April 27, 2005, jebPRELIMINARY

Outline

• The Interstellar Medium
• What is the Interstellar Medium?
• Extinction and Interstellar Reddening
• Nebulae
• Emission Nebulae
• Dark Dust Clouds
• Ultraviolet Astronomy and the "Local Bubble"
• 21-centimeter Radiation
• Interstellar Molecules and Molecular Radiation
• Summary

The Interstellar Medium




The interstellar medium is clearly evident as it blocks the light
from many stars in the Milky Way



From outside the Milky Way, we would observe a galaxy much like Andromeda (M31)




An enlarged view in the direction of the center of our Galaxy, illustrating the blockage of light



Nebulae visible in the above enlarged view

• M8, the Lagoon Nebula
• M16, the Eagle Nebula
• M17, the Omega Nebula
• M20, the Trifid Nebula

What is the Interstellar Medium?




Gas and Dust

• Gas
  • about 90% hydrogen (by no. of atoms)
  • about 9% helium
  • remaining 1% heavier elements
• Dust
  • like cigarette smoke
  • C, N, silicates
  • typically elongated 1/10,000 millimeter particles
  • elongated shape inferred from its polarizing effect on light



Hydrogen Gas

90% of interstellar medium is hydrogen

(about 1 atom per cubic centimeter)

Hydrogen atom = proton + electron (HI)

ionized hydrogen (no electron, just protron - HII)

21 cm line from spin-flip in HI


Temperature and Density
• Temperature
  • few kelvins to few hundred kelvins
    • depends on nearness of stars to heat medium
    • 100 K is typical temperature (COLD)
• Density
  • gas typically 1 atom per cubic centimeter
    • compare this to gas on Earth
    • typical atmosphere = 2.7 x 10¹⁹/cm³
    • best "vacuum" achieved in lab on Earth = 10⁴/cm³
  • dust typically 1 per trillion (10¹²) atoms
    • this is 1000 per cubic kilometer

Total Mass
• while the density is low, the total amount of mass in the interstellar region is nearly as much as the mass contained in the stars themselves

Extinction and Interstellar Reddening



• Extinction
  • Dimming of starlight due to interstellar medium
    • Star's are completely blocked by dust in some directions
  
  
  
• Interstellar Reddening
  • Why is the sky blue?
  • Why is the sunset red?
  • Answer: dust preferentially scatters blue light

Nebulae

Any fuzzy bight or dark patch on the sky

• Absorption nebulae
  • caused by absorbing gas
  • eg. Horsehead Nebula
• Reflection nebulae
  • caused by starlight reflected from surrounding gas
  • eg. Pleiades
• Emission nebulae
  • results when a cloud is heated by stars until it starts to glow
  • eg. Orion Nebula



Many nebula are included in the catalog of Messier Objects, which the 18th century French astronomer Charles Messier compiled to aid him in his search for comets.

Emission Nebulae

• clouds of gas surrounding at least one newly formed O or B type star
• typical temperature of 8000 K
• typically composed of ionized hydrogen, and therefore called HII regions
• Recombination of electron and protons into atom results in emission of radiation => emission lines
  • region of Sagittarius with several fuzzy patches which are emission nebulae M8, M16, M17, and M20
  
  
  
  
  
  • enlargement showing M20 and M8
    • further enlargement showing M20, the Trifid Nebula, and its interstellar environment (notice the blue reflection nebula next to the red emission nebula
  • enlargement showing M16, the Eagle Nebula, and M8, the Lagoon Nebula
  • eg. Butterfly-shaped Emission Nebula, NGC 2346, and emission spectrum
• some very rare lines are seen (referred to as "forbidden lines")
  • example of "forbidden lines" is green light from the Orion Nebula (M42)
    • doubly ionized Oxygen
    • emitted very slowly, only in very "low-density" situation, not realized on Earth, even in laboratory

Dark Dust Clouds

(aka Absorption Nebula)
• Dark dust cloud L977
  • emits strongly at radio wavelengths
• Rho Ophiuchi
• Horsehead Nebula
• Obscures visible light
• Creates absorption spectra
• Typically very cool regions (less than 100 K, down to few 10 K)
  • emit infrared radiation
    • eg. right-most image is Rho Ophiuchi from the Infrared Astronomy Satellite
      • Note - wavelength icon for this image is incorrect

Ultraviolet Astronomy and the "Local Bubble"

• Ultraviolet radiation has helped in the study of the interstellar medium by allowing a map of our local cosmic neighborhood
• Ultraviolet radiation can only be studied by instruments placed in orbit above the Earth's atmosphere
• The International Ultraviolet Explorer (IUE) found a number of regions of interstellar space that are much thinner (5000 atoms/m³) and hotter (500,000 K) than expected
• The Sun seems to reside in such a region
  • a huge cavity, called the "Local Bubble"
  • about 100 pc across, containing about 200,000 stars
  • thought to have been carved out by multiple supernova explosions several hunderd thousand years ago
  • Such supernovae would have been spectacular events in the sky
  
  

21-centimeter Radiation

• A probe of the interstellar medium that does not rely on the heat of the stars
• "radio emission" from hydrogen when the electron flips its spin
  
• This is a critical technique for "seeing" the interstellar medium which is dominated by hydrogen
  particularly when it is not very warm

Interstellar Molecules and Molecular Radiation

When the interstellar medium is cold (say 20 K) molecules can form, and the density often grows (to 10⁶ per cubic centimeter)

• Molecules in space emit radio signals.
• The wavelength of the signals indicate from which molecule it comes.
  • Hydroxyl OH
  • Water H₂O
  • Ammonia NH₃
  • Formaldehyde H₂CO
    • spectra indicate formaldehyde molecules exist around M20
    • contour map of amount of formaldehyde around M20
  • Carbon monoxide CO - useful for spiral arm study
  • Molecular hydrogen H₂
  • Hydrogen isoCyanide HCN - see cloud around center of Milky Way
• Hydrogen is the overwhelmingly dominant component, but it is harder to observe, and the other molecules are the observed tracers of the accumulation
• These radio signals come from limited places in the sky


Molecular Cloud Complexes
• groups of cloud spanning tens of parsecs of space
• containing enough material to make millions of stars like our Sun

Summary



The interstellar medium
• contains nearly as much mass as the stars
• is the region out of which new stars are born
• is the region into which old stars expel their matter when they die
• is typically cold, less than 100 K
• is composed of atomic or molecular hydrogen and helium, and dust
• leads to extinction, reddening, and polarization of starlight

Many nebulae are found in the interstellar media
• Emission nebulae
• Absorption nebulae
• Reflection nebulae

21-cm radiation from atomic hydrogen is used to study cold, dark regions of interstellar space
• penetrates interstellar medium (radio wave)


Cold, dark molecular clouds are often studied by observing "tracer" molecules