Lecture 8: The ISM- Nebula and Interstellar Dust Bunnies

"All we are is dust in the wind..."

Kansas, The Point of No Return

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    • Date: February 16, 1995
    Reading Assignment: pp. 415-423

    Description : interstellar gas and nebula, HII and HI regions

    Objectives

  • be able to describe the size, shape, and composition of a typical interstellar dust particle
  • be able to describe the effect interstellar dust has on star light
  • be able to describe the temperature and density of interstellar space
  • be able to define what a nebula is
  • be able to explain why an emission nebula emits light and what kind of spectra this nebula will have
  • be able to distinguish between HI and HII regions
  • be able to explain what powers and emission nebula
  • be able to explain why an emission nebula will be larger around a O star than one around a B star

    • Lecture Outline

    Slide # 1: The Interstellar Medium (GRAPHICS)

  • Nebula and Interstellar Dust Bunnies
    • Slide # 2: Measuring Stars
  • distance
  • luminosity and brightness
  • motion- radial and transverse velocity
  • diameters
  • color index and Spectral Types
  • HR Diagram
  • stellar masses
    • Slide # 3: Stellar Magnitudes
  • close stars appear brighter than far stars
  • inverse square laws
  • large stars appear brighter than small stars
  • luminosity - size relationship
  • hot stars appear brighter than cool stars
  • Stephan's law
    • Slide # 4: Luminosity and Brightness (GRAPHICS)
  • concept map
    • Slide # 5: Spectral Classes (GRAPHICS)
  • stars are classified by their spectra
  • line intensity changes with temperature
    • Slide # 6: The HR Diagram (GRAPHICS)
  • HR diagram
    • Slide # 7: The Main Sequence
  • 90% of stars
  • most of these stars are Red Dwarfs
  • K and M stars with low luminosity
  • a few are Blue Giants
  • hot, luminous stars
  • a very few are Blue Supergiant
  • very hot, very luminous stars
    • Slide # 8: Red Giants and White Dwarfs
  • Red Giants
  • cool, luminous stars
  • upper right on HR diagrams
  • must have large radius
  • White Dwarfs
  • hot, low luminosity stars
  • lower left on HR diagram
  • must have small radius
    • Slide # 9: Stellar Masses
  • determined in some binary star systems
  • Kepler's law
  • all stars on the MS with the same spectral type have the same mass
  • hot stars on the MS have higher masses than cool stars
    • Slide # 10: Mass and Luminosity
  • mass and luminosity are related for MS stars
  • all F stars on the MS have the same luminosity
  • all F stars on the MS have the same mass
  • therefore, all stars with the same mass on the MS have the same luminosity
    • Slide # 11: Stellar Lifetime
  • O stars are more luminous than M stars
  • 1,000,000 brighter
  • O stars are more massive than M stars
  • 100 times more massive
  • O stars must be burning their fuel very fast
  • O stars will run out of fuel before M stars
    • Slide # 12: A Simple Question
  • Where have your atoms been?
    • Slide # 13: Star Clusters
  • formed out of same gas clouds
  • formed at about the same time
  • seen at about the same distance
  • IDEAL "labs" for studying stars
    • Slide # 14: The Pleiades (GRAPHICS)
  • a young star cluster and nebula
    • Slide # 15: Nebula
  • fuzzy patches in the sky
  • may be either light or dark
  • visible parts of the ISM
    • Slide # 16: The ISM
  • the Interstellar Medium
  • the dust and gas between the stars
  • material that stars form from
    • Slide # 17: The Interstellar Medium
  • conditions in interstellar space
  • interstellar dust grains
  • reddening and extinction
  • emission nebula and HII regions
  • HI gas
    • Slide # 18: Interstellar Temperatures
  • typically temperatures 100K
  • ranges from 20K to 1 million K
    • Slide # 19: Interstellar Density
  • normally 1 atom per cubic cm
  • ranges 0.01 to 1000 atoms per cubic cm
  • dust particles 10-12 dust particles per cubic cm
  • 1000 per cubic kilometer
  • much more dust than Earth's atmosphere
  • Typical Earth Densities
  • best laboratory vacuums 104 atoms per cubic cm
  • atmosphere of Earth 3 x 1019 atoms per cubic cm
  • rocks on Earth 3 x 1022 atoms per cubic cm
    • Slide # 20: Interstellar Composition
  • 90% of the ISM is atomic and molecular hydrogen
  • 9% is helium
  • 1% is heavier elements
  • carbon, oxygen, nitrogen
    • Slide # 21: Interstellar Dust (GRAPHICS)
  • an interstellar dust grain
    • Slide # 22: Composition of Dust Particles
  • carbon, oxygen, silicon, magnesium, and iron
  • some might be ammonia, methane, water ice
  • similar to comet dust
    • Slide # 23: Dust Grains Polarize Light
  • ISM magnetic fields align dust particles
  • aligned particles polarize light
  • only part of EM radiation passes through
  • polarization measurements determine
  • particle sizes
  • shapes
  • orientations
    • Slide # 24: Constellation Corner (GRAPHICS)
  • Constellation De Jour
    • Slide # 25: Canis Major (GRAPHICS)
  • Feb 15 - 9pm - S - 4.0
    • Slide # 26: Canis Major (GRAPHICS)
  • Feb 15 - 9pm - S - 4.0
    • Slide # 27: Canis Major (GRAPHICS)
  • Feb 15 - 9pm - S - 5.0
    • Slide # 28: Interstellar Absorption
  • the density of dust particles is very low
  • space is very big
  • if you look far enough, dust begins to block light
    • Slide # 29: Looking through Fog (GRAPHICS)
  • "..visibility is 1/2 mile with fog..."
    • Slide # 30: Interstellar Absorption
  • stars appear dimmer from interstellar dust
  • APPARENT MAGNITUDE
  • blocks light from some distant stars inside
  • not uniform in all directions
    • Slide # 31: Interstellar Absorption (GRAPHICS)
  • some light is absorbed by dust
    • Slide # 32: The Southern Milky Way (GRAPHICS)
  • dark nebula
    • Slide # 33: Interstellar Reddening
  • blue light is absorbed more than red light
  • shorter wavelength is more affected by small dust particles
  • stars appear redder because the blue light is absorbed
    • Slide # 34: Interstellar Reddening (GRAPHICS)
  • Blue light is absorbed more than Red light
    • Slide # 35: Sunsets and Blue Skys
  • blue light from the Sun is scattered by atmospheric dust particles
  • the Sky is BLUE!
  • red light from the Sun is less absorbed by dust particles than blue light
  • Sunsets appear RED!
  • Effect is more pronounced when the Sun is low in the sky.
    • Slide # 36: Interstellar Reddening
  • color index will be affected by interstellar reddening
  • blue light absorbed more than red
  • spectral classification will NOT be affected
  • spectral lines will not change in intensity
    • Slide # 37: Effects Interstellar Dust
  • stars appear redder
  • appear dimmer
    • Slide # 38: What happens to absorbed light?
  • some heats up the dust particles
  • dust particles emit thermal radiation
  • detected by infrared telescopes
  • some light is just reflected
  • not actually absorbed
  • similar to scattered light in the blue sky
    • Slide # 39: A Reflection Nebula (GRAPHICS)
  • the Pleaides - reflection nebula
    • Slide # 40: Types of Nebula
  • reflection nebula
  • reflect starlight and appear blue
  • emission nebula
  • emit line radiation and usually appear red
  • some appear greenish
  • dark nebula or dust lanes
  • absorb light and appear black
    • Slide # 41: Emission Nebula (GRAPHICS)
  • M42 - the Orion Nebula - an HII region
    • Slide # 42: Emission Nebula (GRAPHICS)
  • the Vela Nebula - Supernova Remnant
    • Slide # 43: Nebula (GRAPHICS)
  • M20 - the Trifid Nebula
  • emission, absorption, and reflection
    • Slide # 44: Emission Nebula (GRAPHICS)
  • M57 - the Ring Nebula - a planetary nebula
    • Slide # 45: Messier Objects
  • catalog of nebula and galaxies
  • observed by Messier in 1787
  • created the catalog of things NOT to observe
  • he was looking for comets
  • designation M1 through M103
    • Slide # 46: Exciting Line Emission
  • collisional excitation
  • collisions from gas pressure
  • radiative excitation
  • photons excite or ionize gas
    • Slide # 47: Emission Nebula
  • collisions are very rare
  • very low density
  • very low collision rate
  • radiative excitation
  • photons from stars excite the atoms
    • Slide # 48: Emission Nebula Power Sources
  • high luminosity stars
  • lots of photons to ionize atoms
  • hot stars
  • much more blue light (Wien's law)
  • more ultraviolet photons
  • lots of ultraviolet photons needed
  • photons must ionize the gas
  • ultraviolet photons have lots of energy (photoelectric effect)
    • Slide # 49: The HR Diagram (GRAPHICS)
  • HR diagram
    • Slide # 50: Colors of Emission Nebula
  • mostly red color
  • ionized hydrogen lines
  • HII regions
  • some greenish color
  • doubly ionized oxygen lines (OIII)
  • a "forbidden line"
    • Slide # 51: Forbidden Lines
  • doubly ionized Oxygen de-excites very slowly
  • most atoms de-excite in 10-8 seconds
  • OIII takes HOURS to de-excite
  • we can't observe this in laboratories
  • collisions happen before for the atoms de-excites
  • the collisions change the spectral lines
  • lines like this are called "forbidden"
    • Slide # 52: Ionization States of Hydrogen
  • molecular hydrogen, not ionized = H2
  • two atoms in the molecule
  • cool, dense gas
  • atomic hydrogen, not ionized = HI
  • electrons still bound to the atom
  • most of the ISM
  • ionized hydrogen = HII
  • electron removed from atom
  • found around hot, bright stars
    • Slide # 53: The Contest
  • OBAFGKM
  • Oh Be a Fine.... contest
    • Slide # 54: The Winner (GRAPHICS)
  • Our Brother Andrew Found Green Killer Martians
  • Bethany Kirsch