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The Interstellar Medium
Astronomy 315Professor Lee
CarknerLecture 18
Exercise 17 – Dark Matter Use Kepler’s Third Law to find the total
mass of the galaxy a = (20000 pc)(206000 AU/pc) = 4.12X109 AU P = 460X106 years M = (4.12X109)3/(460X106)2 =
Mass-to-light ratio Mass of stars = 1X1010 solar mass total mass/visible mass = 3.3X1011/1X1010 = The galaxy contains 33 times as much mass as
we can see
Upcoming Events
Panel discussion: “The End of the Universe” Tonight (April 25) Wallenberg Hall, 5pm
Planetarium Open House Saturday, April 30, 8:30-10 pm
Roys Science Lecture: Dr. Ralph Harvey speaks on Martian Meteorites from Antarctica Thursday May 12, 7:30pm, Olin
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
Next Time
Read Chapter 24.1-24.4