The Milky Way Galaxy

Structure of the galaxy• The Milky Way is a

spiral galaxy• The galactic center

is the thickest part.• Around a dozen

arms spin around the center.

• The Solar System is on the Orion Arm.

Dimensions of the Milky Way

• Distances in the galaxy are measured in kiloparsecs (kpc).

• Most of the stars are found near the galactic plane.

• Globular clusters are found above and below the plane.

Parts of the Galaxy

What We Actually See From Earth

• On Earth, the Milky Way appears to be a white band across the night sky.

• In the constellation Sagittarius, we look into the galaxy center and thousands of stars appear.

A Visible View of the Milky Way, pieced together from many pictures.

Photographs using all parts of the spectrum:

How Do We Know the Size of the Galaxy?

• Henrietta Leavitt discovered that the period of Cepheid variable stars depends on their absolute magnitudes.

• If we know the magnitudes, we can determine distances: D = 10(Mapp - Mabs +5)/5

• You can also read the distance off the graph!

Cepheid Variables are Giants that have left the main sequence.

• Polaris is a good example of a Cepheid variable that is found in our region of the galaxy.

• Polaris’s magnitude goes up and down 0.15 in around 3 days on a regular cycle.

Hubble Records a Cepheid

Leavitt Studied Variables in the Small Magellanic Cloud

All these stars are around the same distance to us, so those that are bright are really bright.

Dr. Leavitt discovered a variable in the small Magellanic Cloud with an apparent magnitude of +15.5, and its absolute magnitude is -4.5.

D = 10(Mapp - Mabs +5)/5

D = 10(15.5- -4.5 +5)/5 = 1025/5 = 105

D = 100,000 pc = 100 kpc

Globular Clusters

• Globular Clusters are found on the edges of the galaxy.

• Harlow Shapley found the distance to these clusters and he plotted their positions.

• For them to fit, the Milky Way must be around 30 kpc across. (Shapley miscalcuated to around 40 kpc.

Modern Globular Cluster Plot

• Once the globular clusters are plotted, using correct magnitudes, they form a halo around the galactic plane.

• This helps us determine that the center is 8 kpc from the Solar System and that the galaxy is 30 kpc in width.

HALO

The Andromeda Galaxy

We can’t see the entire Milky Way Galaxy, so we have to use pictures of other spiral galaxies to help us predict its structure.

2 million light yrs. away

The Rotating Galaxy

• The long spiral arms of the galaxy tell us that the galaxy is spinning.

• It takes around 230 million years for one revolution of the Solar System.

• We are not anywhere near the center of the galaxy; we’re just a satellite system.

Radio View of the Milky Way

Evidence of Rotation

• The Doppler Shift tells us if an object is moving toward or away from us.

• Red shift indicates moving away; blue shift towards us.

The Doppler Shift can be caused by

another star’s motion or our motion, or both.

We’re catching up to them.

We’ve already past them.

They’re catching up to us.

They’re already past us.

Determining Radial Velocity

Vradial = shift x c

rest

Vradial = 200 x 300,000

5400

Vradial = 10714 km/sec10714 km/sec

(Away from us.)

Another Example…

The Solar System is overtaking a star on the Perseus Arm, causing a blue shift of 0.14 nanometers.

Vradial = shift x c

rest

Vradial = 0.14 nm x 300,000

700 nm

Vradial = 60 km/sec

A problem to try for yourself…

An observer near the galactic center sees a red shift for the Solar System of 0.4 nanometers. The rest wavelength is 600 nm. What is the velocity of the Solar System through the galaxy?

Vradial = shift x c

rest

Vradial = 0.4 x 300,000 km/sec

600

Vradial = 200 km/sec

Center

Motions within the Milky Way

Population I Stars• The stars of the galaxy

are often divided into two populations.

• Population I stars are “metal-rich.”

• This means they have elements other than hydrogen and helium

• They are found mostly in the galactic plane, and are mostly likely second generation young stars.

Population II Stars

• These older stars are found mostly in the halo of the galaxy. They have very eccentric orbits.

• They are “metal-poor.”• They contain almost only hydrogen and helium.

Summarizing Properties of the Milky Way

GALACTIC DISK GALACTIC HALO GALACTIC BULGE

Highly flattened Roughly spherical—mildly flattened

Somewhat flattened and elongated in the plane of the disk ("football shaped")

Contains both young and

old stars Contains old stars only Contains both young and

old stars; more old stars at greater distances from the center

Contains gas and dust Contains no gas and dust Contains gas and dust,

especially in the inner regions

Site of ongoing star formation

No star formation during the last 10 billion years

Ongoing star formation in the inner regions

Gas and stars move in circular orbits in the

Galactic plane

Stars have random orbits in three dimensions

Stars have largely random orbits but with some net rotation about the Galactic center

Spiral arms No obvious substructure Ring of gas and dust near

center Galactic nucleus

Overall white coloration, with blue spiral arms

Reddish in color Yellow-white