1. Binary Stars
Palomar Observatory

2. Star Systems Star systems can involve both single and multiple stars.
Double stars common
Star systems within 10 Pc have been cataloged by RECONS (Jan 2012).
Total systems 259
Singles 185
Doubles 55
Triples 15
Quadruples 3
Quintuples 1

3. Visual Binaries
Visual binaries occur when the centers are separated by more than 1”.
Atmospheric effects
Apparent binaries occur when two stars are near the same coordinates but not close in space.

4. Orbit Kepler’s third law is used as compared to the sun.
Mass M in solar masses
Period P in years
Semimajor axis a in AU
The semimajor axis depends on knowing the distance and tilt.

5. Mass Gravity attracts each star towards the center of mass.
Larger mass closer to the center
Separate masses come from observing the center of mass.
Magnified to show gravitational pull

6. Spectroscopic Binaries
As binary stars orbit each other their radial motion changes.
Doppler shift
Spectral lines shift red then blue
The period of time to make the shift and back is one orbital revolution.

7. Eclipsing Binaries
Some stars are too close to separate with telescopes or spectra.
Fluctuating spectrum
Fluctuating light
Periodic eclipses diminish the total light.

8. Stellar Sizes
The length of the eclipse is related to the size of the star.
A few close, big stars can be seen in a telescope as measurable disks.
Speckle interferometry
Betelgeuse with infrared interferometry
X. Haubois, Observatorie de Paris