Week 4, clip 12

Timepoint:  07:42.6

1) What are some of the physical quantities of stars that we can learn from observing just the spectra (plural of spectrum) of binary stars?  Check all that apply.

- Distance (from Earth)
- Luminosity
- Mass
- Orbital radius
- Orbital period
- Stellar radius
- Temperature
- Radial velocity

Answers:
1) Orbital period and radius, radial velocity, mass and temperature can all be found from spectra.

Explanation:
Luminosity L requires the distance from Earth D, and vice versa, from L = 4 * pi * D^2 * b.  Stellar radius R requires the luminosity, L = 4 * pi * R^2 * sigma * T^4. 

The correct options (period, velocity, orbital radius, mass, and temperature) can all be found in an ideal system, as Dr. Plesser described, with circular orbits that are not tilted with respect to the line of sight to Earth.