Death of a Giant

This site attempts to depict the evolution of a sun-like star as it moves off the main sequence, through gianthood, and on to its final fate as a white dwarf.

In constructing this site, I have tried to draw on the most current sources and the most accurate information available in order to present a scientifically accurate picture of the star as it evolves. As the study of stellar evolution is still very young, there is still a great deal that is not known. Consequently, I have tried to make note of those cases where I have had to resort to conjecture, "averaging of several opinions" or "educated guesswork" to arrive at my conclusions. While some of the information in this site may well prove to be innaccurate, I sincerely believe that it does give a reasonably accurate portrayal of its subject matter, at least as reflected by the latest theory. Ongoing research has tended to support this belief.

The star being considered in this site is Mira, also known as Omicron Ceti. Discovered in 1596 by the noted Dutch pastor and ameteur astronomer David Fabricus, Mira was the first long-period variable star found. It has given its name to its particular stellar type, explaining why long-period variables are often called "Mira variables." Today, thousands of these variables are known. Our study of Mira begins 300 hundred million years ago, when Mira and its somewhat more massive companion were still on the main sequence. It concludes more than 50 billion years from now, when both stars have expired, leaving slowly cooling white dwarves as their epitaph.

Perhaps fifty to seventy million years ago, this star was on the "intermediate main sequence." Though Mira is believed to have originally been between three and four times as massive as the Sun, it actually had much in common with the Sun in terms of internal structure and its expected evolutionary path. Mira has moved through its evolutionary process much more quickly than the Sun will, because it contains about 2.5 to 3 times the Sun's mass. In spite of this, it is expected that the Sun's evolutionary path will be similar, though, of course, more drawn out. In examining Mira's evolution, we are examining what is likely to be the fate of our own Sun.

Given a mass of about 3 times that of the Sun during its main sequence lifetime, Mira was probably a star of early type A. According to the included HR Diagram, Mira is assumed to have been between type A1 to A3 during its main sequence lifetime. Mira has now exhausted the hydrogen, and subsequently, the helium in its core. Now, with no energy source to support it, Mira's tiny, superdense carbon-oxygen core is contracting and heating under the force of gravity, that heat pouring into the helium and hydrogen fusing layers surrounding it. The processes of nuclear fusion in these layers ultimately dump more carbon and oxygen "ash" into Mira's core, causing its mass to increase, which intensifies the effect of the gravitational contraction, which, in turn, intensifies the heating effect and so on. This goes on in a vicious cycle that will (and possibly already has) resulted in the collapse of the core into a degenerate state where the electrons of the carbon and oxygen of which it is composed are stripped away from the shells of their parent atoms and allowed to float freely in a sort of "electron soup". The star lacks the mass necessary to be able to generate sufficient gravitational pressure to heat its carbon/oxygen core to the temperatures necessary to ignite fusion.

Located between 400 and 420 light years from our solar system, Mira is a highly evolved asymptotic giant branch (AGB) star of type M7 IIIe. Its diameter varies over a period of approximately 332 days*. oscillating between about 400 and 700 times that of the Sun. Were Mira in place of the Sun, its surface would fluctuate between two AU (Farther by 25% than Mars' orbit) to about 3.5 AU (about 67 percent of the diameter of Jupiter's orbit). Along with this dramatic variation in the diameter, Mira's luminosity also varies between slightly less than that of the Sun to 625 times that of the Sun on average, with occasional peaks of as much as 1560 times that of the Sun. This represents a variation of about 7 magnitudes on average, 8 or more magnitudes on occasion. There is at least one documented case where Mira brightened to first magnitude--a brightening of 9 magnitudes, or almost 3100 times the Sun's brilliance!

It is important to note that Mira's variation in luminosity (at least in the visible spectrum) is roughly inverse to that in its diameter. Thus, when Mira is at its brightest, it is also at its or near its most condensed state. By the same token, when it is at its dimmest, Mira's diameter is at or near its most distended state. The reason for this apparent contradiction is that when Mira shrinks under the compressive force of gravity, its surface area is dramatically reduced (as the square of its radius), and thus the amount of energy radiated per unit surface area increases--and the star's surface becomes brighter and hotter. The variation in surface temperature is probably not that great--a matter of between a few hundred to perhaps 1000 degrees kelvin at most. At its coolest, however, the temperature at the star's surface drops to a low of about 2200 to 2300 degrees, a level low enough to allow the formation of large quantities of titanium oxide (TiO) in its upper layers.

Titanium oxide is the active ingredient in sun-screen, and it works in Mira's case just as it would when a person applies it to her body in preparing for a day of sunbathing: It stops radiation, particularly ultraviolet radiation. In Mira's case, it prevents much of the visible radiation and almost all of the ultraviolet radiation eminating from deeper in the star from ever reaching the surface. Additional dust, compounds and molecules forming in Mira's upper layers are carried away from Mira's surface by the star's prodigious wind, forming a vast, obscuring haze around it. Among these is very fine carbon "soot" which also contributes to the absorbtion of much of Mira's radiation. This effect is most pronounced when Mira is at its coolest and largest.

Mira as it appears to us today (and as it actually was 400 to 420 years ago) is probably best exemplified by the illustrations for Stages 16 through 18. To obtain an idea of what Mira might look like if it were to be viewed at close range as it varies from its maximum to its minimum (in terms of luminosity; the reverse for its size), one should compare stages 17 and 18. Stage 16 can be used as an example of what Mira might look like on those occasions when it reaches the first or second magnitude.

THIS "DEATH OF A GIANT" SITE, ALL NOTES, EXPLANITORY TEXT, AND ALL ILLUSTRATIONS CONTAINED HEREIN ARE ORIGINAL WORKS COPYRIGHTED AND OWNED BY D. AARON FREED. ALL RIGHTS ARE RESERVED. Permission to use these images and the accompanying text in an educational environment such as a college or university is hereby granted, so long as the original copyright is retained and acknowledged. Permission to use these works for commercial purposes may be arranged through correspondence with me at cyclopes@lafeyette.net

Any errors, innacuracies, or omissions are the fault of the author (D. Aaron Freed). This is especially true of spelling and grammatical errors as I type faster than I spell! ;-)

No animals, persons, or stars were injured or harmed in any way in the creation of this work.

I have happened upon several sources that suggest that Mira's mass may be about the same as, or slightly more than, that of the Sun. I have also found one source that indicates a mass of 15.7 times that of the Sun. It has proven rather difficult to pin down any type of reliable or consistent figure. In spite of this, there are several points to consider:

I have seen this period quoted as being anything from 330 to 335 days, with most of the quotes between 331 and 332 days. I can only assume that Mira's variability is slightly irregular, and that it really does vary over a period of 330 to 335 days.

This site is dedicated to the following individuals:
Professor James B. Kaler
University of Chicago Astronomy Department
Your willingness to help me with answers to my numerous questions about stars, astrophyscis, Rigil Kentaurus, and Mira has been invaluable and I am most grateful! Your books, "Stars", "100 Greatest Stars", and "Extreme Stars" have been, and continue to be, invaluable tools in my research and astronomical studies.

Ben Lin
SolStation.com
Your website has been, and continues to be, an invaluable resource for me during my continuing study of astronomy, astrophysics and stellar evolution. Your pages on Gamma Cruxis, as well as those on Alpha Centauri were especially invaluable as was your willingness and interest in my first attempts at "computer-aided-graphic" representations of stars.

Tomas Stimac
(Tom, I apologize for not properly accenting your name!)
Your wonderful artwork at http://www.inet.hr/~tstimac/planets/index.htm, along with that of John Watmaugh (Mentioned below) inspired me to try my hand at computer graphic presentations such as this one. Thank you, Tom, and thank you for the CD's of star data you kindly sent to me!

John Watmaugh
I envy you and Tomas Stimac (Mentioned above) your artistic skill in creating your wonderful websites. Your site, http://www.extrasolar.net/mainframes.html has been a fascination for me for quite some time. I recall the frustrations of several years ago when you seemed to have your site hosted on the world's slowest web-server. Thank God you moved it! It is truly an exemplary site, and, I believe an inspiration for many others--including this one.

Professor R. Earle Luck
Department of Astronomy, Case Western Reserve University
Thank you, Sir, for sending me the table on stellar diameters/masses/and life-times and the paper on same, as well as for answering a number of questions I had about this subject.

Rafael Millan-Gabet and Sveva Besana
Thank you, Rafael, for your interest, advice and commentary on my first drawings, and thank you, Sveva, for your interest and for allowing me to show the drawings to you and to Rafael!

Bernadette Ernalie "Ming", my wife
You, most of all, for putting up with me, feeding me, making sure I go to bed at a reasonable hour while I futz with this thing--and most of all, for your love.
Mahal na mahal kita magpakailanman, aking kasintahan!

Thank you ALL! I trully appreciate your kindness, your immeasurable help, your support, and your patience! This project (and more importantly, the knowledge I have gained in working on it) would never have been possible without you! So, here's to you! Enjoy!