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)

~15 kpc

~3 kpc

He used the apparent brightness and proper motion of stars to come to the same conclusion as Herschel.

So why were they wrong?

• Imagine driving through a heavy fog.• To get your bearing perhaps a large watch tower could provide guidance

We need to use globular clusters (collection of galaxies above/below the plane of the galaxy)

The Milky Way Disk acts just like a thick fog. To get our bearings…

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.

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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

Sa

Sb

Sc

Ordinary Spirals

NGC 628Face-On Sc

NGC 891Edge-On Sb

Andromeda Spiral Galaxy

Spiral Galaxy M100

NGC 253 is a Starburst Galaxy

NGC 4414Type Sc

Flocculent & Grand Design Spirals

NGC 628Type Sc

SBa

SBb

SBc

Barred Spirals

E1 E5

Elliptical Galaxies

Elliptical Galaxies

Irregular Galaxies

Large Magellanic Cloud Small Magellanic Cloud

Irregular Galaxy

Galactic Clusters