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What Are the Mira Variables?

The red giant stars in the M10 globular cluster are seen fairly obvious here as the reddish-orange orbs of light. Image credit: Till Credner, Sven Kohle (Bonn University), Hoher List Observatory.
Mira-type variable stars are cool red giant stars that are named for the prototype of the class, omicron Ceti, or more commonly known as Mira. (See the December 1998 VSOTM for Mira.) Such variables are found in the high luminosity portion of the asymptotic giant branch (AGB) in the H-R diagram, along with the semiregular variable stars. (For a review of the semiregular variable stars, see the March 2000 and December 2000 VSOTM for Z Ursae Majoris and Betelgeuse, respectively.) In comparison with solar parameters, stars belonging to the Mira membership of variable stars generally have larger radii, higher luminosities, lower temperatures, and lower surface gravities than our Sun. As a result of the low surface gravity, the outer atmosphere of the star is loosely bound and forms an envelope around the star. Mira stars tend to be of spectral type M, S, and C, and with lengthy periods of 80 to 1000 days, these stars (along with their semiregular relatives) are referred to as long period variables. Mira stars are seen to vary in the visual regime by 2.5 magnitudes or more, and by more than 1 magnitude in the infrared (Mattei 1997). The change in observed brightness of Mira stars is believed to be result of pulsation. The maximum light is associated with the emergence of a shock wave into the atmosphere of the star and the shock wave is viewed as a result of a regular pulsation which is taking place below the visible surface (Willson 1987).

According to Olofsson (1999), the red giant stars are divided into two components: "a small, very hot (~108 cm, ~108 K) core that is strongly gravitationally bound, and a huge, much cooler mantle (~1013 cm, ~103 K) at the surface where the external parts are only weakly gravitationally bound." The Mira stars are typically on the order of about 1 solar mass, but undergo substantial mass loss of about 10-7 to 10-6 solar masses per year, whereby the large circumstellar shells are formed (Lattanzi et. al 1997). This mass loss limits the Mira stage to about 1 million years. Those stars experiencing He-burning in the core populate the AGB branch. The AGB stage of stellar evolution is the final phase for the majority of stars in the Universe following the main sequence. It is through this mass loss, however, that they avoid ending in supernova events. When the envelope of the star is nearly gone, an enhanced time of loss with a rapid velocity produces a planetary nebula, and eventually leaves behind a white dwarf star around 0.6-0.7 solar masses (Willson 1986). During this stage of life (AGB) the mass given off by these stars is going into enriching the interstellar medium. Hence, contributing to the sea of stellar life. The red giant stage is ultimately where the Sun will evolve to.

For more information about Mira stars, see the Proceedings of the AAVSO Session on Mira Stars Commemorating the 400th Anniversary of the Discovery of Mira in a special edition of the Journal of the AAVSO. The papers in this publication are available online.

Breaking the Spherical Mold

The above image of Mira in visible light taken by the Hubble Space Telescope (AURA/STScI) for AAVSO Council Member Margarita Karovska (Harvard-Smithsonian Center for Astrophyics) reveals its oblong shape.

In a 1996 press release (no. STScI-PR96-26), NASA revealed that Hubble's Fine Guidance Sensors (FGS) had retrieved data showing that perhaps not all Mira stars are spherical, but may be somewhat egg-shaped. The report issued stated that "FGS measurements showed that R Leonis' apparent diameter (in visible light) is 70x78 milliarcseconds (eight by nine hundred million miles at the star's distance of about 390 light-years) along the star's long and short axis, respectively, and 76 by 91 milliarcseconds (with linear dimensions similar to those of R Leonis) for W Hydrae." In addition, an impressive account of size of these objects was also given: "If placed within our solar system, both of these stars would extend well beyond the orbit of the Earth and almost to Jupiter."

The observed asymmetry is believed to take place in the extended atmosphere of the star, but the exact cause is currently not understood. It is speculated that the odd shape may be a result of nonradial pulsations (the star not pulsating equally in all directions) or could possibly be an optical illusion as a result of large dark spots on the star, perhaps caused by giant convection cells (Lattanzi et al. 1997). The first indications of this oblong shape were detected in Mira, but through interferometric means have been observed in the stars R Cas, R Leo, and W Hya as well (Karovska 1997). Further investigation is needed to determine the cause of the nonspherical shape and whether more stars can be added to this list.

Save the Miras!

In a world of faster is better, many observers feel that the shorter period, but more unpredictable dwarf nova-type stars are more rewarding to observe since their entire range of variation can typcially be seen within a few days, weeks, or months. Another common misconception is that the long period variable stars are very regular, have been overobserved, and don't need any further monitoring. Nothing could be further from the truth. Longterm data have shown that many of these stars have undergone a dramatic change in visible light output over time. Long period variables are the class of variable star that benefit most from amateur astronomer participation. Automated telescopes simply do not monitor these stars for a long enough span of time. Therefore, we need the help of observers like you to help maintain and extend our database into the future.

To help observers plan their observing time more efficiently, the AAVSO publishes the AAVSO Bulletin 64: Predicted Maxima and Minima for Long Period Variables on a yearly basis. Here you will find anticipated dates of maximum and minimum activity for 562 long period variable stars. This is a valuable tool for those observers looking for bright stars to view with their moderate equipment, or for those with more advanced means looking to go after faint minima.

Borrowing the words from the 1986 Journal of the AAVSO article by AAVSO President and world-renowned Mira expert Lee Anne Willson: "I will end with a plea to the observers: While observations of other classes of variable stars are important and can seem more exciting than the monitoring of slowly varying Miras, continuing to monitor the long period variables is vital for the development of our understanding of stellar evolution. These stars are also capable of surprising behavior, even after 75 or 100 or 200 years of regularity. Please contiue to observe the Miras!" If you haven't yet started with this plight, then you may consider following in the footsteps of Leslie Peltier by making R Leo your first (Mira) variable too!

For More Information

A wealth of Mira-related information may be found in the "Proceedings of the AAVSO Session on Mira Stars Commemorating the 400th Anniversary of the Discovery of Mira" published in the Journal of the AAVSO.