An early attempt to locate the Sun’s position in the Milky
Way was done by William Herschel (who also
discovered Uranus)
Herschels’ Milky Way Map (1785)
Sun
Dutch Astronomer Jacobus Kapteyn Model (1922)
He used the apparent brightness and proper motion of
stars to come to the same conclusion as Herschel.
~3 kpc
~15 kpc
So why were they wrong?
• Imagine driving
through a heavy fog.
• To get your bearing
perhaps a large watch
tower could provide
guidance
We
useDisk
globular
The need
Milkyto
Way
acts clusters
just like
(collection
galaxies
a thick fog.ofTo
get our
above/below
bearings… the plane of the
galaxy)
Globular Cluster – approx 106 stars in a
volume of only a few hundred LY across.
Globular Cluster M55
(located in constellation Sagittarius)
 Observing the luminosity of these stars has allowed us
to determine the distance to the cluster.
 This technique allows us to triangulate the location of
our Solar System.
35
8

The top view is from the ISAS spacecraft
 25,60,100 micrometer wavelengths (interstellar dust)
 Shows the distribution of interstellar dust.

Near IR taken from COBE (1.2, 2.2, 3.4 micrometers)
 Note the bulge at the center of the galaxy
An optical view of the sky
An infrared view of the sky
A radio view of the sky
This is what the Milky Way
would probably look like
from a great distance
Consider a Hydrogen Atom
Spin – Flip transitions
give off a radio photon
The entire sky at 21cm - Colors indicate the density of Hydrogen gas
Spin – Flip transitions in Medicine

Magnetic Resonance
Imaging
Radio Astronomers can map the Milky Way
using the Doppler Effect
 Galaxy
M83
 12 million LY
from Earth
Note that
Hydrogen is also
concentrated in
the Spiral Arms
The Milky Way
So, what are the Spiral Arms?

Consider the following situation
High Density due to slow truck
It is thought that the spiral
arms are high density
regions (waves) where stars
form
As the galaxy
rotates, the
spiral arms get
tighter and
tighter
Ordinary Spirals
Sa
Sb
Sc
NGC 891
NGC 628
Edge-On Sb
Face-On Sc
Andromeda Spiral Galaxy
Spiral Galaxy M100
NGC 253 is a Starburst Galaxy
Flocculent & Grand Design Spirals
NGC 4414
Type Sc
NGC 628
Type Sc
Barred Spirals
SBa
SBb
SBc
Elliptical Galaxies
E1
E5
Elliptical Galaxies
Irregular Galaxies
Large Magellanic Cloud
Small Magellanic Cloud
Irregular Galaxy
Galactic Clusters