Download Outline - March 16, 2010 Interstellar Medium (ISM) Why should you

Midterm Exam #2
Tuesday, March 23
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Closed book
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Will cover Lecture 8 (Special Relativity) through Lecture 14 (Star
Formation) only
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If a topic is in the book, but was not covered in class, it will not be on the
exam!
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Some combination of multiple choice, short answer, short calculation
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Equations, constants will all be given
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Standard calculators allowed
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Cell phones, PDAs, computers not allowed
Outline - March 16, 2010
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Recap: Interstellar Medium
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Recap: How are stars made?
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Stages of star birth
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Protostars, protostellar disks, protostellar jets
Interstellar Medium (ISM)
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Material between the stars (specifically in our own Galaxy)
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Most of space is a better vacuum than can be made in a
laboratory!
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Why should you care about the ISM?
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Stars had to come from somewhere (the Big Bang didn’t
make stars)
About 1/5 as much mass in the ISM as in stars in our Galaxy
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When stars die, their guts have to go somewhere
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Some regions of space contain clouds gas (some clouds are
hot: > 10,000 K, some clouds are very cold: 10 K-30 K)
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If those “somewheres” weren’t the same place, we wouldn’t
be here! (a topic for after Spring Break)
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Chemical composition of ISM: 70% H, 28% He, 2% other
elements (by mass)
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Association Between Cold Clouds and Stars
“Heir ist wahrhaftig
ein Loch im Himmel”
Wm. Herschel
Cold clouds are transparent in the infrared and radio
Milky Way: Optical
Milky Way: Radio
Milky Way: Infrared
Image taken in optical / visible light
Molecular Clouds
Stellar Nurseries
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Very, very cold (10K to 30K)
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Typical density is 300 molecules per cubic centimeter (vastly
less than the density of air at sea level, but vastly more
than the density of the ISM on average in our Galaxy)
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Gas is primarily H2 molecules, but you can’t detect them
directly! (Note: Helium does not form molecules because it is
chemically inert.)
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Most common “tracer” molecule is CO (carbon monoxide)
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About 1% of the mass in molecular clouds is in “dust”
Cold clouds obscure our view at visible wavelengths, but infrared and
radio light penetrates the clouds.
“Dust” in the ISM
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Not dust bunnies, more like the
microscopic particles in smoke
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Size of dust grains is smaller than
bacteria (typical size is 1 micron =
10-6m)
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Dust grains made mostly of some
combination of carbon, silicon,
oxygen, and iron
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Dust blocks wavelengths of light
that are smaller than the size of
the grains (λ < 10-6 m)
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Dust easily blocks UV and visible
light, but IR and radio light can
(usually) pass right through
Horsehead Nebula (in Orion), optical image
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Cloud Structure: Gravitational Equilibrium
What do we mean by “pressure” in a cloud?
A stable cloud has a
balance of two forces:
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Why does a balloon maintain its
shape?
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What happens to a balloon if you
blow it up at room temperature, then
put it in the freezer for a couple of
hours?
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This is what is known as “thermal”
pressure (the common pressure for
gasses)
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Easiest place for gravity to “win” over
pressure is in a cloud of gas that is
very cold (= low pressure)
INWARD: Gravity
OUTWARD: Pressure
No net force => No motion
Collapsing Clouds
Most Stars are Born Inside Clusters
Most molecular clouds contain
MUCH more mass than would make
a single star
This cold, dark cloud is
collapsing and forming
cores that will eventually
become stars
Most molecular clouds are very
LUMPY (not smooth)
Likely scenario is that many lumps
(which are more dense than the
average) contract to form stars at
about the same time
This is a cloud where gravity
has won the tug-of-war!
Pleiades Star Cluster
Single star formation is possible but
probably very rare (because you
need an unusually dense, yet lowmass cloud)
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Cloud Collapse
Star Formation
Basic stages:
* Cloud collapses
* Protostar forms
* Disk forms
* Planets form
Collapse to Protostar
If you compress a gas, it will
heat up.
Protostars vs. Stars
Main difference between a protostar and a genuine star:
Center of a collapsing cloud
becomes denser and hotter.
The energy is gravitational. Half
the gravitational energy goes into
heating the collapsing clout, the
other half escapes as light.
The central object is called a
“protostar”, and they are very
bright! (Because they have very
large radii.)
Protostar
Protostars are hard to see
because they are being formed in
very dusty regions of space.
Stars generate power by nuclear fusion, protostars generate
power by gravitational collapse.
Note: For the most part, stars are stable (neither expanding nor
contracting). Protostars are all contracting.
When does the contraction end?
When the core becomes hot enough and dense enough to start
nuclear fusion reactions.
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Some Protostars
Disk Formation
NGC 6334
IRDC 43
Angular Momentum, L
Conservation Laws
The following quantities are conserved:
1. Total Energy
2. Linear Momentum
3. Angular momentum
m
r
v
For an object of mass m
orbiting with velocity v and
distance from rotation axis r,
the angular momentum L is
L = mvr
The total amount of these quantities never changes
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Spin-up
Disk Formation
Li = mvi r i
Lf = mvf rf
Li = Lf
vf = v i (ri/rf)
vi
ri
vf
L = mvr
rf
r
If the radius of the orbit decreases, and angular
momentum is conserved, the velocity must increase.
Disk Formation
To conserve angular
momentum, an object moving
perpendicular to the rotation
axis must increase speed,
and eventually stop
because it has a finite energy.
r changes here
Disk Formation
L=mvr
r
To conserve angular momentum,
an object moving parallel to the
rotation axis need not change
speed, and is free to move to
the equatorial plane.
r doesn’t change here
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Do disks around stars exist? YES
More Disks around Protostars
HST Image of Beta Pictoris: note star itself has been blocked out
Scale bar shows 1,000 AU
HH-34
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HH-30 (image & model)
Planet Formation
Jets are perpendicular to the disk = rotation plays a role in their
formation. Most likely link of rotation and outflowing gas are magnetic
field lines. We’ll see this again when we talk about “active galaxies”.
It shouldn’t be that hard….
Proplyds are ~99% gas, ~1% dust
Portion of Orion Nebula showing “Proplyds” = Protoplanetary Disks
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Beta Pictoris will have many planets around in the future?
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