Original: By obtaining periodic Doppler shifts in the components of Algol, Vogel proved that the latter was a binary star; thus, Algol was one of the first known spectroscopic binaries (and it is also known to be an eclipsing binary).
Simplified: Volgel proved that Algol was a double star that could not be seen (spectroscopic binary star) and it was one of the first of these double stars proven to exist.