ASTR 1200 Announcements

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ASTR 1200Announcements
Exams are at the back. Please pick up if you
havent yet. Mean score 70.4 Problem Sets 3 and
4 posted. Due on Thursday 23rd. Second exam will
be October 30
Website http//casa.colorado.edu/wcash/APS1200/AP
S1200.html
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Binary Stars

• Optical Double appear close together but arent
  really binary
• Visual Binary orbiting, but we can see them both
• Astrometric Binary proper motion wiggles to show
  orbit
• Spectrum Binary spectra of two stars of different
  type
• Spectroscopic Binary Doppler shift shows
  orbital motion
• Eclipsing Binary light varies

Half of all stars are in binaries.
Binary stars are formed at birth. Both components
will have same age and composition. Can vary in
mass Can be very distant (0.1pc) or touching
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Spectroscopic Binary
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Variable Stars
Some stars just expand and contract.
Eclipsing Binary
Algol The Devil Star
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Russian Variable Star Catalogue
Compilation of all the stars that vary.
Letter starting with R, followed by Constellation
Name After Z starts RR through ZZ, then AA
SS Cygni VY Hydrae W Ursa Majoris Gets funny on
occasion RU Lupi EZ Sextans
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Close Binaries
Gravity Mid-Point
Equal Energy Curves
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Contact Binaries
Very Close
Touching
Common Envelope Two Nuclear Cores
W Ursa Majoris star
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Periods of Contact Binaries
By Keplers Law P R(3/2)
R? (1/200) AU
So P (1/200)(3/2) years 3.5x10-4 years 104
s 3 hours
These contact binaries swing around each
other every few hours!
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Mass Transfer
Can stop evolution of one and speed up other Gets
complicated Dog Eat Dog Scenario
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Mass Transfer Binary
White Dwarf Star
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Mass Transfer Binary
Accretion Disk
Material Swirls In Friction allows the material
to fall and heats while it falls. All the way to
the surface
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Energy Released
Huge amounts of energy are released as the
material swirls in. Material get hot. Really
hot. Like a million degrees Kelvin. Emits
ultraviolet and x-rays. We can see these
accretion disks with x-ray telescopes!
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Material Reaches Surface
Carbon White Dwarf
Layer of H build up on surface
Pressure builds on the hydrogen. Material pouring
in heats it.
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Nova
One day the hydrogen ignites in huge nuclear
rush. Burns like a brush fire from one end of the
star to the other. This is called a Nova. A
new star appears in the sky. Often visible to
the unaided eye. Lasts a few weeks to months.
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Novae
Hydrogen explodes into space to create a shell of
expanding gas. Gas expands outward at 500km/s
The Sun can never go nova! Its not a white dwarf
in a close binary.
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3 Kinds of Novae
Classical Novae Only seen once Recurrent Novae
Seen several times over last few hundred
years Dwarf Novae Pop off every few weeks to
months
Its just a matter of how fast material is
transferring and how much needs to accumulate
before the spark.
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SupernovaeNatures Biggest Explosion

• 10,000BC
• 185AD m
• 396 -3
• 1006 -10 1300pc
• 1054 -6 1800 Crab II
• 1572 -4.1 5000 Tychos I
• 1604 -2.2 7000 Keplers I
• 1667 gt5 3400 Cas-A II
• 1987 4.0 55,000 SN1987A II

We now see a dozen or so every year in distant
galaxies.
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Supernovae
Occur about once every hundred years per
galaxy. Briefly outshines the other 100Billion
stars in the galaxy.
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Type I Supernovae
White dwarf is gaining mass. Over time, the mass
will approach the Chandrasekhar Limit Remember,
at 1.4M?, electron degeneracy fails. What
happens?
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White Dwarf Collapse
As WD starts to collapse, the material falls
through the gravitational field of the star. It
heats very rapidly. In just a few seconds it
reaches gt100,000,000K. Carbon and Oxygen ignite
and burn by fusion to even heavier elements. The
whole star explodes in a frenzy of nuclear
burning. Blows completely apart. All that
remains is an expanding shell of gas that used to
be a white dwarf and the companion star slingshot
into space.
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Explosion
Explosion Starts at Center where pressure is
highest
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Energy Released
Nuclear Energy Generates 2MeV per atom in forming
molecule (burning) 2MeV 3x10-13 Joules Number
of Atoms in Star
Available Energy
About 1044J release in just a few
seconds. Thats as much energy as the Sun emits
during its entire lifetime. In a few
seconds!!!! This is so titanic we can see it
across the universe A billion trillion trillion
atomic bombs Gas returning to interstellar space
has more CNO etc.
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SN1987A Before and After
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The Crab Nebula
Supernova Dominated Sky in 1054 AD Observed by
Chinese (not in Europe) Recovered in 18th Century
by Messier
Called a Supernova Remnant 1pc in
diameter Expanding Rapidly
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Tychos Supernova
Seen in X-ray Gas at 10,000,000K Expanding at
5000km/s
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Type II Supernovae
High Mass Star --- M gt 5M?
In low mass star, envelope is blown off into
space, creating planetary nebula, before Carbon
in core can flash. High mass star has enough
gravity to hold onto the gas. Get a Carbon flash
just like the Helium Flash Carbon burns to
Neon Then Neon flash Gets very complicated
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Onion Skin Model
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Nuclear Reactions
neon shell
12C 12C ? 20Ne 4He
oxygen shell
20Ne g ? 16O 4He
silicon shell
16O 16O ? 28Si 4He
iron core
28Si 28Si ? 56Fe
Iron cannot nuclear burn at any temperature (On
border between fusion and fission) Develops
degenerate iron core than cannot flash Just gets
hotter and heavier down in the middle of the star