Omicron Ceti (Mira, the “Wonderful”)
January 2010 :
Last month, we looked at the prototypical eclipsing binary beta Persei (Algol). This month, we turn to another prototype, the classic long-period variable (LPV) omicron Ceti. This star boasts a rich history, having been discovered by David Fabricius on August 13, 1596. Johann Bayer added it to his Uranometria star catalog as a 4th magnitude star. When it became apparent that this star would miraculously appear and disappear (a stellar behavior unheard of in those days), astronomers gave omicron Ceti the nick-name Mira “the Wonderful.” Mira’s periodicity was first described by Johann Holwarda, who determined a period of 11 months - a figure is close to today’s standard.
Mira is the prototype of a class of pulsating variable stars called “long-period variables (LPVs).” The typical “Mira-type” star is a red giant with a range of 5 or 6 magnitudes and a period of several months to one or two years. The brightest of LPVs, Mira typically varies from magnitude 3 to 9 in a 331 day cycle. At times Mira will rise to 2nd magnitude, and in 1779 was observed by William Herschel to rival the first magnitude star Aldebaran.
With modest means, you can follow Mira through a complete cycle. Naked eye observations will cover magnitude 5 and brighter, binoculars will work for magnitudes 5 through 8, while a small rich-field telescope can handle Mira at minimum. A small scope magnifying 50X will also uncover Mira’s 9th magnitude optical companion, situated 120 arc-seconds away.
In November, Mira reached a peak brightness of about magnitude 3.5. The star has begun to fade, but should still be visible to the naked eye throughout December and the early part of January. The accompanying chart should help you make rough estimates of Mira’s brightness. If you want to follow it into the domain of binoculars and small telescopes, log on to www.aavso.org. First, click on “Make a Chart.” In the box labeled “NAME,” type on “omi Cet.” Next to the “Plot a Chart of this Scale” prompt, scroll to “B” (the scale used for relatively bright variable stars). Click on “Plot Chart” and – voila! – you have a “B” chart for Mira. Last month, I noted that observing and recording an eclipse of Algol should be on every backyard astronomer’s “to-do” list. Add Mira the Wonderful to that list.
Your comments on this column are welcome. E-mail me at gchaple@hotmail.com.
