of the amount of light given off by a star at each wavelength – using infrared light, invisible to human eyes but able to penetrate the veil of dust that obscures the center of the galaxy.
 
The new dataset includes the first public release of data from the SDSS-III APOGEE project, which will map the motion and chemical pattern of stars in the Milky Way and reveal the assembly history of our own galaxy.
 
The current phase of the Sloan Digital Sky Survey, SDSS-III, is under direction of former University of Arizona astronomer Daniel Eisenstein, who started it during his tenure as a professor at the UA's Steward Observatory, one of the survey's partner institutions. SDSS-III is a six-year survey of nearby stars, the Milky Way galaxy, and the distant cosmos.
 
The Sloan Foundation 2.5-meter telescope at Apache Point Observatory in New Mexico conducts observations every night that feed either the Baryon Oscillation Spectroscopic Survey or APOGEE infrared spectrograph.
 
The APOGEE project, which stands for Apache Point Galactic Evolution Experiment, was made possible by a new multiobject infrared spectrograph that UA scientists helped develop. The highly sensitive detectors used on the spectrograph are the same type that are being used in the near-infrared camera on NASA's James Webb Space Telescope, a project led by UA Regents' Professor Marcia Rieke. Rieke and her group provided the detectors and electronic systems for the new spectrograph. 
 
The question of how the Milky Way formed has been the subject of scientific speculation and debate for hundreds of years. APOGEE's three-dimensional map will provide key information for resolving central questions about how our galaxy formed over many billions of years.
 
"APOGEE will accurately measure both the motion and chemical makeup of tens of thousands of stars in different parts of the Milky Way Galaxy," said Xiaohui Fan, a professor of astronomy at the UA's Steward Observatory and a member of the SDSS-III advisory committee. "All elements heavier than hydrogen and helium are made as a result of stellar evolution and death. So the chemical patterns of stars in different parts of the galaxy reveal how stars formed and evolved over the last thirteen billion years of its history, and how these different components, disk, halo and bulge of the galaxy, were assembled during this time."
 
"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."
 
"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, who led the effort to prepare the APOGEE data for Data Release 10. "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."
 
The Milky Way currently has three main parts: a high-density oblong bulge in the center, the flat disk where we live, and a low-density spherical component called the "halo" extending out hundreds of thousands of light years.
 
"Stars in these different regions have different ages and compositions, which means they formed at different times and under different conditions throughout the history of our galaxy," said Gail Zasowski, an NSF Postdoctoral Fellow at The Ohio State University who led the critical effort to maximize APOGEE's scientific potential by selecting the best possible sample of stars.
 
If you look up at the sky from a dark site, far away from the overwhelming glow of city lights, the Milky Way galaxy appears as a luminous band across the sky, overlaid with dark curtains. This band is the disk and bulge of our galaxy, and the curtains are the dust that blocks visible light from more distant parts of the Milky Way.
 
Because of this dust, previous studies of stars in the Milky Way have been limited in their ability to consistently measure stars toward the center of our galaxy. APOGEE's solution is to look in infrared light, which can pass through the dust. This ability to explore previously hidden regions of the galaxy allows APOGEE to conduct the first comprehensive study of the Milky Way, from center to halo.
 
Observing tens of thousands of stars is a daunting, time-consuming task. To accomplish its goal of observing 100,000 stars in just three years, the APOGEE instrument observes up to 300 different stars at a time, using fiber-optic cables plugged into a large aluminum plate with holes drilled to line up with each star. Light passes through each fiber into the APOGEE spectrograph, where a prism-like grating distributes the light by wavelength.
 
"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."
 
APOGEE's spectra of stars will help unlock the history of our galaxy, and the key is learning the compositions and motions of stars in each region. Because elements heavier than hydrogen and helium were produced in stars and spread through the galaxy by exploding stars and stellar winds, astronomers know that stars with more of these heavy elements must have formed more recently, after previous generations of stars had time to create those heavy elements.
 
"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 of Texas Christian University, who coordinated all of the APOGEE observations.
 
APOGEE data also provide a rich context for investigating a wide range of questions about the stars themselves. Because APOGEE observes each target star several times, it can identify changes in each star's spectrum over time. This feature has enabled the APOGEE team to discover unusual types of rapidly variable stars, to pinpoint how many stars are actually binary stars with unseen companions, and even to detect the subtle stellar motions caused by orbiting planets.
 
Data Release 10 also publishes another 685,000 spectra from the SDSS-III Baryon Oscillation Spectroscopic Survey, or BOSS. These new spectra come from galaxies and quasars as seen when our universe was much younger, just as the mysterious force of "dark energy" was beginning to influence the universe's expansion. The new BOSS spectra, and the additional spectra that the SDSS-III will continue to obtain in the final years of the survey, will help scientists in their quest to understand what dark energy might be.
 
"We've been putting out data releases since 2001, and we're not slowing down yet," said SDSS-III spokesperson Michael Wood-Vasey of the University of Pittsburgh. "Public access to data has always been a key goal of our project, and we're proud to continue that tradition today with this new release rich with information about our own galaxy."
 
All of the data gathered through the survey is available to the public, free of charge, at the Sloan Digitial Sky Survey website. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the participating institutions, the National Science Foundation and the U.S. Department of Energy Office of Science.