The Milky Way and Other Galaxies
This set of notes by Nick Strobel covers: the structure of our Galaxy, the
characteristics of other galaxies and finding distances to other galaxies.
Most of these notes will be in outline form to aid in
distinguishing various concepts. As a way to condense the text a bit, I'll
often use phrases instead of complete sentences. Vocabulary terms are
italicized. A photo gallery of different galaxies is at the bottom of the page.
Disk-shaped with spiral arms. Elliptical bulge in center and spherical halo that is denser closer to Galaxy center.
Some stars in last
stages of life pulsate by changing size. Try to have hydrostatic equilibrium but thermal pressure lags gravity collapse. Expanding star overshoots equilibrium
point. Then gravity catches up and contracts star. Contracts beyond equilibrium
point. Then pressure is too high and cycle continues.
- Cepheids--pulsation period 1-50 days. Henrietta Leavitt finds in
1912 that more luminous Cepheids pulsate more slowly-period-luminosity
relation. Later we find that there are two types of Cepheids: a) classical
Cepheids from young high-metallicity stars and more luminous AND b) W Virginis
Cepheids from older low-metallicity stars and less luminous. We can distinguish
between the two types of Cepheids from shape of the light curve profile (plot of brightness vs. time).
- RR Lyrae--pulsation period 5-15 hours. When we found distances to
cluster via main sequence fitting (described in the ``Steps to
the Hubble Constant'' section below), we noticed a) all RR Lyrae in a
cluster have the same average apparent magnitude; b) in different
clusters, get different
average apparent magnitude; c) All RR Lyrae have same average absolute
magnitude (=+0.6 corresponding to 49 solar luminosities) in clusters that are different
distances. Remember that in astronomy ``magnitude'' is the logarithm of the
brightness.
Harlow Shapley (in 1918) uses variable star PL relation
to find distance to 93 globular clusters-spherical clusters of 100,000's stars (looking like a glob) in very elliptical orbits around center of galaxy.
Shapley finds strong concentration of globulars in direction of Sagittarius
so he derived that
we are not at center! He found we are 10 kpc from center. Now taking into
account the effects of dust extinction on brightnesses and that there are
different Cepheid types, we get the distance to the center of the galaxy is
about 8 kpc.
Observe a) globular clusters on one side of the celestial
sphere have red shifts and globular clusters on the other side has blue shifts;
b) stars and H II regions near the sun have small doppler shifts; c) conclude
disk is rotating in organized fashion and globular clusters are not. We now use
21 cm emission from neutral atomic Hydrogen to map the motion of the disk.
A star in orbit
has a balance of centripetal acceleration (outward) and gravitational
acceleration (inward): 
,
where R is the
distance of star from center of galaxy, M
is the mass of galaxy inside
the star's orbit, and
G is the gravitation
constant. So 
.
The highest solid line in the plots above is for all of the galactic
components combined. The other curves (dashed, dotted, and solid) are the
contributions of individual galactic components (the bulge + stellar
halo = ``bulge'', disk, and the dark matter corona) to the rotational velocity.
We see a rigid body rotation close to center, then drop off, then rise,
then flat as far out as we can see. If we can reach the point where the enclosed mass does NOT increase with distance, then we have reached edge of galaxy mass.
Beyond the edge of the Galaxy, the rotational velocity will decrease as
the distance from the center increases. The Galaxy edge is not seen! The stars,
dust, and gas in the disk and stellar halo do not explain all the mass, so there
must be a Dark
Matter corona! Ninety percent of the Galaxy's mass is in the form of
Dark Matter--matter
that does not shine. Could be in form of planets, brown dwarfs
(too small to be a star), black holes, neutrinos with mass, or other exotic
particles we haven't discovered in our laboratories yet.
Gas is compressed in arms (most spiral galaxies usually
have two arms) to form stars. O & B stars (very hot, massive, young stars) and
H II regions enhance spiral outline. Lots of dimmer stars in between arms. Differential rotation (different angular speeds) of arms means
they should wind up and last only 5 x 10
years. Also,
spirals should occupy
only a small part of the disk. But other galaxies have spirals that are not
wound up which implies that
spirals are long-lasting and spirals occupy entire disk. Why?
- Density wave theory: Spiral region of greater gravity concentrates stars
and gas. The spiral regions rotate more slowly than the stars do (by
about a half). Stars behind the region of greater gravity are pulled forward
into the region and speed up. Stars leaving the region of greater gravity are
pulled
backward and slow down. Gas entering spiral wave is compressed. On downstream
side of wave, there should be lots of H II regions (star formation regions).
Unanswered questions: What forms the spiral wave in the first place? What
maintains the wave?
- Transient spirals: Arms come and go. Computer simulation of galactic disks show spiral patterns appearing and disappearing.
- Massive stars in spiral pattern die and have supernova explosion.
Explosion material compresses surrounding ISM causing formation of more stars,
some of which have supernova explosions, etc. The shockwaves keep the spiral
arms visible.
Walter Baade in 1944 finds stars fall into two
basic groups: Young, metal-rich stars (Pop. I) and old, metal-poor stars (Pop.
II). The population classification scheme is now:
- Population II--in spheroidal component (halo, bulge);
old stars (10 to 15 billion years old) so the stars masses less or equal to
0.8 M
;
abundance of metals: 
times that of the sun for stars in the
outer stellar halo and the metal abundance rises as go inward toward the
Galactic center, in bulge see metal content rise to 1-3 times that of sun;
little overall rotation--stars have large random velocities with highly
elliptical orbits.
- Population I--in disk component, the stars have a wide range of
ages (0 - ten billion years) with the youngest ones in the spiral arms;
abundance
of metals: 0.1 times that of the sun in the oldest Pop. I stars that can be
found at heights 1 kpc above the disk plane in slightly elliptical orbits,
0.5-1 times sun in middle-aged stars that can found at heights 350 pc above
disk plane, 1-2 times sun in young stars that can be found at heights 200 pc
above disk plane in circular orbits, 1-2.5 times sun in youngest stars
(< 100 million years) in spiral arms that can be found at heights 120 pc
above the disk plane in circular orbits.
The Galactic center has a strong radio source in Sagittarius (Sagittarius
A).
Non-thermal (synchrotron) radiation from rapidly moving charged particles in
strong magnetic field. X-rays also seen from object less than 1 pc across.
Stellar velocities indicate large mass (one million solar masses) so it is
assumed that there is a massive black hole
formed by mergers of stars and stellar remnants. Also see expanding ring about
3 kpc from center. The compact mass could also be a dense cluster of stars. We
see compact masses in other galaxy cores (like M31,
M32, Sombrero Galaxy, M87,
others).
Galaxy--organized system of 10's millions to trillions of stars sometimes mixed with gas and dust all held together by gravity. We count the number of
stars in a galaxy by dividing the total luminosity by one star's luminosity (can also use luminosity function to get proportions right) OR by dividing the total
mass (from the rotation curve) by one star's mass (or use mass function).
Elliptical, Spirals (regular and barred), &
Irregulars. Most galaxies are small and faint-at far distances only see
luminous galaxies. Edwin Hubble (in 1936) classifies galaxies using
``tuning-fork diagram''. Originally, he thought it was an evolution sequence. We
now know that it is NOT.
- Ellipticals--round or elliptical shapes. a) Much more random
star motion than rotational (ordered) motion. The flattened shape is NOT due to
rotational flattening. b) Little dust and gas left between stars. c) No new star formation. No hot, bright, massive stars. d) No spiral structure.
Subclassified according to shape:
10 x (largest diameter - smallest diameter) / (largest
diameter), so an E7 is flatter than an E0. Most ellipticals are small and faint.
The dwarf ellipticals may be the most common type of galaxy (or maybe the
irregulars are). Examples of ellipticals: M32
(dwarf elliptical next to the
Andromeda Galaxy) and M87 (a huge elliptical in the center of the Virgo
cluster).
- Spirals--flattened disk with a) spiral structure. Spiral arms can
go all the way to bulge or be attached to bar that bisects bulge. b) More
rotational motion than random motion. c) Some or lots of gas and dust between
stars. d) Have new star formation in spiral arms. Most spirals are luminous.
Examples of spirals: Milky Way, M31 (the
Andromeda Galaxy), M33 (a small spiral
in the Local Group).
- Irregulars--no definite structure. Some irregulars have lots of dust and gas (star formation possible!). Most are faint. May be most common type of
galaxy (or maybe the dwarf ellipticals are). Examples of irregulars: LMC, SMC
(two small irregulars that orbit the Milky Way).
Galaxies seem to avoid 5-10 degree band along Milky Way
mid-plane--``zone of avoidance''. Now know that the zone is due to dust in the
disk.
Need to measure distances to determine the
luminosity and mass distribution of galaxies. See the
Steps to the Hubble Constant section below. The width of
absorption and emission lines from
galaxies show the amount of stellar motion in the galaxy. Some stars in a
galaxy are moving toward us (blue-shift spectral lines) and some stars moving
away from us (red-shift spectral lines); all the spectral lines blend together
to form a
FAT line. Same effect seen with gas motions (caused by heat and rotation) in a
single star or gas cloud. More thermal and/or rotational motion
creates a broader
line. More massive galaxies have more gravity so stars need to move faster
yielding the result that a
more massive galaxy has broader line. Also more massive galaxy has higher
luminosity. Use correlation of galaxy recessional velocity and galactic distance (Hubble law) to get distances for far away galaxies.
Most galaxies have large amounts of Dark
Matter--material not producing light but having a noticeable
gravitational effect.
The classical model on the origin of galaxies: 1) slowly rotating,
collapsing gas cloud(s) forms most stars before cloud can flatten into disk
resulting in an elliptical
shape. 2) faster rotating, collapsing gas cloud(s) forms disk before most
stars made so its
spiral formed. A spiral galaxy without a massive dark matter corona may form a
bar across the middle of it. Big ellipticals can form from the collisions
of galaxies. Giant ellipticals (called ``cD galaxies'') found close to the
centers of galaxy clusters formed from the collision and merging of galaxies.
There is a lot of current
research on the formation of galaxies (see for example the work by the
HPCC
group at the University of Washington).
Galaxies love to cluster together! Here are some examples of galaxy
clusters that you'll often hear about in astronomy:
- Milky Way in ``Local Group'' of 3 spirals, 2 ellipticals, 4 irregulars, 8 dwarf ellipticals (maybe more irregular and dwarf ellipticals) and is about 1 Mpc across.
- Virgo cluster--many 100's galaxies (mostly spirals and irregulars) in
irregular-shaped cluster about 15-18 Mpc from us. Some ellipticals in central
part. See cD galaxy at center (M87) formed
by mergers of galaxies via
``dynamical friction'' (galactic cannabilism). The cluster is about 3 Mpc
across. Virgo is the closest large cluster to us; Local Group ``falling''
toward Virgo because of Virgo's large mass and proximity.
- Coma cluster--1000's galaxies [mostly ellipticals and borderline spirals
(lenticular galaxies)] in a large, regular-shaped cluster 100 Mpc away.
Ellipticals in central part while the few spirals on outskirts. See two giant
cD's (NGC 4874 & NGC 4889) formed by dynamical friction that has slowed
down
galaxies colliding with NGC 4874 & 4889 so that they merge together (the cD
galaxies ``gobble up'' passing galaxies). Evidence for this cannabilism is seen
in the multiple cores found in cD galaxies.
Cluster of galaxy clusters! 10-100's clusters bound
together into long filaments roughly 100-300 Mpc long, 50-100 Mpc wide, 5-10
Mpc thick. We use doppler shifts and Hubble law to find distances to galaxies.
In between filamentary superclusters are HUGE voids of very few galaxies. Why?
Why do we care so much about finding distances in astronomy? If we know the
distance to a star, we can determine its luminosity and mass. We then can
discover a correlation between luminosity, mass, and temperature for
main sequence stars that our physical theories must account for. If we can
measure the angular size of a star, we can then find its geometric size (how
many kilometers in diameter it is). That gives us another clue to what is
happening with the stars. Finding distances to stellar explosions like planetary
nebulae and supernovae enables us to find the power needed to make the gaseous
shells visible and how much was needed to eject them at the measured speeds.
Stellar distances and distances to other gaseous nebulae are necessary for
determining the mass distribution of our galaxy. We then have been able to
discover that most of the mass in our Galaxy is not producing light of any kind
and is in a dark halo around the visible parts of the Galaxy.
Finding distances to other galaxies enables us to find their mass, luminosity,
and star formation history among other things. We're better able to hone in on
what is going on in some very active galactic cores and also how much dark
matter is distributed among and between galaxy cluster members. From galaxy
distances, we're also able to answer some cosmological questions like the
large-scale geometry of space, critical density
(
), age of the
universe, and whether or not the universe will be expanding. This is only a
quick overview of the reasons for distance measurements and is by no
means an exhaustive list of reasons why we care about distance measurements.
Now let's take a look at the distance scale ladder. The bottom foundational rung of the ladder is the most accurate and the most certain of all the distance
determination methods. As we climb upward, each rung depends on the previous
rung and is less certain than the previous one.
The Earth and Distance to the Sun. Use radar reflections
from Venus and angular separation from Sun to get Astronomical Unit (AU). Find
distances out to 50 AU.
Geometric Methods. Determine trigonometric parallax to
nearby stars using their angular shift throughout the year and the Astronomical
Unit. Find distances to nearby clusters (like Hyades or Pleiades) via trig.
parallax or moving clusters method (another geometric method). Calibrate the
cluster's main sequence in terms of absolute magnitude (luminosity). Find
distances out to 100 pc.
Main Sequence (M.S.) fitting and Spectroscopic Parallax.
Find spectral type of star and measure flux. Use calibrated color-magnitude
diagram to get its luminosity and then its distance. Plot cluster's
main-sequence on color-magnitude diagram with apparent magnitudes not absolute
magnitude. Find how far the unknown M.S. needs to be shifted to match the
calibrated MS. Age affects M.S. An older cluster only has fainter stars left on
the M.S. Also stars on the M.S. brighten at a constant temperature as they age
so they move slightly vertically on the M.S. Model M.S. evolution to get back to Zero-Age M.S. Assume: all Zero-Age M.S. stars of given temp. (mass) start at the same luminosity. Find distances out to 50 kpc.
Variables Period-Luminosity Relation. Find Cepheids and/or
RR-Lyrae in stars clusters with distance known through M.S. fitting. Or use
more direct Baade-Wesselink method (uses the observed expansion speed along the
line of sight from doppler shifts with the observed angular expansion rate
perpendicular to the line of sight). RR-Lyrae have same time-averaged
luminosity
( ~ 49 L
which corresponds to an absolute
magnitude MV = +0.6).
They pulse with periods < 1 day. Cepheids pulse with periods > 1 day and
are more luminous than RR-Lyrae stars. The longer the pulse, the more
luminous they are.Two types Cepheids: classical (brighter, type I) and W
Virginis (fainter, type II) have different light curve shapes. Find distances
out to 4 Mpc (40 Mpc with Hubble telescope).
Galaxy Luminosity vs. Another Bright Feature. Find
Cepheids in other nearby galaxies to get distance. Use galactic flux and inverse square law of brightness to get galactic luminosity. Find geometric size of H-II regions in spiral and irregular galaxies. Calibrate possible H-II region
size-galactic luminosity relation. Or calibrate correlation between width of
the 21 cm line (neutral H emission line) and spiral galactic luminosity. Width
of 21 cm line due to rotation of the galaxy. This correlation is called the Fisher-Tully relation:
L = 180 
solar luminosities if
in
km/sec. Elliptical galaxies have a correlation between their luminosity and
their velocity dispersion,
, within the
inner few kpc called the Faber-Jackson law:
km/sec, where
solar luminosities in the visual band and H
=
50-100 km/sec/Mpc (the Hubble constant).
Luminosity or size of Bright Feature. Find Cepheids in other nearby galaxies to
get distance. Calibrate supernova type 1a maximum luminosity in any type of galaxy.
Calibrate globular cluster luminosity function in elliptical galaxies. Calibrate
blue or red supergiant stars relation in spirals and irregulars. Calibrate maximum
luminosity-rate of decline relation of novae in ellipticals and bulges of spirals.
Calibrate planetary nebular luminosity function in any type of galaxy.
Find Rung 5 distances out to 50-150 Mpc depending on the particular method.
Galaxy Luminosity and Inverse Square Law. Calibrate Hubble
law using rung 4 methods for nearby galaxy distances and rung 5 methods for
larger galaxy distances. If those rung 5 galaxies are like nearby ones (or have
changed luminosity in a known way), then using flux and/or angular size and
estimated luminosity and/or geometric size, we can find their distance. Need to
take care of the effect on the measured velocities caused by the Milky Way
falling into the Virgo Cluster. Also find galaxy cluster luminosity function.
Hubble law relates a galaxy's recession (expansion) speed with its
distance: speed = H x
distance. Measuring speed from Doppler shift is easy, but measuring distance is
not. Calibrate Hubble law out to 500 Mpc.
Use Hubble law for all far away galaxies. Find geometry of universe.
Rung 4 is the critical one now for the distance scale ladder. With the fixed
Hubble telescope, we'll be able to use the Cepheid P-L relation out to distances
ten times further than what we can do now on the ground. The current measurements
of the Hubble constant are 50-100 km/sec/Mpc (a factor of two in range!). With
Cepheid observations at farther away distances, we're able to constrain its value
to 75-85 km/sec/Mpc. Since 1/H
is a rough upper limit on the age of the universe (assuming constant recession
speeds!), the new
Hubble constant measurements are implying an universe age of only 12-13
billion years. This is in conflict with the ages derived for the oldest stars
(found in globular clusters) of about 15-16 billion years. Right now, there
is a lot more confidence in the age determinations for the oldest stars than
for the age of the universe. This is because we are still quite uncertain as
to the history of the expansion speeds and what all can affect the expansion
speed. So the recent Hubble Telescope distance measurements have forced
astronomers to attack the deficiencies in the theory of the universe expansion.
Stay tuned for more late-breaking announcements!
The pictures come from various sites around the world and several are quite
large (over 400 kilobytes) so they take a while to upload. Even a fast internet
connection will take over ten minutes to upload them all so I have the gallery
on a separate page. If you are willing to wait for some gorgeous pictures then
go to the galaxy picture gallery.
last updated 17 Nov 95
Nick Strobel --
Email:
strobel@astro.washington.edu
(206) 543-1979
University of Washington
Astronomy
Box 351580
Seattle, WA 98195-1580