ISP205 Lecture #25, April 8, 2001

  1. Announcements:
    1. Homework:
      1. Set 7: Due April 10, 23:59 (TODAY !!!)
      2. Set 8: Due April 17, 23:59
      3. Set 9: Not handed out yet; will be due: April 24
      4. No Set 10 this time (sorry :) )
    2. New Bonuspoint project:
      Find black hole property on the Web
      Address on Website
      Worth 6 bonus points
             
  2. Review:
    1. Special Relativity
      1. Basic principle: Speed of light is constant
        (no matter how light is moving relative to observer)
      2. A consequence: Moving Clocks run slower (as seen from
        observer at rest)
      3. Time travel is possible ! But only into the Future ...
      4. Special relativity is a theory that works well and is used everyday
        by scientists.
        (This does not mean it will maybe replaced with a
        better theory sometimes, but the better theory has
        to explain all the weird phenomena relativity
        is able to explain.)
    2. General Relativity
      1. Basic principle: Equivalence principle
        Acceleration generates gravity in opposite direction.
      2. Consequence 1: Gravity is nothing else then curved space
      3. Consequence 2: The stronger the gravity the slower run clocks

        If you are watching from far away (at low gravity), then at a 
        place with high gravity:

        1. Motion seems slower
        2. Light emitted seems redder
        3. This is most extreme as one approaches the event horizon
          of a black hole
      4. General relativity is a theory that works well and is used everyday by
        scientists and rocket engineers
            
    3. Black holes
      1. A point of mass in space with zero size and infinite density
      2. Inside the event horizon light cannot escape
      3. Clocks at the event horizon seem to stand still
        if seen from outside (frozen astronaut)
      4. Do black holes exist ?
        1. There is no direct proof yet
        2. There are compact objects orbiting stars that
          are more massive than 3 solar masses and can
          therefore not be neutron stars (determined from orbit)
        3. Matter in the center of many galaxies swirls around
          a compact object of several million solar masses |
          generating X-rays.
          Best explanation: supermassive black hole 
             
  3. Our home Galaxy
    1. The Milky Way (pictures)
    2. History of Milky Way discovery
      1. 1610: Galileo discoveres that it's made up individual stars
      2. 1785: William Herschel discovers that our sun is part
        of a disk shaped star cloud - Milky Way is seen when
        we look into the disk. (picture)
        But wrong conclusion that sun is in the center.
      3. 1917: Harlow Shapley shows that sun is 26,000 Ly off center
        by mapping distances to 97 globular clusters
            
    3. Properties of the Milky Way (example, picture)
      1. spiral galaxy
      2. "top" view:  spiral arms and bulge
      3. "side" view: disk, bulge, and halo
      4. has about 200 billion stars
      5. diameter: 100,000 Ly
      6. location of the sun: 26,000 Ly away from the center in a
        secondary spiral arm
          
    4. Motion of Stars in the Galaxy
      1. Every object follows Keplers Law and orbits around the
        center of the Galaxy

        Mass of stars within orbit = semi major axis ^ 3 / Period ^ 2
      2. The sun orbits around the galactic center in 225 million years
      3. Stars at the edge of the galaxy should move slower the further
        away they are from the center (like planets around the sun)

        But they don't ! (picture)
      4. Conclusion: The Galaxy is much bigger than what we see. 90%
        of our Galaxy is made of some unknown, invisible dark matter

        Dark matter is invisible but does not block the view to distant objects

        Some suggestions:
        1. Brown dwarfs (found to contribute but less than half !)
          (also called Massive Compact Halo Objects - MACHOS)
        2. Weakly interacting massive particles (WIMPS)
        3. Cold neutrinos (if they have some mass)
             
    5. Spiral Structure
      1. Differential rotation is NOT the explanation  - it would wind
        up the spiral tighter and tighter until it disappears
      2. Density Wave Theory: Stars and dust pass through spiral arms on
        their Kepler orbits but slow down because of higher density
      3. Region of high density has lots of star formation - spirals are bright
      4. Spirals rotate but slower than individual stars
               
    6. Things in different parts of the Galaxy
      1. Disk (spiral arms)
        1. Lots of dust and star formation (blue)
        2. OB associations (young and massive stars)
        3. Lots of gas: HI and HII regions
      2. Halo
        1. Old Stars and globular clusters (more red)
        2. No gas and dust
        3. Halo stars seem to move fast with respect to earth because
          they are not orbiting along with us in the disk
          "high velocity stars"
      3. Bulge
        1. Mixed: old and young stars star
      4. Center
        1. Center is in the direction of the constellation Sagittarius (picture)
        2. Strong radio emission from galactic center (Sagittarius A)
          (pictures)
        3. Sagittarius A is smaller than Jupiter's orbit (10 AU) and from
          Keplers law one finds it is as massive as 2.3 million solar masses
          Best Theory: black hole
                     
    7. Formation of the galaxy
      1. Contracting gas and dust cloud
      2. Stars from in dense regions - globular clusters
        these objects from spherical halo
      3. Remaining dust and gas flattens into disk
        ends star formation in halo (~10 billion years ago)
      4. Star formation continues until today in disk
            
  4. Other Galaxies
    1. There are billions of galaxies in the universe
       (Hubble deep field view)
    2. Classification: (scheme)
      1. Spirals (examples)
        1. open spirals (picture)
          1. S0 (mostly bulge, some spirals)
          2. Sa (large bulge, small spirals)
          3. Sb
          4. Sc (small bulge, large spirals)
        2. barred spirals
          1. SBa (long spirals off the bar, tightly wound)
          2. SBb
          3. SBc (short spirals off the bar)
      2. Ellipticals (picture)
        1. E0: spherical
        2. ...
        3. E7: cigar
      3. Irregulars
        1. Not any of the above
        2. There are many irregulars, for example because of
          galaxy collisions (picture)
    3. Properties
      1. Spirals:
        1. Old and young stars
        2. Gas, dust, OB stars, ongoing star formation
        3. Most are similar in luminosity and mass
      2. Ellipticals:
        1. Most common
        2. No gas and dust
        3. Old stars
        4. Vary greatly in luminosity and mass
    4. Distances to other Galaxies
      1. Next neighbours:
        1. Small and Large Magellanic Clouds (irregulars)
          (pictures)
          ~200,000 Ly away
        2. Andromeda (spiral)
          ~2.2 million Ly
        3. Size of the universe: ~13 billion Ly's
      2. The distance ladder:
        1. Parallax
        2. HR diagram (spectroscopic parallax)
        3. Variable stars (with HST up to 65 million Ly)
        4. Tully-Fisher relation (100 million Ly)
          luminosity of spiral galaxy is related to rotational
          velocity (Kepler)
        5.  Brightness fluctuations in elliptical galaxies (100 million Ly)
        6. Type Ia supernovae (8 billion Ly)
        7. Brightest galaxy in cluster (13 billion Ly)
        8. Redshift (13 billion Ly) see next lecture
             
    5. The famous Shapley Curtis debates of 1920
      1. Debate about size of the Universe - and whether "Galaxies" are
        fuzzy nebulae in our Galaxy or similar Galaxies than ours far away
           
      2. Harlow Shapley: 
        (Harvard Astronomer famous for determining size of the galaxy)
        1. Thought our Galaxy is ~100,000 ly wide (correct)
        2. Thought that is the whole universe and everything else is in this
          one Galaxy (incorrect)
      3. Heber Curtiz: (From Muskegon, Michigan)
        1. Thought our Galaxy is much smaller - only 20,000 ly (incorrect)
        2. Thought that "fuzzy nebulae" are other Galaxies far away (correct)
      4. Shapleys arguments:
        1. Observed Novae in other Galaxies - from typical Nova brightness
          they determined the distance
        2. Astronomers observed that "fuzzy nebulae" move over time - so
          they have to be close
        3. Using Cepheid variables he estimated that the magellanic clouds
          (closest galaxies) are only 75,000 Ly away and therefore inside
          our Galaxy
      5. What was wrong with Shapley's arguments ?
        1. Novae were much brighter supernoave
        2. Observation of moving galaxies were wrong (nobody knows why)
        3. There are 2 classes of Cepheid variables - the ones Shapley found
          belong to the much brighter ones, which he did not know