The Interstellar Medium Astronomy 315 Professor Lee Carkner Lecture 19

Exercise 18 – Dark Matter  Use Kepler’s Third Law to find the total mass of the galaxy  M = a 3 /P 2  a = (20000 pc)( AU/pc) = 4.12X10 9 AU  P = 460X10 6 years  M = (4.12X10 9 ) 3 /(460X10 6 ) 2 = 3.3X10 11 solar masses  Luminosity of galaxy  F = L/4  d 2, L = F4  d 2  L = (3.88X )(4)(  )(5.58X10 23 ) 2 = 1.52X10 37 W  1.52X10 37 /3.8X10 26 = 4X10 10 L sun  Mass-to-light ratio  M/L = 3.3X10 11 /4X10 10 = 8.25

The Stuff Between the Stars   Called the interstellar medium  Concentrated in the disks of spiral galaxies   Exists with different temperatures, densities, size scales, and radiation environments

Composition  We can get an idea of the composition of the interstellar medium through spectroscopy  There are two components: gas and dust  Gas    Dust   Account for most of the opacity

Gas  Interstellar gas is abundant in hydrogen and helium    Heavier elements have condensed to form the dust  What form is the gas in?

Radiation Environment  Different parts of the ISM experience different external radiation fields   Location of bright stars   Density of cloud  The inner part of a dense cloud is shielded from most external radiation

Effects of Radiation  Clouds that experience little radiation are known as HI regions   Clouds in higher radiation environment are known as HII regions   Some clouds have so little radiation that they can form some molecules   Called giant molecular clouds

Dust  The clouds are much more opaque than you would expect just from the gas   Dust particles are small (~1 micron or 1 millionth of a meter)  Some different types of dust:     All these different types identified from complex absorption properties

Extinction Curve

What Does Dust Do?  Absorption   This causes interstellar extinction   Scattering   Dust scatters blue light better than red   Stars behind the cloud look redder

Reddening in the ISM

Reddening

Nebulae  What is a nebula ?    There are three basic types of nebula, each with a distinct appearance based on the way it interacts with light

Dark Nebulae  Dense clouds can completely block out the light of stars behind them   Can see stars in front of the cloud projected on it   Smallest are called Bok globules

Star Cluster and Dark Cloud

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   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

Observing the ISM  With optical telescopes we see the effects of the ISM in different ways    We also need to observe at other wavelengths

Multiwavelength ISM  Radio   Penetrates dust and allows us to map much of the galaxy  Millimeter   Some are very complex and must be protected from UV radiation

More Multiwavelength ISM  Infrared   Dust is still very cold (<100K or -150 C)  X-ray   Formed from supernova

Types of Clouds  HI clouds   Molecular Clouds   Coronal gas   Intercloud medium  Hot, low density, about 50% of total mass  A lot of mass between the well-defined clouds

Structure of ISM  If we look in any direction we see many different parts of the ISM   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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