1. Binary star systems
redshift

2. Redshift of binary star systems
60% of stars are in binary systems
Two stars ‘orbit’ a common centre of mass

6. spectroscopic binaries

7. Half an orbit later

8. Question
A binary star system is studied where it is concluded that both stars are of the same mass.  Their orbital period is 2.4 years.  A certain element is known to give a spectral line of 460 nm.  And this is observed at time zero.  0.6 years later, the same line is observed to be at nm.  At the same time another spectral line is seen.
(a) Where is the second spectral line seen?
(b) Explain the observation.
(c) What is the orbital speed of the star?
(d) What is the radius of the orbit?
(1 year = days)

9. Stars of different masses
Sketch the pattern you would observe in the absorption spectra

10. Question
Two stars are orbiting each other as a spectroscopic binary.  One star has more mass than the other.  A spectral line of an element merges once every 1.5 years.  It has a laboratory wavelength of 486 nm.  The line then splits so that there is a difference of nm and nm.  Calculate:
(a) the orbital speed of each star;
(b) the radius of each orbit.