Mapping the galaxy is tricky business because of the inherent difficulties in measuring distances to stars. The problem boils down to determining the inherent brightness of a star, a property called its absolute magnitude. Then the difference between this and its observed magnitude on Earth gives you some idea of its distance.
But how to determine absolute magnitude? One way is to use the famous Hertzprung-Russel diagram (also known as the colour magnitude diagram), which plots the relationship between a star’s colour and its absolute magnitude. The process is relatively straightforward: measure a star’s colour and observed magnitude, then determine its absolute magnitude from the colour magnitude diagram. A straightforward calculation should give you its distance (provided there’s no dust obscuring your view).
The problem of course is that the colour magnitude diagram is complex and many-valued. So a star with a given colour could belong to one of several different stellar populations with different luminosities.
Until recently, all that had been more or less academic because nobody had measured the colours of most stars in the galaxy.
All that changed with the Sloan Digital Sky Survey, an effort now ten years old to scour the skies using a 2.5 metre telescope at Apache Point Observatory in New Mexico. One part of this project called the Sloan Extension for Galactic Understanding and Exploration or SEGUE which was completed earlier this year was to measure the colour of some 240,000 stars in the Milky Way.
Now Jelte De Jong at the Max-Planck-Institut fur Astronomie in Germany and a few pals have used this data to estimate the distance of each of these stars using the colour magnitude diagram. The result is a new 3-D map of the galaxy, part of which is shown above (each map is a slice through the galaxy, centred on the Sun).
That’s an important stepping stone towards a better understanding of our galaxy’s evolution. For example the map immediately shows how the metal content in stars varies in three dimensions across the galaxy
But there’s far more to be mined from this map. Astronomers believe that galaxies form and evolve through accretions and mergers with other smaller bodies. These events leave their mark on the structure of the galaxy and the movement of the stars within it.
De Jong and friends say they can see in their map various structures such as the Sagittarius stream which is the result of the Milky Way’s interaction with the Sagittarius dwarf galaxy.
However the origin of other features is far less well understood. In particular, the increased density of stars in the direction of Virgo and Monoceros are puzzling. The promise of the new map is that it might reveal the origin of these features. Exciting stuff.
Ref: http://arxiv.org/abs/0911.3900: Mapping The Stellar Structure of The Milky Way