binary stellar evolution

Binary Stellar EvolutionHow 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

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

•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

Star A’s Roche Lobe

Star B’s Roche Lobe

No man’s land

Roche Lobes

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

Accretion Disks

•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

Second star

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

A is heavier than B is heavier than C. When could this system first become “interesting”?A) When A becomes a giant starB) When B becomes a giant starC) When C becomes a giant star

C B A

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




How to Make a Nova

White Dwarf•White dwarf

•Hydrogen gets added to carbon/oxygen layer•Builds up•Ignites and explodes•Cycle repeats




How to Make a White Dwarf Supernova•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

How to Make a Black Hole X-Ray Binary•White dwarf:

•Nova•White dwarf supernova



•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

Black Hole

X-Rays

What happens to the gas after it crosses inside the event horizon?A) It heats up even more, contributing more X-raysB) It impacts on the surface of the black hole, producing additional energyC) It disappears forever

How to Make a Black Hole X-Ray Binary•White dwarf:




•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

X-rays

How to Make a Neutron Star X-Ray Binary•White dwarf:




How to Make an X-Ray Pulsar•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




How to Make an 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




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

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!

Questions?

End of Material for Test 3

4 H + 2e - He + 2 neutrino + energy

L = 4d2B

3.26 lyd

p

4 2L T R

L T R

Test 3 Review Online

binary stellar evolution

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