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   Heavier elements have condensed to form the dust  How gas looks depends on the amount of external radiation

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

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

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

Extinction Curve

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

Reddening in the ISM

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

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

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

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

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

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

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

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 

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