The Structure of the Milky Way Galaxy What is the size, shape, and nature of the system of stars in which we live?

You can learn a lot by looking at the sky and thinking

From this picture, let’s figure out the shape of the Milky Way. Characteristics to go on are: Sprinkling of stars everywhere Thick band of light across the sky (the Milky Way) Band brightest in the direction of Sagittarius Band faintest in the direction of Taurus- Auriga What’s the answer?

The Answer

Let’s get quantitative about the thickness and diameter of the galactic disk. We start with the thickness of the galactic plane.

Measure the distance over which the density of stars significantly falls off with height above, or distance below, the galactic plane. The distance depends on the type of objects considered, but is roughly 100 – 200 parsecs. The galactic plane is much thinner than it is wide.

How far is it to the galactic center? How could we know? Hint provided by globular star clusters. Great summertime objects, few in winter

How globulars indicate the distance to the galactic center

Globular Clusters are Distant It must be a long way to the galactic center Globular clusterDistance (light years) M13 (Hercules)21,000 M15 (Aquarius)34,000 M92 (Hercules)26,000 M4 (Scorpius)14,000 M5 (Serpens)26,000