The Algol
System

The most famous eclipsing binary star is visible to the naked eye and has a rich mythology associated with it.

Winking Demons

In the constellation Perseus there is a moderately bright star called Algol. It is remarkable in that every 68.75 hours its light dims rather suddenly for several hours before returning just as quickly to its former brightness. This change in brightness is sufficiently large to be apparent even to the naked eye. Very careful observation also indicates a small dip in light output halfway in between the large dips, and detailed inspection of the spectrum indicates that Algol is a spectroscopic binary. ( Additional evidence indicates that Algol is actually a triple star system, but two of the stars are very close together. It is these two stars that concern us here.)

Algol is called the Winking Demon Star because of its light variation and because Perseus is according to mythology holding the severed head of the Gorgon or demon.

The Geometry of the Algol Binary

The following figure illustrates, to scale, the Algol binary.

The Algol System


A blue spectral class B8 star with a diameter of 3 solar diameters and red-yellow spectral class K2 star of about 3.5 solar diameters are in very close orbit around each other (See the earlier discussion of spectral classes). The orientation of the orbits is such that a large percentage of each star is eclipsed during the primary and secondary eclipses. The blue star (because it is hotter) emits more light from each square centimeter of its surface than the yellow-red star, so the primary eclipses occur when the blue B8 star is occulted by the K2 star.

From the light curve, we see that the change in light output corresponds to a change of more than one order of magnitude, easily visible to the naked eye.

In the lower right image we see that the two stars are so close together that tidal forces are distorting the shape of the K2 star, distorting it into a teardrop shape. In fact, there is evidence that some of the matter of the K2 star is being pulled onto the B8 star, as we shall discuss further later. The dotted line with the figure-8 shape in the lower right image corresponds to what are called the Roche lobes of the gravitational potential between the stars. We shall have more to say about that later.

Animation Algol Eclipses

The following animation illustrates the motion in the Algol system that leads to the eclipses (Source).

The orange line between the two stars is a schematic indication that there appears to be matter streaming between the components of the binary. We shall return to this later when we discuss accreting binaries.

Here is a virtual reality simulation of the eclipses in the Algol system.

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