On the last day of 1919, revelers gathered to toast 1920 as a new, 5-foot-diameter iron ball lit with 100 25-Watt light bulbs was lowered down the flagpole of One Times Square. It was the 12th time a ball had been lowered and the last New Year's celebration before the 18th Amendment, Prohibition, took effect on Jan. 16.
Unknown to the revelers, 547 trillion miles above their heads, an aging star swollen in its death throes eclipsed its close companion, a smaller, brighter star, dimming the light from the pair that we see as a single star, Algol (AL-gall), the demon.
The stars of the winter constellations seem to remain the same throughout our lives. Orion hasn't flexed a muscle since his glittering belt was first pointed out to us by our parents or scout leaders. With the exceptions of the wandering planets like Jupiter looping through
This map shows the sky directly overhead (above SSW horizon) in Tupper Lake at 9 p.m. today. Jupiter beams brightly between the Hyades and Pleiades. Algol is near the zenith, directly overhead. It is usually as bright as Almach in Andromeda but, when eclipsed, dims to a magnitude between neighboring Rho and Kappa Persei. Star clusters h and Chi Persei, and the galaxy in Andromeda are visible to the naked eye in dark skies such as those in the Adirondacks.
(Made using Starry Night software)
Taurus this year, we look at essentially the same patterns that the ancients did as they built the pyramids, the Great Wall and Cohokia Mounds. Century after century the constellations and stars seem to be frozen in their places as they march across our seasonal skies. And yet there is change, even among the stars, that have been known, and generally feared, for generations.
Algol is one strikingly changeable star is nearly at our zenith (point directly above) during the evenings of January. As shown in the diagram, it resides in the constellation of Perseus, the hero who killed Medusa and is returning with her head. Algol is the eye of Medusa and I warn my students not to stare too long at this star lest they be turned to stone.
Given the temperatures we can plunge to on clear January nights, this is not a completely remote
possibility! Telescopic views, however, are completely safe since they use Perseus' trick of viewing the star reflected in a mirror.
Most of the time, Algol is fairly bright, having a magnitude of 2.1. Our scheme of stellar magnitudes dates from the Greek astronomer, Hipparchus, who lived in the second century B.C.
He sensibly ranked the brightest stars as magnitude 1 and dimmer stars as magnitudes 2 through 6. Sensible as it was, this system results in the counter-intuitive fact that high magnitudes are associated with dim objects. Distant galaxies seen as faint smudges in Hubble Telescope images have magnitudes in the 30s, whereas the Sun has a magnitude of -26.74. The system remains in wide use among astronomers but has been modernized to tie it to measured brightness values. This has resulted in Sirius, the brightest star in the sky, having a magnitude of -1.47. Vega of the Summer Triangle, low in the western sky just after dusk tonight, has a
magnitude of 0.0, "zeroth" magnitude. Overhead tonight, Capella in Auriga is nearly as bright as Vega with a magnitude of 0.06.
Algol typically shines at magnitude 2.1, a hair dimmer than 2.09 magnitude Almach (AL-mock) in Andromeda and a hair brighter than 2.12 magnitude Gamma assiopeiae, the center of the "cosmic W". However, every 68 hours, 48 minutes and 56.5 seconds, its magnitude drops over a period of about 4 hours to 3.4, where it remains for two hours, then brightens over another four hours back to 2.1. At its dimmest, it's bracketed by its Perseus neighbors, brighter Rho and dimmer Kappa.
According to a Finnish team, this behavior may have been noted by the Egyptian authors of the 3200-year-old Cairo Calendar. However, it took until 1881 for its cause to be discovered.
Algol is actually two stars separated by a mere 5.8 million miles, 6.2 percent of Earth's distance from
the sun, while a third star orbits at a distance 2.69 times greater than Earth's orbit (about that of the asteroid belt between Mars and Jupiter). The two close stars are a bright mid-life star much like those in the Belt of Orion, and a larger but cooler and dimmer dying star resembling Mirfak (MEER-fack), the brightest star in Perseus. The bright star is 2.9 times the diameter of the sun and 3.7 times more massive. It is actually pulling matter off the other star that has only 81percent of the sun's mass, but has swollen to 3.5 times its diameter.
The plane of their orbit is nearly aligned with our line of sight so we see them eclipse each other. There is a slight dip in brightness when the bright star passes in front of the dim star, but the dramatic drop in magnitude is due to the bright star being almost entirely eclipsed by its larger companion.
As the now Waterford crystal ball dropped in Times Square last week, the light from that 1920 eclipse, having crossed 93 light years of interstellar space, finally arrived. I doubt any of the revelers noticed. If any did look up to Algol, they would have seen it begin its four-hour emergence from eclipse two minutes after the stroke of midnight.
As listed in the table, the next eclipse of Algol in the evening hours won't take place until two weeks from tomorrow. So the next two weeks give you time to get familiar with the typical brightness of Algol. On the next clear few nights, face SSW and look directly up to find Perseus with Algol at its brightest, comparable to Almach. If you have a very dark sky, look for the galaxy in Andromeda. Follow Mirach (MEE-rock) to Mu to Nu Andromedae, then scan about the area to pick out the galaxy's fuzzy blur or use binoculars. Pretty star clusters h and Chi Persei (h and Kye PER-see-eye) between Perseus and Cassiopeia should also become visible as your eyes adapt to the dark (20 minutes to half an hour).
Set a reminder for Wednesday, Jan. 23 between 8:45 and 10:45. At that time, look up to see Algol dimmed to the brightness of its nearest neighbors Rho and Kappa Persei. Unlike the Egyptians who declared Algol eclipse days to be unlucky, we have the great fortune to know that a dim star eclipsed its bright companion in early January of 1920. For me, the fact that there are eyes and minds on this small, blue world to observe, understand and wonder at an event from our grandparents' time is the most amazing thing of all.
If you miss that eclipse, the table lists the dates and times of mid-eclipse that occur between sunset and midnight for the Adirondacks this winter. Within an hour before or after the listed time, Algol will be at its dimmest. Sky and Telescope Magazine also offers a "Minimal
of Algol" calculator on line (Google that phrase to find it) that lists minima dates and times.
Beyond this column, local astronomers are anxious to share our love of the sky with you.
Check out the Adirondack Public Observatory website at apobservatory.org for events. Listen for me on North Country Public Radio about once a month during "The Eight O'clock Hour" or email me with any questions at firstname.lastname@example.org.