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About Mira - The Extended Edition

Last updated 8.12.2007
Written by Mark Seibert


Mira
Mira's location in the sky.
Credit: Stefan Seip and APOD
Discovered by David Fabricius in 1596, Mira is one of the most extensively studied stars because it was the first periodic variable star ever found. Over the course of 332 days Mira regularly changes in brightness. For several months it is bright enough to be seen by the naked eye but then dims by a factor of 1,500 (8 magnitudes) to become invisible. Because it appeared to turn on and off it was given the name Mira (The Wonderful). Dorrit Hoffleit has written a nice history of Mira's discovery. When it is bright you can see Mira for yourself without a telescope or binoculars while the constellation Cetus (the whale) is visible. You won't be able to see the tail from the ground. The next time it reaches maximum brightness will be January 1, 2008 and should be visible for at least 1 and a half months before and two and half months after maxima. Cetus is most easily seen during November. So look for Mira to appear in mid November. Fittingly, Mira is located in the tail of the whale. Let me just apologize up front... It may sound like a whale of a tale but in the tail of the whale is a star with a whale of a tail. There I did it. Now we can all just move on.

To be accurate, we now know that Mira is a mildly-symbiotic thermally-pulsing asymptotic giant branch star with the same amount of mass as the Sun. That is a mouthful. An explanation is in order.

evolution1
Basic evolution of a solar mass star.
Credit: ATNF/CSIRO
Mira is an old and evolved star that was once just like the Sun. The Sun will be just like Mira in several billion years. Mira is in the fourth of six major stellar evolution phases it will experience. Unlike our comparatively youthful Sun which balances the force of gravity by producing energy and light from an internal nuclear furnace that fuses (or burns) hydrogen into helium within its core - what is called the main sequence (MS) phase on the Hertzsprung-Russell diagram - Mira has exhausted the supply of hydrogen in its core, the event which marks the beginning of post-main sequence evolution. Mira has also passed through the red giant branch (RGB) phase when a star burns hydrogen in a shell around a helium core causing it to dramatically increase in brightness by up to 1000 times, expand in size by hundreds of times, and decrease in surface temperature.
evolution2
Detailed AGB evolution schematic.
Credit: Frost et al. 1996
When the core temperature becomes hot enough to ignite helium in a violent event called the helium flash, the third major phase that Mira has passed begins - the horizontal branch (HB). During the HB phase a star is powered by burning core helium into carbon and oxygen, causing it to slightly shrink in size and increase in surface temperature. Mira has burned most of its helium produced while on the main sequence and red giant branch leaving a dense core of carbon and oxygen. Having only as much mass as the Sun, Mira is unable to generate high enough temperatures to create energy by processing carbon and oxygen into heavier elements. It now resorts to producing energy by burning thin layers of hydrogen and helium that surround the carbon-oxygen core. This is called the asymptotic giant branch (AGB) phase which again causes a star to swell in size, increase in brightness, and decrease in surface temperature. Mira is the closest and brightest AGB star.

An early AGB (E-AGB) star generates power predominantly by hydrogen shell burning creating a helium ash that adds to the helium shell. When a sufficient mass of helium accumulates, the helium shell becomes unstable and ignites in a violent helium shell flash which is powerful enough to dredge up carbon and oxygen from the core into the extended stellar atmosphere. Helium shell flashes mark the start of the thermally-pulsing AGB (TP-AGB) phase that Mira is currently in. The helium shell flashes, or thermal pulses, occur every few 1,000 to 100,000 years with durations of a few hundred years. Thermal-pulses are difficult to study because of the long time scales involved. Mira's tail, which documents at least the past 30,000 years, might be able to provide new insights into the thermal-pulse history of Mira - if we can decode it.

Mira's light curve
The size of a Mira-type star.
Credit: NOAO and A&A
As an AGB star, Mira has grown so large it would swallow Mars if it was located at the position of the Sun. The surface is cool enough to allow molecules and dust to form in vast quantities in its atmosphere. Furthermore, the atmosphere is so weakly held by gravity that pressure from the light it generates actually ejects some of the molecular material into space - forming a strong stellar wind. Mira ejects the equivalent of the Earth's mass every 10 years. It has released enough material over the past 30,000 years to seed at least 3,000 Earth sized planets or 9 Jupiter sized planets!

Mira's light curve
Mira's variability over 10 years.
Credit: AAVSO
As previously mentioned, Mira changes in brightness by more than a factor of 1,500 over the course of 332 days. Mira is the prototype of all stars that exhibit this type of long period (100 to 1000 days or longer) nearly regular variable behavior. They are often called Mira-type variables or just Miras. All normal stars with masses between one half and a few times the mass of the Sun will enter a phase like this as they begin to approach the end of their lives.

Why do Miras exhibit periodic behavior? They are pulsating (not to be confused with the infrequent thermal-pulses discussed above). As with thermal-pulses, it is a result of trying to maintain hydrostatic equilibrium - the balance of outward pressure force against inward gravitational force. However, the mechanism responsible for the periodic behavior is very different than what drives thermal-pulses. Theory suggests that the temperature and internal structure of AGB stars allows the formation of layers of helium and hydrogen partial ionization zones (PIZ). These layers have an interesting property, when they expand or compress they do not change in temperature, but decrease or increase in opacity - the ability to transmit light. Once a PIZ layer forms, the high opacity causes an increase in the outward radiation pressure force below it and this exceeds the gravitational force from the material above it. The star must expand to maintain equilibrium. As the star expands it cools (becomes fainter).
partial ionization zone
Pulsations and PIZs.
Credit: adapted from lecture by A. Belyanin
AGB schematic
AGB schematic.
Credit: J. Hron
As the PIZ layer expands its opacity decreases allowing the pressure beneath it to drop. The star overshoots equilibrium. Now the force of gravity exceeds the outward pressure and the star must contract and get hotter (become brighter). As the PIZ layer contracts its opacity increases and the whole cycle starts all over again just like a pendulum. Pulsing stars are very similar to harmonic oscillators. Because of these radial oscillations, Miras actually change in size by about 20% during a pulsation. PIZ layers are the responsible mechanism for short period variable stars as well. Understanding how stars pulsate required a lot of effort by theorists and the details remain an area of active research. Alfred Gautschy has written a history of stellar pulsation theory. It is the complex interplay between pulsations and the formation of molecules (including a lot of dust) in the cool extended atmosphere that leads to the strong stellar winds. A typical AGB star will loose 50% to 70% of its mass this way, thereby seeding the next generation of stars and planets with molecules. AGB stars play a crucial role in the chemical evolution of galaxies.

The oscillatory change in the size of Mira variables explains their periodic behavior well but is not sufficient to explain the change in brightness by factors of more than 1000. A recent theory suggests it requires the application of sun screen. Seriously. Mira is considered an oxygen-rich star.
Mira's light curve
The benefits of sun screen.
Credit: Sky & Telescope
That is, more oxygen than carbon has been dredged up from the core into the atmosphere. In oxygen-rich Miras, metallic oxides can form deep into the stellar atmosphere when it is in the coolest part of its pulsation cycle. In particular, the amount of titanium-oxide (TiO) significantly increases. TiO has been seen in the atmospheres of Miras for a long time and just happens to be an ingredient used as a sun screen. The net effect is that TiO can absorb much of the visible light Mira generates as it approaches minimum light (maximum size) making it even fainter than expected from radial oscillations.

Mira's light curve
Sharpless 2-188. Mira's fate?
Credit: Chris Wareing
What does the future hold for Mira? In less than 1 million years Mira will loose its 10 billion year long battle with gravity as the hydrogen and helium fueling the shell burning begins to run low. The remainder of its outer gas envelope (10%-20% of it's total mass) will be gently ejected into space and leave the carbon-oxygen core exposed. The newly exposed hot core will ionize the ejected envelope material causing it to glow as a planetary nebula. Instead of a being symmetric, Mira's planetary nebula will likely become highly distorted due to Mira's high space velocity, possibly like the planetary nebula Sharpless 2-188. After a few 10,000 years the nebula will disappear. The carbon-oxygen core will rapidly cool, contract, and fade as it becomes a bizarre ball of ultra-dense material containing 50% of the mass of the Sun in an object about the size of the Earth - a white dwarf.

But wait there's more! You probably thought I forgot about the "symbiotic" portion of the full classification, which in case you need to be reminded is: mildly-symbiotic thermally-pulsing asymptotic giant branch star.

Mira HST
Mira AB separation over 80 years.
Credit: Prieur et al., 2002
Mira is a binary star system. It has a gravitationally bound companion star. The dominant AGB star we have been discussing is designated Mira A and the companion is called Mira B (or VZ Ceti). It has long been believed that Mira B is a faint white dwarf - but that is controversial. Many astronomers will argue that Mira B is a low mass ordinary main sequence star. The jury is still out. Mira A and B are a physically widely separated (detached) binary system. The distance between them is 70 times the Earth-Sun distance or more than twice the average Earth-Pluto distance. It takes about 500 years for the pair to orbit each other. The orbital parameters are not perfectly determined yet because only a small fraction of the orbit period has been observed.

Mira HST
1997 Hubble image of Mira A & B.
Credit: M. Karovska
In 1997, the sharp-eyed Hubble Space Telescope was used to look at the system and easily resolved the two stars despite their small angular separation. The Hubble data show that Mira A is actually football shaped. Furthermore, in the ultraviolet it had an odd spur-like appendage pointing toward Mira B. No one knows what this is. In case you are wondering, Hubble did not see any of the bowshock, streams, or tail because the ultraviolet detectors on Hubble have a very small field of view compared to the detectors on GALEX. Each GALEX image maps an area of sky 200 times larger than Hubble, but the angular resolution of Hubble is 50 times smaller (better) than GALEX.

Mira HST
Mira B accretes Mira A's wind.
Credit: M. Ireland
Mira B feels the breeze of the stellar wind from Mira A and a very small fraction of the dust and gas in the wind are gravitationally captured by Mira B. In January of 2007, Mira was in the news because evidence was found of a possible proto-planetary disk around Mira B formed by the capture of Mira A's wind. A binary pair is termed symbiotic when a white dwarf star orbits a mass-shedding giant red star at such a large distance that the mass exchange between them is due only to the red giant's wind. Mira is often refered to as mildly-symbiotic because there has been no strong evidence for any violent episodic outbursts from Mira B due to the accretion of Mira A's wind, not to mention that Mira B may not even be a white dwarf at all! Mira B has been known to mildly brighten in the ultraviolet at times, which is assumed to be connected to changes in the wind accretion rate.
Mira HST
2004/5 Mira AB in X-rays and UV.
Credit: M. Karovska

When a white dwarf orbits much closer to a companion, it can pull material directly from the surface of the red giant - this is how some violently explosive type Ia supernovae, novae, and dwarf novae form. Recent Chandra X-ray imaging data from 2005 and additional Hubble ultraviolet data from 2004 has shown that Mira A has experienced an outburst in X-rays which is unusual for a red giant star. To complicate matters, these images even hint that one can see a bridge of heated gaseous material connecting Mira A and B.

Last but not least, in 2007, a shockingly long tail, bow shock, and "streams" were found around Mira. Return to the page about these discoveries.

GALEX ultraviolet image of Mira.
Credit: Martin, Seibert, Neill et al., 2007
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