PES 106        Spring 2003 Previous Lecture General Astronomy II Next Lecture

Lecture Notes:

Galaxies: Milky Way Overview and Components

text: Chapter 15 Sections 1 - 4

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Galaxies:

• many stars (100 billion )which interact gravitationally
• clouds of gas and dust
• "dark matter"

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Observing our Galaxy: The Milky Way

Observe a dense band of stars in the sky (the Milky Way)

this suggests a disk-like shape

See Figure 15.1 [Link to Figure 15.1]

Here is a link to a 360 degree view of the Milky Way from Earth: http://www.pbs.org/wgbh/nova/universe/media/optical_mw.html

Further study suggests the following structure for our galaxy:

(dimensions in kiloparsecs (kpc) where 1 kpc = 1000 pc

top view:

side view:

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Properties

• number of stars
  • about 100 billion
  • most have mass less than Sun
  • average mass about same as Sun
• rotation rate
  • 220 km/sec at the radius of the Sun (130 miles/sec)
  • (takes 240 million years for Sun to go around once!)
• stars independently orbit the nucleus
  • obey Kepler's Laws
    • complicated motion from broad distribution of mass in the galaxy
      • generally faster rotation near center
      • generally slower rotation near edges
• density
  • stars closer together near center - farther apart near edges
• mass
  • 10¹¹ x Mass of Sun (100 billion solar masses)
    • based on
      • Kepler's Law for mass inside orbit of Sun
      • observation of stars, gas and dust
  • 5 x 10¹¹ x Mass of Sun from larger scale observations of gravity
  • suggests that
    • 20% of Milky Way is stars, gas and dust
    • 80% of Milky Way is "dark matter"
• age
  • oldest stars are 11 - 15 billion years old
• future
  • galaxy is forming about 3-5 new stars per year
  • has enough gas to keep forming stars for 10 billion years

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Components of Galaxies

major components of galaxies are

• stars
• clusters of stars
• interstellar matter: gas and dust
• dark matter

star types

Population I stars blue stars (hot, young) found in disk mainly H and He contain heavy elements probably formed from gas recycled from stars orbits: circular orbits in the disk

Population II stars red stars (cool, old) found in the bulge and halo mainly H and He almost NO heavy elements probably formed from original gas cloud orbits: elliptical - tilted out of the disk

star clusters

groups of stars gravitationally bound within the galaxy

Two types:

open clusters 10 - 5000 stars loosely scattered - not tightly bound together eventually will break up 4 - 15 pc across found in disk (Population I stars) Milky Way has about 20,000 open clusters from birth of stars in large cloud

globular clusters 100,000 - 1,000,000 stars tightly bound in spherical shape 20 - 100 pc across found in halo (Population II stars) Milky Way has150 - 200 globular clusters

Figure 15.10 shows clusters [Link to Figure 15.10]

interstellar matter

• 99% gas
• 1 % dust
• mainly in the disk
  • Figure 15.11 shows this for another galaxy
• often concentrated into clouds (1 - 100 pc diameter)

  dust

  • small particles (< 1 micrometer)

    mainly forms of Silicon and Carbon

    dust scatters light

    • blue scattered more than red
      • light from stars behind cloud looks more red
    • intensity of light is reduced
      • hard to see stars in disk with visible light (infrared and radio are OK)

  gas

  • gas is 71% Hydrogen and 27 % Helium (from absorption spectra)

    if gas is hot (10,000 K) - usually heat from nearby star

    • ionized H atoms
    • it emits emission line spectra (with strong red and radio lines)

      Figure 15.17 shows this [Link to Figure 15.17]

    if gas is cold

    • H atoms
      • temperatures tyically about 100 K
      • radiation in the radio (from quantum mechanical spin flips)
        • imagine the electrons orbiting the atom as being able to spin on their own axis (like the Earth spins as it orbits the Sun). The electrons can flip so that their spin changes from clockwise to counterclockwise or vice versa. When the electron changes from spinning in the same direction as the proton in the nucleus to spinning the opposite direction, it emits radiation in the radio part of the spectrum.
    • H₂ molecules
      • temperatures typically below about 15 K
      • H₂ emits in the ultraviolet (absorbed easily)
      • collisions of hydrogen molecules with other molecules produce radio emissions from the other molecules

  nebulas

• planetary - ejected outer layers of a star
• reflection - blue light scattered from dust clouds
• dark - dark region from light unable to penetrate a thick, dense dust cloud
• emission - hot gas cloud (often red)

dark matter

may be up to 80% of the matter in the galaxy

mainly located outside of the disk and bulge

may be about half brown dwarfs (balls of gas that did not quite make it to be stars) and half something else (maybe black holes, cool white dwarfs, sub-atomic particles (WIMPS), .....)

Here is a description of dark matter: http://www.pbs.org/wnet/hawking/strange/html/dark.html

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