May 29, 2008

Astronomers map the metals in millions of Milky Way stars:
Solving mysteries about the birth and growth of the galaxy

Examining stars as far as 30,000 light years away from the Sun, researchers from the Sloan Digital Sky Survey (SDSS-II) measured the metal content of millions of stars in our galaxy, the Milky Way.

Accurate measurements of the metal content and motions for an unprecedented number of stars allow astronomers to decipher how our galaxy formed and how it evolved over the 13 billion years since its formation.

The position and size of the mapped region, relative to the rest of the Milky Way, is illustrated in the top right corner. This new map is overlaid on an image of the Andromeda galaxy, the closest galaxy that looks like our own.

Most of the stars in the Milky Way are found in a disk-like feature, whose vertical cross-section is shown by the gray scale background; brighter shade means more stars.

The new metallicity map, shown as the colored inset, indicates that the disk is composed of high-metallicity stars that are typically just a few billion years old (red and yellow shades). The disk is embedded in a low-density stellar halo composed of lower metallicity stars with ages over 10 billion years (blue shades).

The Milky Way is still growing by cannibalizing other nearby galaxies. A good example of a victimized galaxy is the Monoceros stream, marked by the arrow. The fact that the Monoceros stream stars have somewhat different metal content than other nearby stars (green shade instead of blue) helps to delineate its extent and reveal its origins.

High-resolution pdf available at

(Credit - SDSS Collaboration, Zeljko Ivezic, University of Washington)