Transcript charts_set_9

The Milky Way Galaxy
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
Distance (pc) = Parallactic angle (arcsec)
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)
Sun
.
from above ("face-on")
see disk, with spiral and
bar structure, and bulge
(halo too dim)
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
- 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.
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:
Traffic jam on a loop caused by merging
Not shown – whole pattern rotates
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.
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) 5
Solar System Rotation Curve: when
almost all mass at center, velocity
decreases with radius ("Keplerian")
1
1
10
20
30
R (AU)
observed curve
Milky Way
Rotation
Curve
Curve if Milky
Way ended
where radiating
matter pretty
much runs out.
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.
Perseus arm
from above ("face-on")
see disk and bulge (halo
too dim)
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
increasing apparent flatness
disk and large
bulge, but no spiral
Spirals
Ellipticals
barred unbarred
SBa-SBc Sa-Sc
E0 - E7
Irregulars
Irr I
"misshapen
spirals"
Irr II
truly
irregular
bulge less prominent,
arms more loosely wrapped
Irr
increasing apparent flatness
disk and large
bulge, but no spiral
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
1010 - 1013 stars
10's of kpc across
Dwarf Elliptical NGC 205
Spiral M31
Dwarf Elliptical M32
vs.
Dwarf
106 - 108 stars
few kpc across
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.
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 GROU
DISTANCES TO:
GALACTIC CENTER
8.5 Kpc
LARGE MAG. CLOUD 55 Kpc
LOCAL ISM
M31
~ 800Kpc