In this second type of binary, the presence of two stars is indicated by periodic eclipses. Eclipsing binaries are one type of
variable star. Algol is a prime example of a classical eclipsing binary. Algol was the first star recognized to be an eclipsing binary
by John Goodricke back in 1782, and is a very easy star to find for the rest of us. Sky & telescope magazine provides a list of the
times of minimum light for Algol.
Eclipsing binaries are fairly easy to understand. The two stars in these systems need to be rather close to each other in order
to block each other’s light. We plot this changing brightness on what is called a light curve. The orbital periods would be on the order of
days, if not hours. Because of the periodic eclipses, these binaries appear as variable stars.
Some eclipsing binaries have dramatic changes in brightness, whereas some don’t change by more than a hundredth of a magnitude. So
much depends on the orbit. Try this simulation. The most famous eclipsing binary is Algol, discovered by John Goodricke two hundred years
ago. Here is a video about The Demon Star.
You will be doing the lab on eclipsing binary stars, using the simulation found in the link above.
Eclipsing Binary Stars
In a spectroscopic binary, the two
stars appear as one point of light,
but the existance of two stars is
revealed by their shifting spectral
lines.
Spectroscopic binaries will be in
very close orbits, more often than
not. A spectroscopic binary could
also be an eclipsing binary if the
two stars just happen to pass in
front of each other. Otherwise, the Doppler
Effect reveals their orbital motion. You, of
course, know about the Doppler Effect.
Also, this type of binary will show the
spectra of two different stellar spectral
types from a single point of light.