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Astro 101 Fall 2013 Lecture 10 Galaxies (part I) T. Howard. The Galaxy – part I -- Overview. 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

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Astro 101

Fall 2013

Lecture 10

Galaxies (part I)

T. Howard

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.


Parallactic angle (arcsec)

Distance (pc) =

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

1 pc = 3.3 light years

= 3.1 x 1018cm

= 206,000 AU

1 kiloparsec (kpc) = 1000 pc

1 Megaparsec (Mpc) = 10 6 pc

The Milky Way Galaxy

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

All-sky projection

1.2, 1.6, 2.2


Atlas Image [or Atlas Image mosaic] courtesy of 2MASS/UMass/IPAC-Caltech/NASA/NSF.

Core of Galaxy

(NASA – Spitzer)

(false color)

How was Milky Way size and our location determined?

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


Herschel’s Milky Way drawing



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.

Take a Giant Step Outside the Milky Way

Artist's Conception

Example (not to scale)

Artist's Conception

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 kpcdiameter

- 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

young stars

Gas, dust


Type: SBc

~ 1011 stars

(mass ~ 1012Msun)

Age ~ 13.2 GYr

8.3 kpc

Morphology of the Milky Way

Disk: - 30 kpcdiameter

- 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)

Halo: - at least 30 kpc across

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

- roughly spherical

Bulge: - About 4 kpc across

- old stars, some gas, dust

- central black hole of 3 x 106 solar masses

- spherical

Galaxies (believed to be) structurally similar


NGC 891

NGC 891

Close up


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.

from above ("face-on")

see disk and bulge (halo

too dim)

Perseus arm

Orion spur


Cygnus arm

Sagittarius arm

Carina arm

from the side ("edge-on")

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.

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 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

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.

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

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. Individualobjects come and go.

observed curve

Milky Way Rotation Curve

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

90% of Matter in Milky Way is Dark Matter

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


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










R (AU)

Rotation curves of many normal galaxies are similar

-- allows galaxy mass to be estimated

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.

Rotation curve of M31

The M31 major axis mean optical radial velocities and the rotation curve, r <120 arcmin, superposed on the M31 image from the Palomar Sky Survey. Velocities from radio observations are indicated by triangles, 90< r <150 arcmin. Rotation velocities remain flat well beyond the optical galaxy, implying that the M31 cumulative mass rises linearly with radius.

(Image by Rubin and Janice Dunlap.)

Source: Physics Today, Dec. 2006

Several alternative explanations
  • Matter is “missing” (i.e., can’t be seen – nonluminous)
    • Probably view most prevalent is that of Cold Dark Matter (CDM)
  • What can’t it be?
    • Dust – no. Why??
    • Gas – no. Why??
  • What can it be?
    • Normal (i.e., baryonic) matter
      • MACHOs – MAssive Compact Halo Objects
        • White dwarfs, Red/Brown dwarfs
        • Neutron stars, BHs
    • Non-baryonic matter
      • WIMPs – Weakly Interacting Massive Particles
      • Weak interactions w/baryonic matter, no significant interactions w/light
Ruling Explanations In or Out
  • White & red dwarfs – HST can’t find enough of them
  • BHs – large – would probably leave signatures, disruption of Globulars in the halo  not seen
  • BHs – small – can’t be excluded, but how did they get there?
      • Compact gas clouds in halo  why not seen? Why don’t they form stars?
  • Cosmology theory predicts baryonic matter should be ~ 4% of the critical density of universe; luminous matter seems to be only about 0.8%
    • But when hot matter (i.e., intracluster gas) added  probably close
    • Although, error bars on this are large
  • Note, “CDM” vs. “HDM” (cold vs. hot) means:
    • Non-relativistic vs. relativistic
More on Dark Matter
  • Dark matter thought to be necessary in cosmology theory to explain clumpy distributions of matter w/ enough mass  galaxies form
    • Could have been either CDM or HDM
  • Usual predictions of CDM candidates require modifications or extensions to Standard Model of particles
  • Non-baryonic candidates
    • Neutrinos (neutrino oscillation  implies neutrino mass)
    • WIMPs – hypothetical
      • Possibly “heavy” neutrinos or something equally exotic
    • Axions
MOND – MOdified Newtonian Dynamics
  • Introduced c. 1983, M. Milgrom (Weizmann Inst., Israel)
  • ad hoc phenomenological fit to explain two things:
    • Galactic “flat” rotation curves, and
    • Tully-Fisher relation for spirals: M = (const.)*Va, a ~ 4
  • Modifies Newtonian force for very small accelerations
  • Successfully predicts several astrophysical observations
  • Doesn’t require “missing mass”
    •  but some attempts made to derive it as an effect of CDM
    •  not completely successful
  • Not tied to any physical theory, i.e., a “fit to observations”
    • Some attempts to modify Newtonian gravity, inertia, or GR
    • Not complete
  • Not entirely popular but not ruled out; has a few vocal adherents
Massive Black Holes

-- leading theory to

explain the emission

and properties of many

active galaxies (AGNs)

-- Central BH may be

Billions of solar masses

-- Accretion disk made

of large clouds of gas

And dust

-- X-rays and gamma

rays from BH are

converted to longer

wavelengths by clouds

Milky Way -- Evidence for Central BH
  • Dynamical – orbits of stars near
  • Galactic center, esp. S2, indicate
  • Central mass of ~ 4 million Msun
  • Only a BH dense enough to have
  • this much mass in such a small
  • volume of space
  • Corresponds to location of
  • radio source Sgr A
  • Smaller compact source Sgr A*
  • at center, diameter 0.3 AU
  • Observations also in x-rays
  • and gamma rays
Milky Way – Central Black Hole

Central area

(enlarged) 

Center of Galaxy and

Constellation Sagittarius (Sgr)

Other views of the

Galactic Center


X-rays (Chandra)


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.

More on bars…

Milky Way schematic

showing bar

Another barred galaxy

A bar is a pattern too,

like a spiral.

bulge less prominent,

arms more loosely wrapped


increasing apparent flatness

disk and large bulge, but no spiral

Galaxy Classification

Hubble’s 1924 "tuning fork diagram"

Spirals Ellipticals Irregulars

barred unbarred E0 - E7 Irr I Irr II

SBa-SBc Sa-Sc "misshapen truly

spirals" irregular

bulge less prominent,

arms more loosely wrapped


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)


poor beginnings

of spiral arms

Large Magellanic Cloud

Small Magellanic Cloud

These are both companion galaxies of the Milky Way.


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)
    • -- 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


Jupiter’s Orbit

Radius 5.2AU





M31 ~ 800Kpc