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.


Galactic Structure


Disk-shaped with spiral arms. Elliptical bulge in center and spherical halo that is denser closer to Galaxy center.

A. Period-Luminosity Relation for Variable Stars

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.


  1. 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).


  2. 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.

B. Our Location


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.

C. Our Motion

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.

D. Deriving the Galactic Mass from the Rotation Curve

A star in orbit has a balance of centripetal acceleration (outward) and gravitational acceleration (inward): V^2_{orbit}/R 
= G*M/R^2, 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 M_{encl} = V^2 * R/G. 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.

E. Spiral Arms


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 10^8 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

  1. 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

  2. Transient spirals: Arms come and go. Computer simulation of galactic disks show spiral patterns appearing and disappearing.

    Supernova creating spiral arms

  3. 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.

F. Populations of Stars


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

  1. 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 _sol; abundance of metals: 10^-3 - 3*10^-2 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

  2. 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.

G. Galactic Center


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).

Other Galaxies


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).

A. Types of Galaxies

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.


  1. 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).


  2. 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).


  3. 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).

B. Positions


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.

C. Distances to Galaxies

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.

D. Masses of Galaxies

Most galaxies have large amounts of Dark Matter--material not producing light but having a noticeable gravitational effect.

E. Origin of Galaxies


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).

F. Clusters of Galaxies

Galaxies love to cluster together! Here are some examples of galaxy clusters that you'll often hear about in astronomy:

    Local Group

  1. 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

  2. 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

  3. 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.

G. Superclusters


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?

Steps to the Hubble Constant


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 (Omega), 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.

Rung 1


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.

Rung 2


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.

Rung 3


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.

Rung 4


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_sol 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).

Rung 5a


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 V_rot^4 solar luminosities if V_rot in km/sec. Elliptical galaxies have a correlation between their luminosity and their velocity dispersion, sigma, within the inner few kpc called the Faber-Jackson law: sigma approx 220*(L/L_*)^(1/4) km/sec, where L_* = 1.0*10^10 * (H_o/100)^-2 solar luminosities in the visual band and H _o = 50-100 km/sec/Mpc (the Hubble constant).

Rung 5b


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.

Rung 6


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_o x distance. Measuring speed from Doppler shift is easy, but measuring distance is not. Calibrate Hubble law out to 500 Mpc.

Rung 7


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_o 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!

Photo Gallery of Galaxies


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.

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last updated 17 Nov 95

Nick Strobel -- Email:

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