Download Lecture 10 February 13

ASTR 1120 – February 13
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First Exam: Thursday, February 20
Recitation will be held in Duane G131,
Wednesday Feb 19 at 5pm
NOTE CHANGE FOR NEXT WEEK!!!
Website
http://casa.colorado.edu/~wcash/APS1120/APS1120.html
Review Sheet and Sample Exam posted
What’s Left?
Eventually all the envelope is blown away.
The planetary nebula dissipates.
All we have is the hot, degenerate core.
Settles down to become a White Dwarf
Gorgeous Planetary Nebulae
from Hubble Space Telescope
Notice Rings.
Star has “episodes”
Globular Clusters
Very Old. G stars becoming giants.
All the same age and composition
Can actually see evolution off the MS
White Dwarfs
•
•
•
•
Held up by electron degeneracy
About the size of the Earth R~5000km
Mass Typically 0.8M~
Luminosity ~ .001 L~
Thin layer of “normal” H
Degenerate Carbon
Some Famous White Dwarfs
• Sirius B
• 40 Eridani B
• Procyon B
All in binaries around nearby stars.
Establishes the WD is close and small.
Earth vs White Dwarf
Earth vs. Sun
Mass Radius Relation
1
R∝ 3
M
As mass increases star gets smaller.
Like ball of foam.
WD Density
2 x1033
6
ρ=
=
= 1.5 x10 g / cc
8 3
4 3 4 x(7 x10 )
πR
3
M
Water has a density of 1 g/cc
Lead 11 g/cc
Gold 19 g/cc
100,000 times density of gold!
NOT NORMAL MATTER!!
1 cubic centimeter masses one ton!
Surface Gravity
GM 7 x10 −11 x 2 x1030
6
2
a= 2 =
=
3
x
10
m
/
s
R
(7 x10 6 ) 2
This 300,000 gees
If you weigh 150lbs on Earth, you would weigh
45 million pounds on a White Dwarf!
What would happen to you and your spaceship?
Escape Velocity
2GM
2 x6.7 x10 −11 x 2 x1030
13
6
Ve =
=
=
4
x
10
=
6
x
10
m/s
6
R
7 x10
Speed of light is 3x108 m/s, so escape velocity is .02c.
Gravitational Redshift
Even light loses energy climbing out of this hole.
GMm
2
= αmc
R
α = 2x10-4
At 5000Å have 1Å shift to red
Looks like a 60km/s Doppler Shift
Magnetic Field
R
B = Bo  
R
3
When a star shrinks from 109m to 107m
R0
= 100
R
So B increases from 1Gauss to a Million Gauss
A million Gauss can rip normal matter apart!
Chandrasekhar Limit
A peculiarity of Degeneracy Pressure is that it has a maximum mass.
Each electron added must find its own quantum state by having its
own velocity.
But what happens when the next electron has to go faster than light?
The Chandrasekhar Limit for a White Dwarf is 1.4M~
No White Dwarf Can have more than 1.4M~
Otherwise it will groan and collapse under its own weight.
We’ll come back to this later.
WDs are Common
Every star with less than 5M~ will end up as a White Dwarf
Most stars with mass above 1.3M~ have reached end of MS life.
White Dwarfs are VERY common ~ 10% of all stars
Closest is only 2.7pc away. (Sirius B)
Will become increasing common as universe ages.
Immortal Stars
Regular stars need thermal pressure to balance gravity, and they
need nuclear reactions to maintain the pressure, so the die when they
run out of fuel.
Not so White Dwarfs. They are as stable as a rock. Literally.
A quadrillion years in the future all the stars will be gone, but the
White Dwarfs will still be here.
Their glow is fossil energy left from their youth as a regular star.
Might die in 1031 years if protons prove to be unstable themselves.
That’s 10,000,000,000,000,000,000,000,000,000,000 years!
Really don’t know if universe will still be here.
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