Astronomy 100 -- The Milky Way Galaxy
OUR GALAXY: THE MILKY WAY
An All-Sky Picture.
A Spectacular Image at the APOD Website!
Early "Model" of the Milky Way:
An infinite "sheet" of stars.
This was before astronomers knew about:
- Dust in space.
- Separate star systems
- All stars are not like the Sun.
First Step in Unveiling the Milky Way (1915)
Harlow Shapley uses Cepheids and RR Lyrae stars to
determine distances to 93 globular clusters.
He makes a 3-D map of their distribution.
He finds many more globular clusters located on one
side of the Sun rather than evenly spread around the Sun.
From this, Shapley reached 3 conclusions:
- 1) The Sun is NOT at the center of the Milky Way.
- 2) The Milky Way was LARGE! Determined the center
of the globular clusters was 25,000 pc away from us.
(Today we know its about 8,500 pc.)
- 3) The Milky Way is MASSIVE! Many GCs, objects that have
about 1,000,000 stars, are held by GRAVITY to the
Milky Way. The Milky Way must be many times more massive!
THE GREAT NEBULA DEBATE
In 1920, a "Great Debate" was held on the nature of the
"Spiral Nebulae" between Shapley and Curtis.
In 1923-4, Hubble finds Cepheids in the Andromeda Nebula.
From this he determined the distance to the Andromeda Nebula
(a few million light-years) and showed it was
a separate star system outside of the Milky Way!
MAPPING THE MILKY WAY
1) OPTICAL LIGHT: Dust limits how far we can see.
Map of O-star distribution shows:
2) RADIO WAVES
This works for distances up to 1.5 kpc (1,500 pc or about
5000 ly) away from the Sun.
Not affected by dust. Can "see" all of the Milky Way.
Important radio emission from
at a wavelength of 21 centimeters.
Hydrogen radio map reveals the distribution of the hydrogen gas.
Very structured and clumpy (not smooth)
Kind of a spiral arm pattern.
COMPONENTS OF THE MILKY WAY
Total Mass = 2 x 1011 Solar Masses!!
- Disk -- the flat part; like a pancake.
Diameter = 25 kpc (80,000 light years)
Thickness = 1 kpc (3,000 light years)
Contains: Gas, Dust, Open clusters, most stars.
Spiral arms are located here.
- Bulge -- "ping-pong ball on the pancake"
- Nucleus -- located at the very center.
- Halo -- Spherical volume around the disk.
- Globular Clusters.
- Old Stars
- DARK MATTER
(Discussed in next section)
A Top and Side View Diagram of the Milky Way.
Is our Milky Way Galaxy a Barred Spiral?
Rotation Curves and Dark Matter
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