Digital Universe Guide - Hayden Planetarium

3.3. MILKY WAY DATA GROUPS 141
this is that they have incredibly long lifetimes. The Sun, a G dwarf, has a lifetime of around 10 billion
years. After this time, its primary source of fuel, hydrogen, is exhausted and the star enters into the final
stages of its existence. M dwarfs live for tens of billions and perhaps even trillions of years, meaning
most M stars that were ever created in the Galaxy are still around. While the hotter, brighter, more
massive stars come and go, the cooler, red dwarfs remain.
The Bow Tie Effect By now, you may have noticed that these stars, as seen from outside the
Galaxy, appear to form the shape of a bow tie whose center is at the Sun. These wedges describe the
patches of sky observed by the telescope. If you return home to the Sun, you will see where these stars
were observed in the night sky. The stars are not everywhere, but in select patches in the sky. As you
move away from the Sun, you can see these patches transform from a 2-D view to a 3-D distribution. If
the telescope were to observe the entire sky, we would see a spherical distribution of stars around the
Sun, rather than the triangular shapes presently seen.
Galactic Structure Astronomers study these stars to understand the structure of our Galaxy.
Because we are in the midst of the Milky Way’s disk, it is difficult to grasp the true shape and
configuration of our own Galaxy.
From a point where you see all these stars (you may have to brighten them), notice the contours of
this data set. Stars at the “edge” of the data set, i.e., those stars farthest from the Sun, form a circular
arc. Conversely, the higher density region closer to the Sun has an asymmetric shape. If you do not see
this shape, jump directly to this point:
jump -9000 12000 1500 -50 -80 -100
From this vantage point, you will see a distinct increase in the stellar density parallel to the Galactic disk
and in the direction of the Galactic core. This rising profile reflects the vertical structure of the Milky
Way. You may also notice a small increase in density in the lower half of the data, less pronounced
because there is less data in this part of the survey.
Astronomers use 3-D star counts to realize the structure of the Galaxy. While we call the Milky Way
our home, we still know remarkably little about the overall structure of the galaxy. Knowledge of these
star counts in various locations allow us to map the disk, bulge, and halo, and better understand the
Galaxy we live in.