The Milky Way

An artist's conception of the Milky Way Galaxy, overlaid with points showing the location of APOGEE stars. The color of the points represents the overall abundance of heavier elements in the star: blue points have relatively fewer heavy elements than red points.

The SDSS is mapping the structure and chemical makeup of our Galaxy through its Apache Point Observatory Galaxy Evolution Experiment (APOGEE) surveys. With these observations and prior SDSS results, SDSS researchers measure the abundances of multiple elements in the periodic table found in each of hundreds of thousands of stars across the Galaxy.

This chemical cartography provides information about the formation and evolution of the Milky Way, because the chemistry of stars encodes information about the gas from which they formed, and this gas is enriched in different elements in a variety of astrophysical environments. APOGEE is particularly good at studying the stars in the flat disk of the Milky Way — which contains the vast majority of Milky Way stars — because it works in infrared light that is better able to penetrate the dust that is also found in the disk of the Milky Way. In addition, APOGEE takes advantage of another, identical spectrograph as a "second eye on the sky," located at the Irénée du Pont Telescope at Las Campanas Observatory in Chile. Having one spectrograph in each hemisphere allows the SDSS to simultaneously observe the entire Galaxy, both bulge and disk, from Earth's Northern and Southern Hemispheres.

The new picture of the Milky Way that the SDSS reveals shows a galaxy criss-crossed by long streams of stars, each associated with a former satellite galaxy that has since been absorbed into the Milky Way. By studying the distribution of these streams, astronomers can reconstruct the history of how our Galaxy evolved by absorbing smaller galaxies.

SDSS measurements of the motions of stars in the disk and stellar halo have yielded the most precise determinations of the mass distribution of the Milky Way's dark matter halo, implying a total halo mass of approximately one trillion solar masses, lower than many previous estimates.