ASTR 1200 Announcements Website 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 23 rd. Second exam will be October 30

Binary Stars Optical Doubleappear close together but aren’t really binary Visual Binaryorbiting, but we can see them both Astrometric Binaryproper motion wiggles to show orbit Spectrum Binaryspectra of two stars of different type Spectroscopic Binary Doppler shift shows orbital motion Eclipsing Binarylight 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 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 Compilation of all the stars that vary.

Close Binaries Gravity Mid-Point Equal Energy Curves

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 = 10 4 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 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 Carbon White Dwarf Layer of H build up on surface 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 NovaeOnly seen once Recurrent Novae Seen several times over last few hundred years Dwarf NovaePop 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 10,000BC 185ADm pc CrabII Tycho’sI Kepler’sI 1667>53400Cas-AII ,000SN1987AII We now see a dozen or so every year in distant galaxies.

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 = 3x Joules Number of Atoms in Star: Available Energy About J 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 18 th 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 12 C + 12 C  20 Ne + 4 He 20 Ne +   16 O + 4 He 16 O + 16 O  28 Si + 4 He 28 Si + 28 Si  56 Fe neon shell oxygen shell silicon shell iron core 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