This is NOT the Milky Way galaxy! It’s a similar one: NGC 4414.

The Milky Way Galaxy

New distance unit: the parsec (pc).

Using Earth-orbit parallax, if a star has a parallactic angle of 1",it is 1 pc away.

Distance (pc) = 1Parallactic angle (arcsec)

1 pc = 3.3 light years = 3.1 x 1018 cm = 206,000 AU

1 kiloparsec (kpc) = 1000 pc1 Megaparsec (Mpc) = 10 6 pc

Closest star to Sun is Proxima Centauri. Parallactic angle is 0.7”, so distance is 1.3 pc.

If the angle is 0.5", the distance is 2 pc.

The Milky Way Galaxy

Artist's Conception

Take a Giant Step Outside the Milky Way

Example (not to scale)

from above ("face-on")see disk, with spiral andbar structure, and bulge (halo too dim)

from the side ("edge-on")

.Sun

The Three Main Structural Components of the Milky Way

1. Disk

- 30 kpc diameter

- contains young and old stars, gas, dust. Has spiral structure

- vertical thickness roughly 100 pc - 2 kpc (depending on component. Most gas and dust in thinner layer, most stars in thicker layer)

3. Bulge

- About 4 kpc across

- old stars, some gas, dust

- central black hole of 3 x 106 solar masses

- spherical

- at least 30 kpc across

- contains globular clusters, old stars, little gas and dust, much "dark matter"

- roughly spherical

2. Halo

Globular Clusters

- few x 10 5 or 10 6 stars

- size about 50 pc

- very tightly packed, roughly spherical shape

- billions of years old

Clusters are crucial for stellar evolution studies because:

1) All stars in a cluster formed at about same time (so all have same age)

2) All stars are at about the same distance

3) All stars have same chemical composition

Herschel’s Milky Way drawing

How was Milky Way size and our location determined?

Herschel (late 18th century): first map of Milky Way. Sun nearcenter.

3 kpc

.

Sun

Shapley (1917) found that Sun was not at center of Milky Way

Shapley used distances to Globular Clusters to determine that Sun was 16 kpc from center of Milky Way. Modern value 8 kpc.

Stellar Orbits

Halo: stars and globular clusters swarm around center of Milky Way. Very elliptical orbits with random orientations. They also cross the disk.

Bulge: similar to halo.

Disk: rotates.

How Does the Disk Rotate?

Sun moves at 225 km/sec around center. An orbit takes 240 million years.

Stars closer to center take less time to orbit. Stars further from center take longer.

The "rotation curve" of the Milky Way

=> rotation not rigid like a phonograph record. Rather, "differential rotation".

Over most of disk, rotation velocity is roughly constant.

Spiral Structure of Disk

Spiral arms best traced by:

Young stars and clustersEmission NebulaeAtomic gasMolecular Clouds(old stars to a lesser extent)

Disk not empty between arms, just less material there.

Problem: How do spiral arms survive?

Given differential rotation, arms should be stretched and smeared out after a few revolutions (Sun has made 20 already):

The Winding Dilemma

So if spiral arms always contain same stars, the spiral should end up like this:

Real structure of Milky Way (and other spiral galaxies) is more loosely wrapped.

Proposed solution:

Arms are not material moving together, but mark peak of a compressional wave circling the disk:

A Spiral Density Wave

Traffic-jam analogy:

Now replace cars by stars. The traffic jams are due to the stars' collective gravity. The higher gravity of the jams keeps stars in them for longer. Calculations and computer simulations show this situation can be maintained for a long time.

Traffic jam on a loop caused by merging

Not shown – whole pattern rotates

Gas clouds pushed together in arms too => high density of clouds => high concentration of dust => dust lanes.

Also, squeezing of molecular clouds initiates collapse within them => star formation. Bright young massive stars live and die in spiral arms. Emission nebulae mostly in spiral arms.

So arms always contain same types of objects, but individual objects come and go.

90% of Matter in Milky Way is Dark Matter

Gives off no detectable radiation. Evidence is from rotation curve:

RotationVelocity (AU/yr)

Solar System Rotation Curve: when almost all mass at center, velocity decreases with radius ("Keplerian")

R (AU)

10

5

1

1 10 20 30

Curve if Milky Way ended where radiating matter pretty much runs out.

observed curve

Milky Way Rotation Curve

Not enough radiating matter at large R to explain rotation curve => "dark" matter!

Dark matter must be about 90% of the mass!

Mass of Milky Way

6 x 1011 solar masses within 40 kpc of center.

Composition unknown. Probably mostly exotic particles that hardly interact with ordinary matter at all (except gravity). Small fraction may be brown dwarfs, dead white dwarfs.

Most likely it's a dark halo surrounding the Milky Way.

from above ("face-on")see disk and bulge (halotoo dim)

from the side ("edge-on")

Orion spur

Perseus arm

Cygnus arm

Carina arm

Sun

Sagittarius arm

Galaxies

First “spiral nebula” found in 1845 by the Earl of Rosse. Speculated it was beyond our Galaxy.

1920 - "Great Debate" between Shapley and Curtis on whether spiral nebulae were galaxies beyond our own. Settled in 1924 when Hubble observed individual stars in spiral nebulae.

Early drawings ofnebulae by Herschel (1811). Stars, gas orboth? Distances?

The Variety of Galaxy Morphologies

A bar is a pattern too, like a spiral.

More on bars…

Milky Way schematicshowing bar

Another barred galaxy

Galaxy Classification

Spirals Ellipticals Irregulars

barred unbarred E0 - E7 Irr I Irr IISBa-SBc Sa-Sc "misshapen truly spirals" irregular

Hubble’s 1924 "tuning fork diagram"

bulge less prominent,arms more loosely wrapped

Irr

disk and large bulge, but no spiral

increasing apparent flatness

Still used today. We talk of a galaxy's "Hubble type"

Milky Way is an SBbc, between SBb and SBc.

What the current structure says about a galaxy’s evolution isstill active research area.

Ignores some notable features, e.g. viewing angle for ellipticals, number of spiral arms for spirals.

bulge less prominent,arms more loosely wrapped

Irr

disk and large bulge, but no spiral

increasing apparent flatness

Messier 81 – Sa galaxy Messier 101 – Sc galaxy

Sa vs. Sc galaxies

Irr I vs. Irr II

Irr I (“misshapen spirals”) Irr II (truly irregular)

Large Magellanic Cloud Small Magellanic Cloud

These are both companion galaxies of the Milky Way.

bar

poor beginningsof spiral arms

Similar to halos of spirals, but generally larger, with many more stars. Stellar orbits are like halo star orbits in spirals.Stars in ellipticals also very old, like halo stars.

Orbits in a spiral

An elliptical

Ellipticals

A further distinction for ellipticals and irregulars:

Giant vs. Dwarf

1010 - 1013 stars 106 - 108 stars 10's of kpc across few kpc across

Dwarf Elliptical NGC 205

Spiral M31

Dwarf Elliptical M32

In giant galaxies, the average elliptical has more stars than the average spiral, which has more than the average irregular.

What kind of giant galaxy is most common?

Spirals - about 77%Ellipticals - 20%Irregulars - 3%

But dwarfs are much more common than giants.

• Local solar neighborhood, typical stellar distances ~ 1pc (3.26 ly)-- 50% are double/multiple systems-- Most in near neighborhood (local arm) are cooler/fainter

• Most of Galaxy within ~ 20 kpc (1011 - 1012 stars)

• 800 kpc to M31

• Local group (about 20), radius ~ 1 Mpc

• Local supercluster (about 2500), distance ~ 15 Mpc to Virgo cluster

• Several other galaxy clusters to ~75 Mpc

• Increasing numbers of AGNs out to radius ~ 100 Mpc

• QSOs to extent of observable universe, ~10000 Mpc

ASTROPHYSICAL “DEMOGRAPHICS”

SIZE SCALES

Jupiter’s OrbitRadius 5.2AU

LOCAL ISM

MILKY WAY

LOCAL GROUP

DISTANCES TO:

GALACTIC CENTER 8.5 Kpc

LARGE MAG. CLOUD 55 Kpc

M31 ~ 800Kpc