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ASTR 1120 – February 13 . 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 • • • • • • 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