Transcript 19ism1s

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 = a3/P2
a = (20000 pc)(206000 AU/pc) = 4.12X109 AU
P = 460X106 years
M = (4.12X109)3/(460X106)2 = 3.3X1011 solar masses
Luminosity of galaxy
F = L/4pd2, L = F4pd2
L = (3.88X10-12)(4)(p)(5.58X1023)2 = 1.52X1037 W
1.52X1037/3.8X1026 = 4X1010 Lsun
Mass-to-light ratio
M/L = 3.3X1011/4X1010 = 8.25
The Stuff Between the Stars
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Called the interstellar medium
Concentrated in the disks of spiral galaxies
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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
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Dust
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Account for most of the opacity
Gas
Interstellar gas is abundant in
hydrogen and helium
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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
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Location of bright stars
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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
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Clouds in higher radiation environment are
known as HII regions
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Some clouds have so little radiation that they can
form some molecules
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Called giant molecular clouds
Dust
The clouds are much more opaque than you
would expect just from the gas
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Dust particles are small (~1 micron or 1
millionth of a meter)
Some different types of dust:
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All these different types identified from
complex absorption properties
Extinction Curve
What Does Dust Do?
Absorption
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This causes interstellar extinction
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Scattering
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Dust scatters blue light better than red
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Stars behind the cloud look redder
Reddening in the ISM
Reddening
Nebulae
What is a nebula ?
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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
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Can see stars in front of the cloud
projected on it
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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
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The UV light ionizes the gas
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Like a florescent light
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Emission nebulae are HII regions
Reflection Nebulae
Dust preferentially scatters blue light
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Same reason sky is blue
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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
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We also need to observe at other
wavelengths
Multiwavelength ISM
Radio
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Penetrates dust and allows us to map much of the
galaxy
Millimeter
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Some are very complex and must be protected
from UV radiation
More Multiwavelength ISM
Infrared
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Dust is still very cold (<100K or -150 C)
X-ray
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Formed from supernova
Types of Clouds
HI clouds
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Molecular Clouds
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Coronal gas
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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
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The sun is actually in a large region of hot gas called the
local bubble
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The denser parts of the ISM are the sites of star
formation
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Next Time
Read Chapter 24.1-24.2