• The Milky Way
  • Molecular Gas
  • The Galactic Center
  • Mass of the Galaxy
  • The Star-Gas-Star Cycle
• Discovering galaxies
  • The "great" debate
  • Hubble's discovery
• Types of galaxies

• Molecular "Ring"
  • from 4-8 kpc and concentration on GC
  • Thickness ~ 120 pc.
• Giant Molecular Clouds (GMCs):
  • Size ~ 10 - 50 pc, Mass ~ 10³ - 10⁶ M_sun
  • Stars form in cores of GMCs.
• Mass ~ 3 x 10⁹ M_sun, ~2/3 inside the orbit of the Sun around the Galactic Center.

• What lies at the center of our Galaxy?
  • Dust obscures the visible light from us
  • Use radio and infrared observations
• Dense star cluster peaks at the center.
  • ~ 2 x 10⁶ M_sun within 1 pc
  • Stars only 1000 AU apart
  • A collision every 10⁶ years!
  • Bright radio source. (black hole?)
• A massive "molecular ring" of gas and dust rotates around this star cluster
  • Extends from ~ 1 to 5 pc from the center
  • "Leaking" matter into the center
• Structures outside the molecular ring
  • 20 pc long linear structures tracing Galactic magnetic fields
  • isolated star forming regions

• The ISM provides the matter from which stars form.
• Stars evolve and create "heavy" elements
  • Through stellar nucleosynthesis and supernovae
• These elements are returned to the ISM.
  • Stellar winds, planetary nebula, and supernovae
  • Not all material is returned resulting in the gas being “used up”
• The “enriched” gas is used by the next generation of stars.

• The stars and gas rotate about the center of the Galaxy.
• The rotation speed varies with distance from the center.
• From the speed at a given point, we can deduce the mass.

• The total mass of the galaxy can be computed from Newton's laws
  • Like the mass of binary stars
• From Lecture 16 (Binary Stars), we have Newton's version of Kepler's third law

• For a circular orbit

• Combining this with Newton's version of Kepler's third law gives.

• A rotation curve represents the velocity of particles versus distance from the center of rotation.
• Two examples are:
• Merry-go-round - the velocity increases (linearly) with increasing distance. This is also called "solid body rotation". This is shown below.

• Solar system - the velocity decreases (as 1/square-root(r)) with increasing distance. The rotation curve follows Kepler's law, as shown below.

Figures from "The Cosmic Perspective" by Bennett et al.

• The rotation curve for the Milky Way is relatively flat.
• It is more like the merry-go-round than that of the solar system
  • Thus there is no dominant central mass

Figure from "The Cosmic Perspective" by Bennett et al..

• Using Newton's form of Kepler's third law we can deduce the mass of the Milky Way at different distances from the center.
• For the Sun, v = 220 km/sec at a radius of 8.5 kpc.
  • Orbital period = 240 million years.
• Mass of MW = 10¹¹ M_sun within 8.5 kpc.

• The mass seen in stars is much less than that derived from Newton's laws.
• Conclusion: there must be some additional mass which is non-luminous!
• The is unseen mass is call Dark Matter.
• It is called missing mass because starlight cannot trace it.

• The Galaxy collapsed from a cloud of gas and dust due to its own self-gravity.
• Some (Pop II) stars formed first.
• Remaining gas collapses into a disk - angular momentum conservation!
• First generation massive stars eject metals into the disk so that
  • Pop I stars have higher metallicities

• April 26, 1920
• Debate on "The nature of spiral nebulae" & "The size of our galaxy"
• Heber Curtis vs. Harlow Shapley
• Shapley claimed spiral nebulae were "close"!!

• a.k.a. Classical Cepheids
• Luminosity: 400 to 20,000 L_sun
• Location: Open clusters and the galactic disk (Pop I stars)

• a.k.a. W Virginis Stars
• Luminosity: 100 to 5,000 L_sun
• Location: Globular clusters (Pop II stars)

• Measured Period gives:
  • Luminosity
  • M_v (absolute magnitude)
• Measure m_v (apparent magnitude)
• M_v and m_v => distance from distance modulus equation
  • m_v - M_v = - 5 + 5 log₁₀ (d)
• A Hubble "key project" is to determine the distances to galaxies w/ Cepheids.