In fact, the star appears so ordinary at first glance that no one even recognized it as a binary system until this century. It wasn’t until 1921 that German astronomer Cuno Hoffmeister discovered that SS Lac varied regularly in brightness. The pattern of the variations showed the characteristic signature of an eclipsing binary, proving the dual nature of the star.
In an eclipsing binary, two stars orbit each other in a plane that lies nearly edge-on as seen from Earth. Once during every orbit of the pair, the brighter star, called the primary, passes in front of the fainter secondary and blocks some or all of its light from view. Then when the stars reach the opposite ends of their orbits, the secondary star eclipses the primary. For most of the stars’ orbital period, astronomers see the combined light from both stars, but the light dims slightly when the primary blocks the secondary from view and to a greater extent when the secondary eclipses the primary.
The best known eclipsing binary in the sky is Algol, the Demon Star in Perseus, whose brightness changes can be followed easily with the naked eye (see “Demon Variables,” October 1992). Every 2 days, 20 hours, and 49 minutes, this 2nd-magnitude star fades by 1.2 magnitudes, dropping to just a third of its normal brightness. The secondary eclipse, with a light loss of a meager 3 percent, is much more difficult to see.
Following Hoffmeister’s announcement on the nature of SS Lac, other astronomers began studying the binary. Using an extensive set of photographic plates taken at the Harvard College Observatory between 1890 and 1935, Raymond Dugan and Frances Wright showed that the stars revolved around each other once every 14 days and 10 hours. Unlike Algol, however, the primary and secondary eclipses were nearly equal in depth — the 10th-magnitude system faded by about 0.5 magnitude, or a little over a third in brightness, during both eclipses.
The equal light loss during both eclipses implied that the two stars had to have nearly identical surface temperatures. Otherwise, the system would appear dimmer when the hotter star (which emits more light from each unit of area) was blocked from view. Spectra of SS Lac showed that the two were main-sequence stars of spectral type B9, implying masses about three times that of the Sun and surface temperatures around 11,000 kelvins.
Dugan and Wright also discovered that the secondary eclipse did not occur precisely halfway between primary eclipses, but instead at phase of 0.57. This indicated that the orbits of the two stars had to be elliptical, because circular orbits would cause the secondary eclipse to occur halfway between (at a phase of 0.5).
The Eclipses Stop
That’s the way things stood throughout the first half of the 20th century. Whenever an observer exposed a photographic plate at the time a primary or secondary eclipse was predicted, SS Lac appeared fainter. But the eclipses seem to have stopped around the middle of the century. Astronomers have observed the binary system both photographically and photo-electrically thousands of times in the past 45 years, but they have not detected any change in brightness at any phase.
Obviously SS Lac no longer qualifies as an eclipsing binary, but could it still be a binary star? One way this could happen would be if the inclination of the system’s orbit changed just enough so that eclipses no longer take place.
Astronomer Thomas Lehmann has analyzed many of the old photographic observations and concludes that the eclipses ceased around 1950 after slowly decreasing in intensity for several years. He developed a model for the decreasing eclipse depth by assuming that the system’s orbital inclination had been changing slowly but steadily. In this scenario, the eclipses stopped once the orbits tilted far enough that the stars passed either above or below and not directly in front of each other. Lehmann found that he could fit the observations best if the orbits were exactly edge-on around the year 1911 and the inclination changed by approximately 0.18 degree per year.
This model can be tested by analyzing the spectrum of SS Lac, If the two stars, still revolve around each other, their orbital motions would cause one star to move toward Earth as the other one moves away. These so-called radial velocities would show up in the spectrum as features that shift toward the blue or red, respectively. The radial velocities would be greatest if we viewed the system edge-on and drop to zero if the system appeared face-on.
If the eclipses stopped because the binary system’s orbital inclination had shifted slowly, spectra still would reveal the changing radial velocities by showing two sets of features. However, only one is seen. Second, the radial velocity of the system should vary as the two stars continue to orbit each other, yet spectra show no significant variations in the radial velocity above the uncertainties in the measurements. Both findings suggest that SS Lac is no longer a binary system.
If not a binary, then what? The first thing to keep in mind is that both stars that belonged to original eclipsing binary system still must be there. SS Lac now shines as brightly as the eclipsing binary did when not in eclipse, so the combined light of the two stars must still be reaching us. The key difference is that those two stars no longer appear to orbit each other.
Astronomers have far to go before understanding SS Lac, but they have come up with two scenarios that may help explain its unique behavior. Perhaps this was not just a binary system, but a system of three stars in orbit around one another. In that case, interactions among the trio could have caused the orbits to become suddenly chaotic, disrupting the entire system.
Even more intriguing is the possibility that another member of NGC 7209 collided gravitationally with SS Lac. This inconspicuous interloper could have flung the two stars apart, throwing the entire system for a loop. And despite the cosmic chaos you might expect from such a collision, there wouldn’t yet be any obvious sign that this had happened.
The spectrum of SS Lac reveals only one star. That implies that the two stars must be separating perpendicularly to our line of sight or nearly so, otherwise the different radial velocities of the stars would cause two sets of spectral features. That in turn suggests that the two components should appear to separate rapidly from one another. But the cluster lies so far away, at a distance of roughly 3,000 light-years, that the motion would not yet give rise to an observable separation. After more than 40 years, however, the two stars may well be getting far enough apart to be identified by the Hubble Space Telescope’s sharp eye.
Until that day comes, the stars of SS Lacertae will reveal their fates only through more and better spectra. Maybe then astronomers will be able to puzzle out what happened to the mysterious eclipsing binary that in the middle of the 20th century suddenly and unexpectedly ceased to exist.