We may imagine stars to be unchanging beacons of stability, but of course they’re not. Their habitual reassuring return notwithstanding, the seasonal constellations harbor stars that are not the same each time we look their way. These are the variables, and October offers the year’s best.
Variable stars, whose brightness changes in more or less regular periods, are not just fascinating; they are accessible--and not only to researchers or amateurs with expensive equipment. Many of the finest variables perform their quick-change acts before the naked eye and require nothing but a simple star chart or the most basic knowledge of the sky.
Stars alter in brightness for many reasons: they may be old and unstable or (more like humans) young and unstable. They may simply be binary systems whose components eclipse each other. The most famous of the eclipsing type is Algol, high overhead these nights and the subject of continued study. This second brightest star in the constellation Perseus normally boasts about the same brilliance as the North Star. Yet every 2 days, 20 hours, and 49 minutes it loses most of its light, a bewildering trait obvious to early observers. Its very name is Arabic for the ghoul. Algol’s short period indicates that its components whirl in a frenetic dance, the stars being within a few solar diameters of each other. Its eclipses can be monitored with nothing more sophisticated than a Timex.
Algol is an ongoing pleasure, but perhaps even more striking is Chi Cygni, sitting prettily along the neck of Cygnus the Swan, just west of the Little Dipper. Swelling up every 408 days like a monstrous red beach ball, it ranges from being a target for a small telescope to easy naked-eye visibility. No other star duplicates its ability to increase its luminosity 10,000 times over, again and again.
Or try Delta Cephei, whose snappy 5.4-day period makes it an effortless target. Using a triangle of stars near the Little Dipper for comparison, you can observe its variability as John Goodricke did for the first time, in the eighteenth century. Early in the twentieth century the astronomer Henrietta Leavitt found that the brightness of this type of star was related to its period and could therefore be used as a cosmic measuring rod. Now, monitoring the pulsations of similar variables in other galaxies, we can readily calculate the stars’ luminosity and thus their distance from us.
Mira, in the sprawling constellation Cetus, also delights observers: this famous supergiant contracts and expands like a runaway balloon to become one of the largest stars known. The very shape of Cetus changes as Mira comes and goes. (This month it’s gone.)
The monitoring of such variables is one of the last areas in which amateurs can still make important scientific contributions. The American Association of Variable Star Observers (AAVSO) coordinates this effort from Cambridge, Massachusetts; its thousand members tirelessly track these mercurial suns. When the shuttle Endeavour flew its Astro-2 mission this past March, thrice-daily updates from AAVSO and its amateur sky watchers let astronauts know which variables were active and worth observing.
If you would like to perform useful science, call AAVSO for a beginner’s kit (617-354-0484). If you’d rather just casually catch the dependable beat of Delta Cephei or Algol or the here-today, gone-tomorrow antics of Mira and Chi Cygni, simply glance upward when out for an October stroll. Implausible as it may seem, it’s a thrill to notice suddenly that a star is not there.