Washington, May 29 : An international team of astronomers from the Sloan Digital Sky Survey (SDSS-II) has unveiled the most complete and detailed map yet of the metals present in more than 2.5 million stars in the Milky Way.
Previous chemical composition maps were based on much smaller samples of stars and didn't go as far from a region extending from near the Sun to about 30,000 light years away.
"Older sky surveys that did include a lot of stars were not accurate enough to measure their chemical composition," explained study leader Zeljko Ivezic, a University of Washington astronomer.
"With the new SDSS map, astronomers can begin to tackle many unsolved mysteries about the birth and growth of the Milky Way," he added.
Astronomers use the term "metals" to describe all elements heavier than hydrogen and helium, including the oxygen we breathe, the calcium in our bones, and the iron in our blood.
Although hydrogen, helium and traces of lithium were created at the beginning of the Universe in the Big Bang, all other elements (such as iron and carbon) were forged in the cores of stars or during the explosive deaths of massive stars.
As a result, stars that formed early in the history of the Galaxy (some 13 billion years ago) were made of gas that had few metals created by the generations of stars that came before. These "metal-poor stars" provide astronomers 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.
"By mapping how the metal content of stars varies throughout the Milky Way, astronomers can decipher star formation and evolution, just as archaeologists reveal ancient history by studying human artifacts," explained University of Washington graduate student Branimir Sesar, a member of the research team.
To make this new map of the Galaxy, the SDSS-II team used the colors of millions of stars to infer their metal content - often referred to as "metallicity".
To estimate the metallicity of so many stars at once, the team compared the star colors with spectroscopic observations for many tens of thousands of these stars.
A group from Michigan State University devised methods to estimate the metallicities of these stars based on their spectra.
"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," explained Ivezic.
"By using two-dimensional images in different colors, we build up a three-dimensional 'tomographic' map that clearly delineates the disk and halo components of the Milky Way," he added.