ASTR 1040 – October 12

1. ASTR 1040 – October 12 Next Observatory Opportunity: Tonight at 8:00 Then next Tuesday at 8:00 Homework 3 posted due next Thursday Homework 4 posted soon Second Mid-term November 2 LA Session UMC235 today . Website http://casa.colorado.edu/~wcash/APS1040/APS1040.html

2. 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.

3. 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.

4. 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.

5. 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

6. Spectroscopic Binary

7. Variable Stars Some stars just expand and contract. Eclipsing Binary Algol – “The Devil Star”

8. 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

9. Close Binaries Gravity Mid-Point Equal Energy Curves

10. Contact Binaries Very Close Touching Common Envelope Two Nuclear Cores “W Ursa Majoris” star

11. 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!

12. Huge Flow of Material from One to Other Mass Transfer Can stop evolution of one and speed up other Gets complicated “Dog Eat Dog” Scenario

13. The Roche Lobe Mass Transfer Binary White Dwarf & Star

14. 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

15. 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!

16. Material Reaches Surface Carbon White Dwarf Layer of H build up on surface Pressure builds on the hydrogen. Material pouring in heats it.

17. 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.

18. 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.

19. 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 It’s just a matter of how fast material is transferring and how much needs to accumulate before the spark.

20. SupernovaeNature’s Biggest Explosion • 10,000BC • 185AD m • 396 -3 • 1006 -10 1300pc • 1054 -6 1800 Crab II • 1572 -4.1 5000 Tycho’s I • 1604 -2.2 7000 Kepler’s I • 1667 >5 3400 Cas-A II • 1987 4.0 55,000 SN1987A II We now see a dozen or so every year in distant galaxies.

21. Supernovae Occur about once every hundred years per galaxy. Briefly outshines the other 100Billion stars in the galaxy.

22. 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?

23. 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.

24. Explosion Explosion Starts at Center where pressure is highest

25. 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. 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.

26. SN1987A – Before and After

27. 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

28. Tycho’s Supernova Seen in X-ray Gas at 10,000,000K Expanding at 5000km/s

29. 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

30. Onion Skin Model

31. 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

32. Collapse When the degenerate iron core exceeds the Chandrasekhar limit, electron degeneracy can no longer support it. It will start to collapse. Electrons do not have individual quantum states left. They hide by merging with protons to form neutrons: P + e- n + n Every time this happens, a neutrino is also created. Neutrinos are free to escape to infinity and carry energy with them.

33. Reversal of the Nuclear Reactions Every iron nucleus in the core was formed in nuclear burning. There is one electron for each proton. After electrons are absorbed, the nucleus consists of 56 neutrons. That’s unstable and the nucleus dissolves into free electrons. Millions of years of fierce nuclear burning is reversed in a few seconds! The star keeps shrinking. By the time it has shrunk from 6000 to 600km, this process is complete. So it’s a ball of neutrons. Still nothing to stop its collapse. Keeps shrinking. Finally, when radius is about 7km, it stops. Has at least 1.4M, but is a speck the size of Boulder

34. Neutron Degeneracy Neutrons, like electrons, must have individual quantum states. What stops the descent is “neutron degeneracy” Conceptually identical to electron degeneracy. Because a neutron is 1838 times more massive than an electron, the radius of the degenerate star is 1838 times smaller. This is called a Neutron Star. It is roughly 14km in diameter and has 1.4 times the mass of the Sun. They are formed in the middle of Type II Supernovae.

35. Energy of Collapse As neutron ball collapses it releases gravitational energy. Sun will only emit 1044 J in its entire life. This is about a thousand times greater than the energy released in at Type I supernova.

36. Where did the energy go? • Neutron Stars were found in supernova remnants in the 1960’s. • Type I and Type II Supernovae have comparable brightness. • Type II’s are NOT 1000x brighter. • Where did 99.9% of the energy go??

37. Neutrinos Ball of neutrons radiates thermal neutrinos the same way that a ball of electrons will radiate photons. These elusive particles carry away 99.9% of the energy. So poorly coupled to regular matter that they travel unimpeded through the Universe at close to the speed of light. In fact, at this moment, each and every one of us has about 10 neutrinos per second passing through our bodies. They were generated in distant supernova in galaxies far, far away – long, long ago.

38. Neutrinos So what if we view this as a personal violation? Let’s go to the Department of Homeland Security and ask them to put up a shield that will protect us from these nasty neutrinos. We’ll make it out of one of the best materials for stopping them – lead. How thick will the shield have to be? About a parsec!! Answer: Looks like neutrinos have a mass about a million times lower than electrons.

39. Core Bounce When star reaches neutrons star size it is collapsing so fast that it overcompresses. It reverses direction and grows some. This outward shock wave couples into the rest of the star and drives the stellar envelope into space. That’s the 1044J we see.

40. Explosive Nucleosynthesis Expanding shell starts out very hot. Nuclear reactions are taking place rapidly. As it expands it cools rapidly. Some very heavy elements can’t survive long at high temperature. A few get frozen in during cooling. That’s where all the elements heavier than iron come from. That’s why heavy elements are expensive. They’re made in supernovae! And they’re trace resultants at that.

41. SN1987A • First naked eye supernova since 1604. • Discovered by Ian Shelton. Feb 23, 1987 UT 23.316 • Showed hydrogen escaping at 30,000km/s • In Large Magellanic Cloud • Tiny galaxy orbiting the Milky Way. • Huge international response of the astronomy community.

42. Progenitor Star that exploded was tracked down. Not terribly prominent. SK-69 202 B3 Supergiant m=12.4 M=-7.8 T=16,000K, R= 40R Distance 55,000pc – actually outside Milky Way.

43. Stellar History • Burned H 10,000,000 years • He 1,000,000 • C 300 • Ne 5months • O 6 • Si 2 days • POW!!!!

44. Rings HST image of SN1987A a few years after the event. Center is dim 3 Bright rings Illuminated by the flash Hourglass shape again Means there was mass loss prior to the explosion

45. Impending Collision Blast wave is about to hit the ring.

46. Big Rings 30 Light Years With this geometry there is only a few light months delay.

47. Neutrino Astronomy Ground Level Fill mineshaft with water Put photo detectors around the inside

48. Neutrino Going Through Water n n n e+ e (scattering) light produced by secondary interactions n

49. Super Kamiokande

50. Detection of SN1987A • Kamiokande II Japan 12 events 15s • IMB Ohio 8 events 5.6s Mont Blanc neutrino detector saw a marginal signal 4.7 hours earlier. Real??? In 1987A we detected the formation of a new neutron star!