The Milky Way I

AST 112

Credit: Stephane Vetter

How do we look at this…

… and decide that it looks like this?

The Milky Way

• Studying the Milky Way is “like trying to draw a picture of your house without ever leaving your bedroom.”

• It helps to compare it to other galaxies.

The Milky Way

• Point a telescope at the cloudy band. It turns into countless stars.

Studying the Milky Way

• Astronomers first tried to determine our location in the Milky Way by counting stars in different directions

– Above and below the band of the Milky Way, they came up with roughly equal numbers of stars in all directions

Structure of the Milky Way

• Herschel looked at 683 regions and counted stars

• Assumed:– Stars have same luminosity– Density of stars is constant– No extinction due to dust– Can see to edge of the

distribution of stars The Sun is the large star nearthe center. Is that suspicious?

Our Location in the Milky Way

• These observations suggested that we are at the center of the Milky Way– And why not? Away from the band, we see equal

numbers of stars in all directions.

• Could it be that we only see our “neighborhood” and nothing beyond?– Why would that happen?

Extinction

• If there is dust between the observer and the object, the object will look dimmer than it is

– This is called extinction, and astronomers did not account for it early on

Are we at the center?

• Seems to me…

– If you look one way and see the band then look the other way and don’t see it, we’re not at the center!

Are we at the center?

• Interstellar dust obscures most of the Milky Way

• The stars in our night sky are a small corner of our galaxy– Can only see so far into the disk– Dust thickest in disk (and we’re in the disk!)– Which direction has the least amount of dust?

• Only thanks to radio and IR can we look further in

Our Location in the Milky Way• Thus far we’ve used stars for

these maps

• Globular clusters are a better indicator of our position in the Milky Way than individual stars

– Brighter

– Spherical distribution and out of the disc!

– Harlow Shapely measured the spherical distribution of globular clusters (1920’s)

Our Location in the Milky Way

• Shapley found:– Sun is 45,000 LY from center of Milky Way– (It’s actually 28,000 LY)

• Shapley’s value was off because he did not account for …? – What would make something look farther away (dimmer)?

• Globular clusters seen mostly toward Sagittarius – Where’s that?

Sagittarius and the Galactic Center

Spiral Arms

• Start by looking at other galaxies for this• The arms are blue with red patches– What does this say about star formation?

Spiral Arms

• Use H II regions and O/B type stars as tracers of spiral arms

• Don’t misinterpret the diagram – not center of Milky Way

• Sun is in Orion-Cygnus Arm

Structure of the Milky Way

Other than dust clouds, can you identify any structures in the band?

Structure of the Milky Way• A spiral galaxy with 100 billion+

stars

• Diameter is 100,000 LY

• The Sun is 28,000 LY from the center

• Components:– Bulge– Disk– Arms– Halo– SMBH (Supermassive black hole)

Structure of the Milky Way

• The bulge is a spherical distribution stars in the center of a galaxy– Note the yellow / red color: they are older

The Bulge

• We use infrared to look through dust and see the bulge

• We can look along lines with minimal extinction– E.g. Baade’s window; can past galactic center

Structure of the Milky Way

• The disk is a 1000 LY thick pancake of younger, bluer stars

• Orbits are “orderly”– Same direction– Confined to disk

• The disk of the Milky Way contains the spiral arms

Structure of the Milky Way

Look closely to see that the spiral arms are structures within the disk.

Structure of the Milky Way

• The halo contains a spherical distribution of:– Stars– Globular clusters– Dark matter

Orbits in the Milky Way

• Orbits of disk stars are roughly circular– They rise and fall

• If it gets too far above the disk, it starts downward

• Overshoots

– This gives the disk its 1000 LY thickness

• Orbits of halo and bulge stars are random

Orbits in the Milky Way

• Orbits of disk and halo/bulge stars indicates that they have different origins, or perhaps formed at different stages.

Mass of the Milky Way

• Kepler’s 3rd Law relates orbital period to distance and mass– Excludes mass not

contained within orbit

• Shapley measured the distance and we know orbital velocities