Eclipsing binaries produce characteristic light curves with two dips per orbit

How the Doppler effect reveals binary systems

• The Doppler effect can be used to identify eclipsing binaryA binary star system where the orbital plane is aligned with the observer's line of sight, causing the stars to pass in front of each other periodically. star systems.
• When observed from Earth, the two stars cross in front of each other as they orbit, periodically blocking each other's light.
• They can be identified from their characteristic light curves, which show periodic dips in observed intensity.

The light curve of an eclipsing binary

• The key part is that the light curve shows two dips per orbit, and these dips are not the same depth:
• The primary minimum (the larger, deeper dip) is caused by the larger star passing in front of the smaller, hotter star. This blocks more light because the hotter star has a greater surface brightness.
• The secondary minimum (the smaller, shallower dip) is caused by the smaller star passing in front of the larger star. It only partially blocks the larger star's light.
• Between the two minima, both stars are visible and the total brightness is at its maximum.

Doppler shifts in binary spectra

• As the two stars orbit their common centre of mass, one star moves towards us while the other moves away.
• This means the spectral lines from one star will be blueshifted while those from the other are redshifted.
• By measuring the magnitude of the Doppler shifts, astronomers can calculate the orbital speeds using $\dfrac{\Delta \lambda}{\lambda} = \dfrac{v}{c}$.
• From the orbital speed and period, the separation of the stars and their masses can be determined.