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The Interstellar Medium

The Interstellar Medium. Astronomy 315 Professor Lee Carkner Lecture 19. The Stuff Between the Stars. Called the interstellar medium (ISM) Gas Accounts for most of the volume Dust Account for most of the opacity. Gas. Modest amounts of C,N,O and other “medium” elements

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The Interstellar Medium

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  1. The Interstellar Medium Astronomy 315 Professor Lee Carkner Lecture 19

  2. The Stuff Between the Stars • Called the interstellar medium (ISM) • Gas • Accounts for most of the volume • Dust • Account for most of the opacity

  3. Gas • Modest amounts of C,N,O and other “medium” elements • Heavier elements have condensed to form the dust • How gas looks depends on the amount of external radiation

  4. Radiation Environment • Amount of radiation depends on: • High mass O and B stars produce a lot of high energy radiation (UV) • The inner part of a dense cloud is shielded from most external radiation

  5. Three Kinds of Hydrogen • Molecular clouds • Form where thick dust layers shield hydrogen • HI regions • Form where radiation is weak • HII regions • Form where there is a lot of radiation

  6. Dust • If the ISM was all gas, you could see right through it • Dust particles are small (~1 micron = 1 millionth of a meter) • Some different types of dust: • Medium silicate grains • All these different types identified from complex absorption properties

  7. Extinction Curve

  8. What Does Dust Do? • Absorption • This causes interstellar extinction • Stars behind the cloud look fainter • Scattering • Dust scatters blue light better than red • Causes reddening (only red light gets through) • Stars behind the cloud look redder

  9. Reddening in the ISM

  10. Extinction • m = M + 5logd - 5 + A • A is the extinction • Can find the distance to a star if we know absolute (M) and apparent (m) magnitudes d = 10X X = (m-M+5-A)/5 • Remember d is in parsecs

  11. Nebulae • What is a nebula? • We now use the term to refer to any part of the ISM in general • There are three basic types of nebulae, each with a distinct appearance based on the way it interacts with light

  12. Dark Nebulae • Clouds that are dense with dust can completely block out the light of stars behind them • Can see stars in front of the cloud projected on it • Often associated with molecular clouds

  13. Emission Nebulae • If a cloud is near bright high mass stars it may shine as an emission nebula • The UV light ionizes the gas • Like a florescent light • The transitions are of the hydrogen Balmer series and so the nebula looks red or pink • Emission nebulae are HII regions

  14. Reflection Nebulae • Dust preferentially scatters blue light • Same reason sky is blue • Need bright star fairly near-by to produce effect

  15. Multiwavelength ISM • Radio • Neutral hydrogen emits at a wavelength of 21cm • Millimeter • For viewing molecular clouds • Some are very complex and must be protected from UV radiation

  16. More Multiwavelength ISM • Infrared • Dust is cold (<100K or -150 C), and shines directly in IR • X-ray • For viewing coronal gas • Formed from supernova • Fills most of the space of the galaxy

  17. Structure of ISM • We may be looking through many clouds when we view a star • The sun is actually in a large region of hot gas called the local bubble • The denser parts of the ISM are the sites of star formation

  18. Next Time • Read Chapter 24.1-24.2

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