1. Introduction to Binary Stars Steve B. Howell NOAO

2. Binary Stars ??

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4. Binary Stars - Binary stars are pairs of stars orbiting each other connected by their mutual gravitational interaction. 50% of stars are binaries: ~100% O/B ~75% M Obs. biases

5. Binary Stars - Binary Stars may have orbital periods from seconds to hours to thousands of years They may contain stars of all kinds, sizes, and masses They may have circular or elliptical orbits They are believed to have formed together - same age (not capture) Close binaries affect each others evolution Long period binaries essentially act as single stars

6. Binary Stars - Types Binary stars come in a few types differentiated by their orbital period and/or observational behavior. For example: –Visual double (actually see both components - LONG period). Do not confuse with optical double –Astrometric binaries - see the wobble on the sky

7. Binary Stars - Types An astrometric and visual binary: Sirius A&B -- an A star (A) and a white dwarf (B) Has a ~50 year orbit. 8.6 light years from Earth Chandra Image

8. Binary Stars - Types or –Eclipsing binaries - one/two eclipses per orbit –Spectrum binaries - spectral properties change –Spectroscopic binaries - radial velocity motions (short periods easier to find)

9. Binary Stars - Types Schematic and real eclipsing binary light curves

10. Binary Stars - Types Spectral variations over time due to the binary nature of this star

11. Binary Stars - Types Or differentiated by other criteria- –Close binaries w/ stellar distortions or mass transfer –Intrinsic brightness changes due to stellar variability –Binaries with components of very different properties: color (WD+RD), mass (Exo- Planet), temperature –Other

12. Binary Stars - Types Binaries can be close (short orbital period) and show light variations due to … Ellipsoidal shape and/or tidal interaction Interactions such as mass exchange / transfer If the smaller (but more massive) star in an IB is a white dwarf, the binary is called a cataclysmic variable

13. Binary Stars - Types All combinations of the previous properties are possible Many are unknown as they simply have not been seen to vary (line of sight / observed) Binary Stars are the fundamental way in which we understand stars (and star systems): their formation, evolution & death. Remember --Three out of every two stars are in a binary system

14. Binary Stars - What can we learn? Eclipsing binaries are the best as the light curve can give stellar masses, radii, temperatures, & age estimates

15. Binary Stars - What can we learn? Eclipsing (or not) Radial velocity curve can give masses (or mass estimate) RV amplitudes give K1 & K2

16. Binary Stars - What can we learn? For a circular orbit Measure semi-major axis, a, from projected orbit & the distance. Relative positions about the center give: M1/M2 = a2/a1 = K2/K1 Keplers Law

17. Mass vs. radius and luminosity

18. The Oddest Eclipsing Binary? 1987 Epsilon Aurigae model from web site -->

19. The Oddest Eclipsing Binary Life?? Elemarkhorsaurus from the Epsilon Aurigae Binary Star System Caption: Ashraf, do you want to meet my new pet? Look, he is behind you and friendly. He loves the climate and his new home in the Eps Aur binary system. (From flickr)