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