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
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 24.1-24.2