AST 180: Lecture 22
Reading: Essentials IV and Chapters 14-15
The Milky Way and Galaxies
The Milky Way is only one of Billions of Galaxies
  • Each Galaxy has millions or even trillions of stars
  • Immanuel Kant suggested in 1755 that vast "island universes" existed beyond the Milky Way
  • Most astronomers of the 19th century thought the Nebulae were in the Milky Way
  • Shapley-Curtis debate in 1920 formalized the problem
  • V.M. Slipher measured velocities of nearby galaxies and showed they were probably not in the Milkey Way
  • Hubble measured distances with Cepheid Variables and clinched the problem
Cepheid Variables led to the Discovery of Galactic Distances
  • Edwin Hubble at Mount Wilson Observatory took photographs of M31
  • By comparing photographs taken at different times, he found variable stars in the galaxy
  • Henrietta Leavitt showed, in 1912, that Cepheid Variables had peroids related to their luminosities.
  • Type I Cepheids (metal-rich) and Type II (metal poor) are variable due to expansion and contraction
  • Interpretations of a Cepheid Variable in M31 showed that it was very distant from the Milky Way (2.2 million LY)
Cepheid Variables in Globular Clusters helped point to the Milky Way's Center
  • Herschel counted stars in the Milky Way and deduced that we were in the center
  • He was wrong because he didn't take dust into account
  • Shapley used the period-luminosity relation to determine distances to 93 globular clusters and mapped out their distributions
  • Shapley discovered the GC were spherically distributed about a center far from Earth.
Radio Observations of the Spin-Flip transition of HI helped map the Galactic Disk
  • The Spin-Flip Transition of Hydrogen produces photons at radio wavelengths (21 cm)
  • This emission passes right through dust and gas
  • The Doppler shift can be used to separate galactic arms superposed in the line of sight.
You Are Here!
  • Gas distribution just hints at Spiral Structure
  • The Milky Way has Stellar Spiral Arms
  • Our Solar System is just inside the Orion Arm of the Milky Way
  • Sagittarius arm between us and center
  • Perseus arm between lies directed away from Galactic Center
Structure of the Milky Way
  • 100,000 LY in diameter, about 2,000 LY thick
  • Nuclear Bulge about 20,000 LY in diameter
  • Sun is 26,000 LY out
  • Bulge surrounds the Galactic Nucleus

The Galactic Nucleus is Violently Active
  • Infrared Views show a thin dust plane, a bright radio region known as Sagittarius A*, and bright stars that rush around it
  • Radio emission from Synchrotron Radiation is especially bright from Sag A*
  • Sag A* is at the focus of the orbits of stars at the galactic center
  • The stars are moving so fast, they indicate 2.5 million solar masses of material are in Sag A*
  • This is only possible if Sag A* hides a black hole!

The Galaxy is surrounded by a Spherical Halo and Satellite Galaxies
  • 99% Halo Stars are in field stars; 1% are in Halo Globular Clusters
  • Satellite Galaxies include the Magellanic Clouds and the Sagittarius Dwarf Galaxy (80,000 LY away).
The Galaxy is Rotating
  • Rotation is Differential (faster towards the center)
  • Using the average of Gloabular Cluster velocities as a reference, we know the Sun moves at about 828,000 km/hr around the Galaxy (once every 200 million years)
  • The rotation curve of the Galaxy is not as expected in the outer regions
  • Some unseen matter must influence the rotation curve, i.e., "Dark Matter"
Spiral Galaxies were classified by Edwin Hubble
  • Spiral Galaxies given "S" designation
  • Tightness of Arms indicated by Sa,Sb, and Sc (as above)
  • The Tighter the arms, the larger the nuclear bulge
  • Thus, Sa, Sb, and Sc also are indicated by the bulge as below

Stars do not move with Spiral Arms
  • Would lead to "Winding Problem"
  • Spiral Arms are waves of Density or Compression
  • Compression-wave analogies occur in traffic

Bars Run through the Nuclei of Some Spiral Galaxies
  • Barred Spirals designated with a "B", e.g., SBa
  • The Milky Way may have a bar
Elliptical Galaxies come in many Shapes and Sizes
  • Designated by "E"
  • Subdivided according to Oval or Circular Appearance
  • E0 are round, E7 are most elongated
  • Giant Ellipticals may be 20 times the size of the Milky Way and Contain 10 trillion solar masses
  • Dwarf Ellipticals are far more common and may contain only a few million stars.
Irregular Galaxies cannot be classified according to shape
  • Generally Rich in gas, dust, and young/old stars
  • Examples are the nearby Magellanic Clouds
Hubble's Galaxy Classification Scheme can fit on a "Tuning Fork" Diagram
  • An SO or SBO galaxy is intermediate between ellipticals and spirals
  • S0 also referred to as a Lenticular Galaxy