Download Lecture 14 - Center for Astrophysics and Space Astronomy CASA

ASTR 1200
Announcements
Exams are at the back. Please pick up if you haven’t 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/APS1200.html
Binary Stars
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Optical Double
appear close together but aren’t 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
Spectroscopic Binary
Variable Stars
Some stars just expand and contract.
Eclipsing Binary
Algol – “The Devil Star”
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
Close Binaries
Equal Energy Curves
Gravity
Mid-Point
Contact Binaries
Very Close
Touching
Common Envelope
Two Nuclear Cores
“W Ursa Majoris” star
Periods of Contact Binaries
By Kepler’s 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!
Mass Transfer
Huge Flow of Material from One to Other
Can stop evolution of one and speed up other
Gets complicated
“Dog Eat Dog” Scenario
Mass Transfer Binary
White Dwarf & Star
The Roche Lobe
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
Energy Released
GMm 6.7 x1011 x2 x1030 x1
E
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 2 x1013W / kg
6
R
6 x10
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!
Material Reaches Surface
Layer of H build
up on surface
Carbon White Dwarf
Pressure builds on the hydrogen.
Material pouring in heats it.
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.
Novae
Hydrogen explodes into space
to create a shell of expanding
gas.
Gas expands outward at 500km/s
The Sun can never go nova!
It’s not a white dwarf in a close binary.
3 Kinds of Novae
Classical Novae
Recurrent Novae
Dwarf Novae
Only seen once
Seen several times over last few hundred years
Pop off every few weeks to months
It’s just a matter of how fast material is transferring
and how much needs to accumulate before the
spark.
Supernovae
Nature’s Biggest Explosion
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10,000BC
185AD
396
1006
1054
1572
1604
1667
1987
m
-3
-10
-6
-4.1
-2.2
>5
4.0
1300pc
1800
5000
7000
3400
55,000
Crab
Tycho’s
Kepler’s
Cas-A
SN1987A
We now see a dozen or so every year in distant galaxies.
II
I
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II
Supernovae
Occur about once every hundred years per galaxy.
Briefly outshines the other 100Billion stars in the galaxy.
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?
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 >100,000,000K.
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.
Explosion
Explosion Starts at Center
where pressure is highest
Energy Released
Nuclear Energy Generates 2MeV per atom in forming molecule (burning)
2MeV = 3x10-13 Joules
Number of Atoms in Star:
Available Energy
M
2 x1030 kg
N
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 1057
 27
m p 1.6 x10 kg
E  3x10 13 x1057  3x10 44 J
About 1044J release in just a few seconds.
That’s 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.
SN1987A – Before and After
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
Tycho’s Supernova
Seen in X-ray
Gas at 10,000,000K
Expanding at 5000km/s
Type II Supernovae
High Mass Star --- M > 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
Onion Skin Model
Nuclear Reactions
12C
+ 12C  20Ne +4He
+ g  16O + 4He
oxygen shell
+ 16O  28Si + 4He
silicon shell
+ 28Si  56Fe
iron core
20Ne
16O
neon shell
28Si
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