PES 106        Spring 2003
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General Astronomy II
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Lecture Notes:

Galaxies: Milky Way Structure and Formation

text: Chapter 15 Sections 5 - 8

Disk and Spiral Arms:

matter in disk is relatively uniformly distributed

brightest stars are found in the spiral arms which is why we "see" them

Milky Way disk: all matter vs. bright stars

Spiral Arms formed by a density wave theory

  • spiral arms have higher density
    • clouds enter into the arm region and are compressed
    • leads to star formation
      • most stars are born in spiral arms
      • hot bright high mass stars tend to be in spiral arms (short lived stars)
      • arms are easy to see
    • stars continue moving through the arm and out the other side
  • Sun was probably formed in a spiral arm
    • but maybe NOT the arm that it is in now
  • Figure 15.19 shows star formation in the arms [Link to Figure 15.19]

    Here is a spiral galaxy you can zoom in on and rotate. Try flipping it up and looking at the disk edge on to see the dust. http://www.pbs.org/wgbh/nova/universe/media/spiral_m83_15.html

Galactic Center (Core, Nucleus, Bulge)

dust blocks visible light - need to observe with Infrared, radio, X-rays, gamma rays . . .

observe a dense cluster of stars and gas - orbiting something

from Kepler's Laws, the "something" has a mass about 1 million x mass of Sun

observationally it does not have a high luminosity - but does emit some radio waves

could it be a huge black hole ?

black hole could become huge because lots of material that could be added to the black hole

possibly even pull in stars since the density of stars is so high

more mass => stronger gravity => pull in more mass => etc.

 

However - we expect X-rays from an accretion disk surroundin the black hole - see none

Figures 15.25 and 15.26 show details of the gas and star structure near the center [Link to Fig. 15.25] [Link to Fig. 15.26]

Formation of the Milky Way

 Two factors:
  • collapse of a huge cloud forming many stars
    • explains rotation of galaxy
    • Population II stars formed early (few heavy elements)
      • have elongated orbits about center because formed during the collapse
    • Population I stars formed later after disk was formed (after massive stars had become supernovas)
    • Figure 15.27 shows this process. [Link to Figure 15.27]
  • collisions between galaxies
    • not unusual
    • Milky Way has three close neighbor galaxies which appear to be getting closer
    • collisions appear to have been more common earlier
      • more small galaxies earlier

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