more than half of all stars have companions

in small percentage of these, two stars are sufficiently close to affect each other

Lecture 14: Binary Stars

Observational Classifications

visual binaries: components can be individually detected

Sirius A & B

white dwarf

AI

Astrometric Binaries

Spectroscopic Binariessingle-lined (only one star’s spectral lines observed) and double-lined (both stars observed)

Eclipsing Binaries hotter star

Visual Doubles Alcor

Mizar

Alcor

Mizar A & B : visual binary

Mizar A: spectroscopic binary

Mizar B : spectroscopic binary

Deriving Stellar Masses

for visual binaries if orbit shape and distance known

double-lined spectroscopic and eclipsing binary with known light and velocity curve

visual binary with known orbital shape and radial velocities for both components (less common)

Roche Lobes & Lagrangian Points

equipotential surfaceLagrangian pt

co-rotating frame

Classification of Close Binaries

1) detached binaries

2) semi-detached binaries

Mass Transfer

one example

3) contact binaries

extreme example is common envelope evolution

what do you expect light curve to look like?

Evolution of Semi-Detached Binaries1) if detached component is normal star, slow mass transfer: evolved, less massive star dumps on unevolved, smaller, more massive star

Algol-like systems

0.81 solar mass star (G or K type) dumping on 3.7 solar mass B star

deep eclipse when?

2) if detached component is white dwarf --> cataclysmic variables

nova: fusion of accreted H on WDdwarf nova: accretion disk instability

3) if detached component is NS or BH --> X-ray binaries

low mass (LMXBs) : evolved low mass star overflowing Roche lobe

high mass (HMXBs) : young, massive star dumping through stellar winds

all sorts of exotic bursting scenarios

Some interesting binaries...

Cyg X-1 (first X-ray source discovered in Cygnus)

B supergiant, 8K light years, 5.6 day orbit

B supergiant --> ~30 solar masses

X-ray source compact (tens of millisecond variability)

orbit implies >6 solar masses

too much for WD or NS