1. Stellar Mass

2. Alcor
Mizar
(Binary star)
Ursa Major

3. Gravitational Orbits Animation

4. Binary star (system): two stars orbiting a common
center of mass (CM). 1 r1 r2 CM 2
r2 = 3 AU, r1 = 1 AU 

5. Kepler’s laws apply here . . .
Yuk!
P: years
a: AU
M: Solar Masses

6. a = r1 + r2 = 4 AU
Suppose: P = 8 yrs . . .
1 M

7. “Seeing” Binary Stars
Visual Binary: Two stars seen in telescope
e.g., Mizar
Albireo

8. Sirius A & B A B

9. 0.0034 L 0.16 M  Distance = 13 ly P = 44.6 yrs a = 9.5 AU A B
Kruger 60
L M A B
0.01 L L
0.28 M 
0.16 M 

10. 2. Spectroscopic Binary: Motion of stars
detected via Doppler shift of spectrum lines.
Mizar
Each star is itself binary!

12. Earth
* Doppler shift
* No Doppler shift

13. Spectroscopic Binary Animation

14. Eclipsing Binary:
Stars mutually
eclipse
each other.
* No eclipse
* Eclipse
Earth

15. Binary ‘light curve’
Brightness
Time
½ orbit period

16. Eclipsing Binary Animation

17. Binary Statistics
* At least 50% of all stars reside in binaries.
* The sun is apparently a loner!