The Structure of the Milky Way

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

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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
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Distribution of globular clusters on the sky
implies a distribution in space like Figure 22.26
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Where (on the sky) is the galactic center?

• In Sagittarius
• RA 17h45m
• DEC-28.3 degrees
• Position on SC1 Chart

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You cant 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.
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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

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Distances to globular clusters and the galactic
center

• 1 parsec 3.26 light years
• Need bigger yardsticks for these objects
• 1000 parsecs1 kiloparsec3260 light years
• Distance to galactic center 8.5 /- 0.50
  kiloparsecs

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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

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

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Galactic coordinates
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Observations indicate the shape and size of the
Milky Way Galaxy
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Structure of the Milky Way

• Disk
• Bulge
• Galactic center
• Galactic halo

Question what simple observation Is consistent
with a part of this picture
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A view of the Milky Way with an artists touch