The sky's changing phenomena have always been a source of wonder. Nearly all the changes seen with the unaided eye result from the movements of solar-system objects. But far in the background, a number of bright variable stars can also be followed through their brightness cycles without optical aid. Compared to most of their telescopic counterparts, the naked-eye variables are easy to find. Once you know a few of them, you can check on how they are doing whenever you look up at the sky.

Thirty-four variable stars have a range of at least 0.4 magnitude and become brighter than visual magnitude 4.0, according to the authoritative General Catalogue of Variable Stars (GCVS) and its supplements the Name-Lists of Variable Stars. (This doesn't include novae or supernovae, which occasionally reach naked-eye brightness.) Among these stars are many eclipsing binaries, Cepheid variables, and semiregular red variables, as well as a few long-period stars of the Mira type and the recurrent nova T Coronae Borealis. As many as 24 do not fade below magnitude 5.1 and so remain visible to the unaided eye all the time. It's interesting that only seven of these are south of the celestial equator, compared to 17 north of it. Could there be several undiscovered naked-eye variables in the southern sky waiting to be noticed?

Here is a personal list of my dozen favorite northern naked-eye variables, in no particular order. Many types of variable stars go unrepresented in this sample, since many are low-luminosity objects too faint to be visible without a telescope or vary too slightly for their changes to be visually noticeable. Small binoculars will help in observing the fainter phases of some of these stars, especially if you don't have really dark skies.

All the light curves in this article resulted from hundreds of naked-eye observations by the author. Each plotted point is the average of between seven and 11 brightness estimates made on different dates. For clarity, data are repeated to cover more than one cycle.

Algol and Lambda Tauri

Algol (Beta Persei), the prototype eclipsing binary, dips from magnitude 2.1 to 3.4 every 2.87 days. Each eclipse, including the partial phases, takes nearly 10 hours. The dimmings are most obvious for the two hours that the star stays close to minimum light. Algol is a star of fall and winter evenings. A comparison-star chart for estimating Algol's magnitude appeared in the November 1996 issue of Sky & Telescope, page 66. (For tips on estimating a variable's brightness, see "The Lure of Variable Stars.")

Nearby Gamma Andromedae, magnitude 2.1, makes a handy comparison for first-glance checks. To monitor Algol more closely, make a visual brightness estimate every half hour for as many hours as possible spanning a predicted eclipse. You can derive the time of mideclipse from a graph of your magnitudes. Such timings provide a useful check on the accuracy of the predictions.

Alternatively, you can estimate the star's magnitude once or twice a night and start making more frequent observations if it is noticeably fainter than normal. In this way, I obtained the star's complete light curve. It showed that the eclipses were coming appreciably later than predicted at the time. My light curve even contains a hint of the secondary minimum halfway between the primary eclipses. This is only 0.05 magnitude deep, though, and I was completely unaware of it at the time of the observations.

Universal Times and dates of Algol's fadings are available online in the companion article, "The Minima of Algol."

Lambda Tauri in the back of the Bull is another Algol-type eclipsing binary, less well known due to its smaller magnitude range of 3.4 to 3.9. The eclipses last 14 hours, too long to cover in a single night. But enough random observations will define the light curve well. The period (3.953 days) is just an hour short of four days, so once the eclipses start coming in the evening, they will repeat every four days for about a month.

In addition to the primary minimum, my light curve shows the 0.2-magnitude secondary minimum. As in the case of Algol, there was a significant difference between the observed and predicted times of minimum light. Accurate photoelectric magnitudes for suitable comparison stars are readily available in modern publications such as Sky Catalogue 2000.0, Volume 1.

Beta Lyrae and Delta Cephei

Beta Lyrae is an eclipsing binary of a different type. Algol-type binaries appear nearly constant between eclipses, because the brighter star of the pair is approximately spherical. But Beta Lyrae's components are so close together that they are distorted into ellipsoids by each other's gravity. As the system revolves in its 12.94-day orbital period, we see continuous change at all phases of its light curve.

The magnitude range is cataloged as 3.3 to 4.4. The mean light curve from my own observations shows a smaller range, perhaps partly because of observational bias. See Sky & Telescope:, June 1993, page 72, and June 1994, page 72, for more about watching this rapidly evolving binary star.