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

New distance unit: the parsec (pc). Using Earth-orbit parallax, if a star has a parallactic angle of 1", it is 1 pc away. If the angle is 0.5", the distance is 2 pc. 1 Parallactic angle (arcsec) Distance (pc) = Closest star to Sun is Proxima Centauri. Parallactic angle is 0.7”, so distance is 1.3 pc. 1 pc = 3.3 light years = 3.1 x 1018 cm = 206,000 AU 1 kiloparsec (kpc) = 1000 pc 1 Megaparsec (Mpc) = 10 6 pc

The Milky Way Galaxy

Take a Giant Step Outside the Milky Way Artist's Conception Example (not to scale)

. from above ("face-on") see disk, with spiral and bar structure, and bulge (halo too dim) . Sun from the side ("edge-on")

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)

2. Halo 3. Bulge - at least 30 kpc across - contains globular clusters, old stars, little gas and dust, much "dark matter" - roughly spherical 3. Bulge - About 4 kpc across - old stars, some gas, dust - central black hole of 3 x 106 solar masses - spherical

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

. How was Milky Way size and our location determined? Herschel (late 18th century): first map of Milky Way. Sun near center. Herschel’s Milky Way drawing . Sun 3 kpc

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. => rotation not rigid like a phonograph record. Rather, "differential rotation". Over most of disk, rotation velocity is roughly constant. The "rotation curve" of the Milky Way

Spiral Structure of Disk Spiral arms best traced by: Young stars and clusters Emission Nebulae Atomic gas Molecular 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.

A Spiral Density Wave 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 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: 10 Rotation Velocity (AU/yr) Solar System Rotation Curve: when almost all mass at center, velocity decreases with radius ("Keplerian") 5 1 1 10 20 30 R (AU) 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! 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. Mass of Milky Way 6 x 1011 solar masses within 40 kpc of center.

from above ("face-on") see disk and bulge (halo too dim) Perseus arm Orion spur Sun Cygnus arm Sagittarius arm Carina arm from the side ("edge-on")

Galaxies

Early drawings of nebulae by Herschel (1811). Stars, gas or both? Distances? 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.

The Variety of Galaxy Morphologies

More on bars… Milky Way schematic showing bar Another barred galaxy A bar is a pattern too, like a spiral.

Galaxy Classification Hubble’s 1924 "tuning fork diagram" bulge less prominent, arms more loosely wrapped Irr disk and large bulge, but no spiral increasing apparent flatness Spirals Ellipticals Irregulars barred unbarred E0 - E7 Irr I Irr II SBa-SBc Sa-Sc "misshapen truly spirals" irregular

Still used today. We talk of a galaxy's "Hubble type" 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 is still active research area. Ignores some notable features, e.g. viewing angle for ellipticals, number of spiral arms for spirals.

Sa vs. Sc galaxies Messier 81 – Sa galaxy Messier 101 – Sc galaxy

Irr I vs. Irr II Irr I (“misshapen spirals”) Irr II (truly irregular) bar poor beginnings of spiral arms Large Magellanic Cloud Small Magellanic Cloud These are both companion galaxies of the Milky Way.

Ellipticals 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. An elliptical Orbits in a spiral

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) ASTROPHYSICAL “DEMOGRAPHICS” 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

SIZE SCALES MILKY WAY Jupiter’s Orbit Radius 5.2AU LOCAL GROUP DISTANCES TO: GALACTIC CENTER 8.5 Kpc LARGE MAG. CLOUD 55 Kpc M31 ~ 800Kpc LOCAL ISM