Unveiling the chemical composition of Milky Way stars
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: May 29, 2008

Astronomers from the Sloan Digital Sky Survey (SDSS-II) have unveiled the most complete and detailed map yet of the chemical composition of more than 2.5 million stars in the Milky Way, allowing them to tackle many unsolved mysteries about the birth and growth of our Galaxy.

The map uses the colors of millions of stars to infer their metal content, or metallicity, (where the term “metals” describes elements that are heavier than hydrogen and helium, such as oxygen, calcium and iron, that were forged in the cores of stars or during the explosive deaths of massive stars), by comparing their colors with spectroscopic observations of the stars.
The color of a star is slightly influenced by the presence of particular chemical elements, which absorb light at specific wavelengths, such that the amount of blue light emitted by a metal-poor star is slightly increased.

The SDSS-II target area extends from a region close to the Sun to 30,000 light years away. The position and size of the mapped region relative to the rest of the Milky Way, and compared to an image of the Andromeda galaxy, is shown in the top right corner. Most of the stars in the Milky Way are found in a disc-like feature whose vertical cross section is shown by the grey-scale background (brighter shade means more stars). The coloured inset shows the new metallicity map and indicates that the disc is composed of high-metallicity stars that are typically just a few billion years old (red and yellow shades), with a low-density stellar halo composed of lower metallicity stars with ages over 10 billion years (blue shades). The victimised galaxy, the Monoceros stream, is marked by the red arrow. Image: Zeljko Ivezic, University of Washington, SDSS-II Collaboration.

By mapping how the metal content of stars varies throughout the Milky Way, astronomers can decipher star formation and evolution. Given that stars that formed early in the history of the Galaxy were made of gas that had few metals created by the generations of stars that came before, astronomers are provided with a chemical fingerprint of the origin and evolution of the elements. As subsequent generations of stars formed and died, they returned some of their metal-enriched material to the interstellar medium, the birthplace of later generations of stars, including our Sun.

"The map of the distribution of metallicity for several million stars reveals the differing content of chemical elements in the stellar populations of our Galaxy," says Zeljko Ivezic of the University of Washington. "By using two-dimensional images in different colors, we build up a three-dimensional ‘tomographic’ map that clearly delineates the disc and halo components of the Milky Way."

Many features of the map confirm standard views of the structure of the Milky Way, but the projected motions measured for metal-poor stars appear to contradict a long-standing hypothesis of galaxy construction that an ancient act of galactic cannibalism gave rise to the "thick disc" of stars enveloping the thin disc in which our star, the Sun, resides. The metal map also shows that galaxies cannibalised by the Milky Way, including shards of one known as the Monoceros Stream, possess stars with different metal content than nearby stars. The differences in metal content with location will help astronomers to delineate the extent of the Monoceros Stream and reveal its origins.

The SDSS-II results also provide a roadmap for future, still larger surveys, such as those planned for the 8.4-metre Large Synoptic Survey Telescope, which could extend the survey ten times further, to the very edge of the Milky Way.