By now, even the mass media will have reported that the other night – Wednesday night at 2:06 a.m. – the bright star Regulus in Leo was eclipsed by an asteroid. This occultation was potentially visible along a narrow path 100 miles wide that happened to pass over our region. It’s very rare for a bright star to be occulted, and extraordinarily unusual for it to be seen from here.
This vanishing of the sky’s 22nd-brightest star lasted about ten seconds as seen from our area. It had the potential to let meticulous observers taking careful timings determine the exact size of the roughly 45-mile-wide asteroid, whose name is Erigone. This goes to press too late to tell you if the weather was clear (but the forecast was pretty dismal).
This is an unusually rich year for celestial objects to get blocked or eclipsed. In just a few weeks, the April Full Moon will venture fully into Earth’s shadow and turn a coppery red (more about that soon). And this fall our region will be treated to a second total lunar eclipse: an unusual twofer.
In truth, the sky is full of stars that wink on and off or periodically lose much of their light. The topic hasn’t appeared here for the past quarter-century simply because very few readers can find these stars. Most of these “variables” have unseen companions that orbit closely around them – too nearby for any telescope to detect visually. Yet the companion star in its orbit blocks some or all of the primary star’s light with a wonderful regularity. By chance, the most famous eclipsing binaries are out these nights.
The most renowned is Algol in the constellation Perseus. The ancient desert-dwelling Arabs clearly knew that something was amiss, because Algol’s very name means “the Ghoul” or demon. Every two days, 20 hours and 49 minutes, Algol loses half its light. The outline of Perseus then changes dramatically. It’s a wonderful if oddly ineffable thrill to glance up during an evening stroll and suddenly notice that Algol is very faint.
Mira, in the southwest these nights in the constellation Cetus the Whale, goes even further: Normally there’s no trace of it at all. But every 11 months, it brightens so much that for a month or two it is strikingly visible.
In Taurus the Bull, the star Lambda, to which the “V” of Taurus points, loses half its light every four days. The famous star Delta in Cepheus, in the north, also loses about half its light, but in this case no eclipse or companion is involved in its five-day periodicity. The star itself grows larger and smaller. Similarly, the famous Betelgeuse in Orion also changes its brightness, but here the period is wildly irregular: generally around three years. On rare occasions Betelgeuse will outshine Rigel to become Orion’s brightest star.
Bottom line: Despite their reputation for permanency and reliability, many of the night’s visible stars change brightness. In many cases the alterations are visually striking with no equipment whatsoever. Some of the folks who love keeping track of them – a sadly diminishing number in our increasingly computer-focused and decreasingly hands-on society – belong to the AAVSO organization (the VS portion stands for “variable star”).
I won’t mention this topic again; I know that it’s an esoteric pursuit. But it was merited now in this special year 2014, when stuff in the sky is getting eclipsed much more than usual.
Stand by for the big Tax Day lunar eclipse.