1. The Structure of the Milky Way
What is the size and shape
Of the system of stars in which
We live?

2. Outline of lecture Lecture taken from Chapter 22
Skip material on interstellar gas
Skip historical stuff
Begin with properties of globular star clusters (crucial for mapping out the galaxy).

3. Globular clusters and why they are important in this context
Big balls of about 100,000 stars
You can see them at great distances
About 150 cataloged
They are unevenly distributed in the sky (great summertime objects, few in the winter). (Figure 22.25)
1/5 of globular clusters
In 2% of the sky

4. Distribution of globular clusters on the sky implies a distribution in space like Figure 22.26

5. Where (on the sky) is the galactic center?
In Sagittarius
RA = 17h45m
DEC=-28.3 degrees
Position on SC1 Chart

6. You can’t determine the distances to globular clusters by parallax
You can’t determine the distances to globular clusters by parallax. They are too far away (think of the theodolite lab). You need another method. If you know the absolute magnitude M of an object and can measure the apparent magnitude m, you know the distance. The distance modulus (m-M) is a direct measure of the distance.

7. Object of choice: Cepheid Variables (after Delta Cephei)
Cepheid variables vary in brightness periodically
Periods range from a couple of days to 100 days.
Absolute magnitude dependent on period

8. Distances to globular clusters and the galactic center
1 parsec = 3.26 light years
Need bigger “yardsticks” for these objects
1000 parsecs=1 kiloparsec=3260 light years
Distance to galactic center = 8.5 +/ kiloparsecs

9. How far is 8.5 kiloparsecs? Let 1 au = 1cm
Nearest star (1.3 parsecs) = 2.68 km = 1.66 miles
8.5 kiloparsecs = 17,532 km = 10,870 miles (2.74 Earth radii)
Light signal transmitted from the GC would take 27,700 years to reach us

10. A view of the galactic center (visible only at radio wavelengths)

11. New Coordinates: galactic coordinates
“imaginary lines on the sky”
A line through the middle of the Milky Way = galactic equator
0 degrees galactic longitude in direction of the galactic center
Coordinates are galactic latitude and longitude

12. Galactic coordinates

13. Observations indicate the shape and size of the Milky Way Galaxy

14. Structure of the Milky Way
Disk
Bulge
Galactic center
Galactic halo
Question: what simple observation
Is consistent with a part of this picture

15. A view of the Milky Way with an artist’s touch