Digital Universe Guide - Hayden Planetarium

3.2. MILKY WAY ATLAS TUTORIAL 53
stars and turn the boxes on, a box will appear around the bright blue stars. Press the Box Toggle
Button to turn the boxes off.
Red Stars Are Cooler Stars Now let’s see the redder stars by typing
see red
The red stars are more numerous (more than 80% of the stars) and are distributed in a spherical shape.
Of course, because these are observed from Earth, the distribution is centered on the Sun, but one can
imagine the entire Galaxy filled with these dim stars.
Red stars are cooler stars that form throughout the Galaxy. They burn their fuel slowly and have long
lifespans; some stars are as old as the Galaxy. The Sun is among these red stars, a relatively cool star
with a (B − V ) = 0.65. It has been burning its fuel for about 5 billion years and has about another
5 billion to go before entering the end stages toward a red giant and a white dwarf star.
Stellar Color Traces Galactic Structure These two contrasting views tell us about the structure of
the stars in our Galaxy. The red stars are typically known as Population II stars and revolve around the
center of the Galaxy in elliptical, sometimes highly inclined orbits. Bluer stars are referred to as
Population I stars and lie in the Galactic disk, orbiting in orderly paths around the Galactic center. While
we plot only the stars we have accurate distances for, you can imagine the disk (turn on the Milky Way
image using the galaxy button) filled with blue stars with many red stars as well. Also imagine a
spherical distribution around the entire Galaxy filled with cool, red stars. This is called the Galactic Halo
(halo button), and we’ll talk about it soon.