Binary Stellar Evolution

1. Binary Stellar Evolution How Stars are Arranged • When stars form, common for two or more to end up in orbit • Multiples more common than singles • Binaries are the most common multiples • Most higher multiples are “hierarchical binaries” • When binaries are far apart (more thana few AU) nothing unusual happens • First star lives, ages, dies • Second star lives, ages, dies

2. Close Binary Evolution • When binaries are close together (AU or less), they can interact extensively • During giant stages, gas can be transferred from one star to the other • This can affect evolution or appearance of star • There will be two distinct periods when something interesting happens • First, when the larger star becomes a giant • Second, when the smaller star becomes a giant

3. Roche Lobes • In a binary system, the region of space gravitationally controlled by each star is called the Roche lobe of that star • Anything within the Roche lobe is likely to be absorbed by the star • The more massive star has a bigger Roche lobe No man’s land Star A’s Roche Lobe Star B’s Roche Lobe

4. Close Binary Evolution: Act I • During main sequence, nothing interesting happens • When the first star becomes a giant, it expands • If it expands enough, it can fill its Roche lobe • Any more expansion leads to mass transfer • This can change the mass balance • The star that was initially lighter may become heavier

5. Accretion Disks Second star • Incoming gas is rotating - from revolution of two stars • Gravity pulling it towards object • Gravity vs. rotation = disk • The system changes • Stars may merge or separate • Second star may become more massive star

6. Close Binary Evolution: Act II, Intro • Compact object: any of the three types of stellar corpses • White dwarf • Neutron star • Black hole • When second star becomes agiant, evolution gets interesting C B A A is heavier than B is heavier than C. When could this system first become “interesting”? A) When A becomes a giant star B) When B becomes a giant star C) When C becomes a giant star

7. Close Binary Evolution: Act II, Outline • We now have a giant star/compact object binary • What you get depends on which type of compact object you have: • White dwarf: • Nova • White dwarf supernova • Black hole: • X-Ray Binary • Neutron star: • X-Ray Binary • X-Ray Pulsar • X-Ray Burster

8. How to Make a Nova • White dwarf: • Nova • White dwarf supernova • Black hole: • X-Ray Binary • Neutron star: • X-Ray Binary • X-Ray Pulsar • X-Ray Burster White Dwarf • White dwarf • Hydrogen gets added to carbon/oxygen layer • Builds up • Ignites and explodes • Cycle repeats

9. How to Make a White Dwarf Supernova • White dwarf: • Nova • White dwarf supernova • Black hole: • X-Ray Binary • Neutron star: • X-Ray Binary • X-Ray Pulsar • X-Ray Burster • During each cycle the white dwarf gains mass • Shrinks slightly • Reaches Chandrasekhar mass • Star begins to collapse • Heats up • Fusion begins • Whole star burns - explodes • Star is completely destroyed • Burns mostly to iron

10. How to Make a Black Hole X-Ray Binary • White dwarf: • Nova • White dwarf supernova • Black hole: • X-Ray Binary • Neutron star: • X-Ray Binary • X-Ray Pulsar • X-Ray Burster X-Rays Black Hole • Accretion disk, as always • Gas is going super fast - enormous gravity • Friction heats up the disk • X-rays from hot gas • Most efficient way to make energy • Even more efficient than fusion

11. How to Make a Black Hole X-Ray Binary • White dwarf: • Nova • White dwarf supernova • Black hole: • X-Ray Binary • Neutron star: • X-Ray Binary • X-Ray Pulsar • X-Ray Burster What happens to the gas after it crosses inside the event horizon? A) It heats up even more, contributing more X-rays B) It impacts on the surface of the black hole, producing additional energy C) It disappears forever

12. How to Make a Neutron Star X-Ray Binary • Neutron star with gas flowing in • Magnetic field of neutron star channels gas to magnetic poles • Large gravity – gas slams into n. star • Spot on neutron star gets very hot • White dwarf: • Nova • White dwarf supernova • Black hole: • X-Ray Binary • Neutron star: • X-Ray Binary • X-Ray Pulsar • X-Ray Burster X-rays

13. How to Make an X-Ray Pulsar • White dwarf: • Nova • White dwarf supernova • Black hole: • X-Ray Binary • Neutron star: • X-Ray Binary • X-Ray Pulsar • X-Ray Burster • The neutron star is rotating as well • As viewed from here, hot spot appears and disappears – it pulses • Over time, the neutron star gains mass

14. How to Make an X-Ray Burster • White dwarf: • Nova • White dwarf supernova • Black hole: • X-Ray Binary • Neutron star: • X-Ray Binary • X-Ray Pulsar • X-Ray Burster • Hydrogen flows ontosurface of neutron star • Hot enough, it burns to Helium • Helium accumulates • Burns explosively,producing X-rays • Cycle repeats • If it passes the maximum mass for a neutron star (2 – 3 MSun) it collapses to a black hole

15. Doubly Dead Stars • A binary system eventually ends as two compact objects • Usually nothing else happens • If very close (neutron stars or black holes) more happens • Stars emit gravity waves – they move closer • Merge to make black hole • Some gamma ray bursters may occur this way • Three ways to make a black hole • Very massive star death (> 30 MSun) • Accretion onto neutron star • Merger of two neutron stars

16. Cosmic Recycling • It is believed that the hydrogen and helium in stars was created at the beginning of time, the “big bang” • What about the other elements? • Red Giants and Double Shell-burning stars lose gas from their outer layers • Add carbon, oxygen and nitrogen to the universe • Supernovae contribute all other elements • Both Massive Star Supernovae and White Dwarf Supernovae • Later generations of stellar systems contain all elements • Like our stellar system!

17. End of Material for Test 3 L = 4d2B Test 3 Review Online 4 H + 2e - He + 2 neutrino + energy Questions?