Omicron Ceti - Mira

Mira is the brightest and most famous long-period pulsating variable in the sky, and gave its name to this whole class of stars. It normally changes in brightness from minima around magnitude 10.1 up to maxima about magnitude 3.5. But occasionally brighter maxima, up to magnitude 2.0, are observed, making Mira among the brightest stars in the constellation Cetus, the Whale.

Mira's name means "wonderful" or "amazing"; the word comes from the same Latin root as "miracle."

Discovery and Historical Observation

David Fabricius, a Dutch amateur astronomer, discovered Mira on August 13, 1596 when he observed a star of the 3rd magnitude in the neck of Cetus unlisted in any of the current, or ancient, star catalogues. When it faded, Fabricius assumed that it had been a nova like Tycho' Star of 1572. However, he re-observed this supposed "new star" on February 15, 1609.

Meanwhile, the German astrocartographer Johann Bayer had plotted the object on the map of Cetus in his 1603 Uranometria, showing it to be of 4th magnitude and labelling it Omicron Ceti. Its periodic variability was discovered in 1639 by Jan Fokkens Holwarda of Holland, who derived a period of about 11 months.

After the periodic nature of its variations had been realized, the name Mira was given to the star by Johann Hevelius in his 1662 Historiola Mirae Stellae. Ismail Bouillaud of Paris then estimated its period at 333 days, less than one day off the modern value of 332 days (and no wonder, as Mira's period is subject to small irregular variations, and may even be slowly changing over time).

Before its discovery, other sightings may have been recorded, first by Hipparchus in 134 B.C., then in 1070 by Chinese observers, and possibly in 1592 - 1594 by Korean observers.

Variability

Mira is the best-known example of the category of variable stars which are named after it. The 6000 or so known stars of this kind are all pulsating red giants which oscillate in brightness over periods ranging from about 80 days to more than 1000.

In the particular case of Mira, the average period is 332 days. Its maximum brightness is usually around magnitude 3.5 and its minimum about 9.5, but individual cycles vary. Well-attested maxima go as high as magnitude 2.0 and as low as 4.9 - a factor of almost 15 times in brightness. Minima range much less, and have historically been between magnitudes 8.6 and 10.1. Mira is the brightest periodic variable in the sky that is not visible to the naked eye for part of its cycle.

Typically, Mira is 6 magnitudes or 250 times brighter at maximum than at minimum. The rise from minimum to maximum takes only 110 days; the fade back to minimum is much more leisurely, occupying two-thirds of the cycle.

Mira lies at a distance of about 420 light years. At this distance, Mira's absolute magnitude range is from about -2.5 at maximum to near +4.7 at minimum. Therefore Mira is only about as luminous as our sun near its minima, but brightens up to about 700, and occasionally over 1500, solar luminosities near its maxima. Interestingly, since Mira emits the vast majority of its radiation in the infrared, its variability in that band is only about two magnitudes (a factor of 6).

Mira, with a temperature just above 2000 K, is one of the coolest stars in the sky. Its spectrum varies as well, being M5e near maximum and a very cool M9 at minimum. Mira-type long-period variables are cool red giants that have strong absorption lines of oxides - particularly titanium oxide - in their spectra. They also typically have emission lines, mainly of hydrogen, in their spectra as they brighten. (In Mira the emission lines appear when the star approaches 7th magnitude.) These emission features disappear soon after maximum light is reached.

Astronomers using the Hubble Space Telescope have determined Mira's angular diameter at about 60 milli-arcseconds at maximum, corresponding to 700 times the linear diameter of our sun. Mira's overall shape has been observed to change, exhibiting pronounced departures from spherical. These appear to be caused by bright spots on the surface that evolve over timescales of 3-14 months.

Evolution

Mira is approaching the last stages of its life. Long ago, the hydrogen fusion that powered its core was fused into helium. Once the core hydrogen supply was exhausted, hydrogen fusion continued along a shell surrounding the inert helium core. This shell generated more energy than did the core, so the star's overall luminosity increased. At the same time, Mira's outer atmosphere expanded to many times its original size, producing a red giant.

As additional helium was generated by the hydrogen-fusing shell, the dormant helium core steadily increased in mass. Once the core reached a temperature and pressure sufficient to begin fusing helium, Mira underwent a runaway process called the helium flash. This initiated the fusion of helium into carbon and oxygen at its core. Gradually the core expanded and cooled. This caused the star's overall luminosity of the star to decrease, while its outer atmosphere shrank and increased in temperature. This stage of a star's evolution is called the horizontal branch.

Now the supply of helium at the core has also run out, forming a new helium-fusing shell around an inactive core of carbon and oxygen. Once more Mira began to expand, increasing in luminosity while its surface temperature decreased. This stage is known as the Asymptotic Giant Branch, and Mira is currently at this phase of its evolution.

Once the temperature above Mira's helium-fusing shell rose to about 10 million K, hydrogen ignited along a shell outside it. However, because energy is less readily transported through the hydrogen-burning shell, pressure builds up between the layers. This causes the hydrogen-burning layer to rise until its fusion is shut off by lower temperatures. Mira then contracts, and the hydrogen layer then re-ignites. The result is an instability known as the thermally pulsing AGB phase. Each pulse lasts a decade or more, and on the order of 10,000 years passes between each pulse. With every pulse cycle Mira increases in luminosity and the pulses grow stronger. This is also causing dynamic instability, resulting in dramatic changes in its luminosity and size over shorter, irregular time periods.

Mira's great size and instability generate a stellar wind that will soon blow away its outer envelope to interstellar space. The remaining core will collapse into a burnt-out white dwarf, a tiny star the size of the Earth. These long period variables help enrich the interstellar medium with chemical elements formed in their nuclear cauldrons; most of the carbon in the universe seems to have come from them. Many billions of years from now, the same things will happen to our Sun.

Companion and Tail

Mira is also the dominant component of a double star, which is separated by only 0.6". As the companion orbits Mira in about 400 years, it has now just once orbited the star since Fabricius discovered its variability. Mira's membership in a binary system makes it interesting because it allows a variety of physical investigations, such as mass determination. Mira's companion also varies in brightness, between magnitudes 9.5 and 12; its variable star designation is VZ Ceti. Its variation is rather complicated: rapid fluctuations, and occasionally a flare of some minutes' duration, are superimposed over a slow variation of about 13 years.

The companion star was resolved by the Hubble Space Telescope in 1995, when it was 70 AU from the primary; results were announced in 1997. The HST ultraviolet images, and later X-ray images by the Chandra satellite, showed a plume of gas rising off Mira in the direction of Mira B. In 2007, observations showed a protoplanetary disc around the companion, Mira B. This disc is being accreted from Mira A's stellar wind, and may eventually go on to form new planets. These observations also revealed that Mira B is most likely a main sequence star of around 0.7 solar masses and spectral class K, instead of a white dwarf, as previously believed.

In 2007, Mira was photographed in the ultraviolet light by NASA's Galaxy Evolution Explorer (GALEX) spaceborne observatory. These photos revealed that Mira possesses a comet-like tail about 13 light-years in length. This tail is probably composed of material ejected from the outer envelope of Mira over the past 30,000 years. The formation of the tail may have to do with Mira's comparatively high velocity (130 km/sec) with respect to the stars in the surrounding part of the Milky Way. As Mira plows through the galaxy, its stellar wind interacts with the gas in interstellar space, creating a a hot bow-wave of compressed plasma; the tail consists of material stripped from the head of the bow-wave.