by Rebecca Rothhaas, 2013-08-08
Omicron Ceti, also known as Mira after the Latin word for “wonderful”, is a binary star system located in the constellation Cetus.  Mira is also a variable star, one of the first discovered.  It’s so well-known that it even has a class of variable stars named after it, called Mira variables. [2, 3] Mira variables, including Mira itself, are red giant stars nearing the end of their lifespan. As the stars age, they run out of helium to burn and become unstable, contracting into red giants. [3, 4] This instability causes parts of Mira to swell and shrink, creating the variations in brightness that makes Mira a variable star.  Because different parts of Mira swell and shrink in different places and at different sizes, there is some variation in how bright and how dim Mira gets in each cycle. 
Mira is a long-period variable star, with a period of approximately 332 days. [1, 2] This means that it takes around eleven months for Mira to complete one cycle of reaching a maximum and minimum brightness. [3, 6] In terms of observation, this means that there are a few weeks every year where Mira is brightest and most clearly visible. 
This graph shows data gathered for Mira’s brightness that was recorded in the AAVSO database from mid-May 2010 to late March 2013.  It shows three cycles, spanning approximately three years. Because the period is around eleven months, each year Mira reaches its maximum brightness approximately one month earlier than the previous year: Mira was brightest in late October 2010, in late September 2011, and in late August 2012. 
Mira has a fairly large variation in magnitude. [1, 2] For most of the year, Mira is so dim that it cannot be seen with the naked eye.  At its brightest, however, Mira is easily visible and is one of the brighter stars in the constellation Cetus. 
1. “Mira (Omicron Ceti)”, The Encyclopedia of Science, [ http://daviddarling.info/encyclopedia/M/Mira.html ], accessed 2013-08-05.
2. “Mira, Omicron Ceti”, Students for the Exploration and Development of Space, [ http://spider.seds.org/spider/Vars/mira.html ], accessed 2013-08-05.
3. “Omicron Ceti (Mira)”, American Association of Variable Star Astronomers, prepared by Aaron Price, [ http://aavso.org/vsots_mira ], accessed 2013-08-05.
4. “The Life Cycles of Stars: How Supernovae Are Formed”, NASA’s Imagine the Universe!, [ http://imagine.gsfc.nasa.gov/docs/teachers/lessons/xray_spectra/background-lifecycles.html ], accessed 2013-08-05.
5. “Variable Stars – II. Pulsating stars”, Dave Kilkenny, National Astrophysics and Space Science Programme, [ http://www.star.ac.za/course-resources/local/david-buckley/var2.pdf ], accessed 2013-08-07.
6. “Time to look for Mira the Wonderful, a famous variable star”, EarthSky, [ http://earthsky.org/astronomy-essentials/mira-the-wonderful ], accessed 2013-08-05.
7. “Light Curve Generator (LCG)”, AAVSO, [ http://aavso.org/lcg/plot?auid=000-BBD-706&starname=MIRA&lastdays=200&start=05/15/2010&stop=04/01/2013&obscode=&obscode_symbol=2&obstotals=yes&calendar=calendar&forcetics=&grid=on&visual=on&pointsize=1&width=805&height=450&mag1=9.5&mag2=1.5&mean=&vmean= ], accessed 2013-08-06.