In 2008 an international team of astronomers from the Sloan Digital Sky Survey completed the largest and most detailed map of stellar metallicity within our galaxy. This map contains over 2.5 million stars and with it "astronomers can begin to tackle many unsolved mysteries about the birth and growth of the Milky Way", as lead researcher Zeljko Ivezic said.

Metallicity, as you learned in the activity of the same name, is the amount of elements heavier than lithium within a star. The big bang produced only the three lightest elements, hydrogen, helium, and lithium. All other heavier elements were produced within the cores of stars through nuclear fussion. When the stars die they eject their matter into space where it condenses into new stars.

These new stars produce more heavy elements and release them into space at the end of their life time. So as time goes on the concentration of heavy elements in space increases and therefore the concentraion of heavy elements within the stars that form also increaes. By measureing this concentration the age of the star can be determined.

"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, another member of the research team.

In order to map the metallicity of such a large number of stars a new, more efficient technique had to be developed. Traditionally the metallicity of a star is determined through studying the star's spectra. However this technique is time consuming and cannot be applied to 2.5 million stars in an efficent way.

What was used instead is a technique developed by a group led by Timothy Beers of Michigan State University. This technique is able to determine the metallicity of a star from simply observing its color, a much simpler process. Using this the SDSS astronomers were able to increase their efficiency over ten fold.

This map is two dimentional prjection of a three dimentional analysis of the galaxy and should be look at as a cross section of the galaxy. With the bottom being the galactic disk and the top being the galactic halo. From the map it can be clearly seen that the stars in the halo are much older than the stars in the galactic disk.

The Monoceros stream is an interesting feture and in a result of our galaxy caniblizing a smaller galaxy sometime during the past. While it has been known for a long time that colliding galaxies will consume one another it was thought that this act gave birth to the thick disk found within galaxies such as ours. However this map seems to contridict this theory, demanding a new and more accurate description of galatic formation.