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The star too tough to die.

Some stars cool down like a lump of hot coal. Others die hard.

On the night of February 20, 1996, Japanese amateur astronomer Yukio Sakurai was searching for comets when he chanced upon something much more remarkable. In a crowded region of Sagittarius, Sakurai photographed a new 12th-magnitude "star." Although it wasn't a comet, he duly reported the oddity to the International Astronomical Union's Central Bureau for Astronomical Telegrams, which, in turn, informed the astronomical community. Sakurai never imagined he had discovered the most rapidly evolving star ever seen--an object so special that it would soon bear his name.

Initially astronomers presumed it was a nova--an explosion on the surface of a white dwarf. But when they took a closer look, they were intrigued. Normally a nova rapidly brightens within a few days, then progressively fades from view over several weeks. But Sakurai's Object continued to shine brightly with no sign of letting up.

If it wasn't a nova, then what was it? The enigma deepened after studies of its spectrum failed to show the usual hydrogen emission lines characteristic of novae, revealing instead a wealth of atomic absorption features including carbon, nitrogen, and oxygen. In addition, astronomers detected a faint glow, strikingly similar to a planetary nebula, around the "new" star.

Today astronomers believe Sakurai's Object (also known as V4334 Sagittarii) is a "born-again" giant, a dying star that temporarily postponed its fate as a stellar ember by using its final bit of fuel to grow to supergiant size one last time. Sakurai's Object offers astronomers the unique opportunity to study the late stages of stellar evolution as they happen, to witness the production of chemical elements through nuclear reactions, and perhaps to catch a glimpse of the fate of our Sun.

The Lives of Stars

In order to interpret the strange behavior of Sakurai's Object, it's important to understand the life cycle of a normal star. This is best represented by the Hertzsprung-Russell (H-R) diagram (see page 51, bottom). A star's position on the graph depends upon its total energy output, or luminosity, and its surface temperature. Bodies migrate in characteristic ways across the H-R diagram. Normally, significant motion occurs only on time scales of millions or billions of years--thus it's impossible to watch a star travel across the diagram in a human lifetime.

Let's focus on stars with no more than eight solar masses, which will not end up as supernovae. Such a sun begins its life somewhere along the H-R diagram's main sequence. Where that point is depends entirely on the star's mass. But once there, it stays put, quietly spending the bulk of its life turning hydrogen into helium through nuclear fusion. The Sun is no exception. It has spent the past 4.6 billion years burning hydrogen and will do so for 7 billion more. Stars more massive than the Sun shine brighter because they consume their nuclear fuel more rapidly and therefore evolve faster.

In time, nearly all the hydrogen in the center becomes helium. However, fusion continues in a shell surrounding the core, where there is still an ample supply of hydrogen. No longer able to counteract gravity, the depleted core compresses and heats up. This causes the outer layers of the star to expand, and its stellar surface cools to about 3,000[degrees] Kelvin (5,000?F). At this point, the star becomes a red giant--dozens of times larger than its earlier main-sequence size.

Meanwhile, the core continues to shrink and heat up. When it reaches 100 million degrees, helium, the "ash" from hydrogen fusion, spontaneously converts to carbon and oxygen. This ignition of helium occurs quickly and "fluffs up" the star's outer layers. The expansion lowers the gas density, and without ample pressure, the hydrogen shell extinguishes.

Helium fusion within the core does provide a stable source of energy, but compared to hydrogen it's only about 1/10 as efficient. Therefore, the fresh supply of helium fuel vanishes 10 times faster, again shifting energy production outward. Once more the star swells and brightens--becoming a red supergiant several hundred times larger than its main-sequence youthful figure.

Now this sun has moved onto the H-R diagram's asymptotic giant branch (AGB), and its days as an active star are numbered. As the end nears, the star's life becomes more and more hectic as it rapidly exhausts its remaining gas. During the later evolution along the AGB, the hydrogen and helium shells surrounding the core (which now consists mainly of carbon and oxygen) take turns providing stellar energy. The hydrogen-fusion shell produces helium, which accumulates in a helium-rich layer below. When the temperature and density in the helium shell again climb high enough to convert helium to carbon and oxygen, another "helium flash" occurs. This explosive event is just a normal part of the aging process. After some 1,000 years, hydrogen burning resumes until the next helium flash 100,000 years later. Therefore, at any given time either hydrogen or helium fusion dominates.

While this trick prolongs the energy production of the star, it also introduces instabilities and causes the star to pulsate. The helium flash brings heavier elements from the stellar interior to the surface. As a second consequence, the star loses between 30 and 80 percent of its mass in a series of shock waves induced by the helium flashes. Therefore, only one or two dozen flashes occur before the outer layers are completely blown away. Left over is a stellar remnant--a carbon and oxygen core surrounded by a layer of helium and a skin of hydrogen.

What's left of the star then leaves the AGB, getting hotter while shrinking in size. Eventually it becomes hot enough--30,000[degrees]K to 100,000[degrees]K--to ionize any gas in its vicinity, and the lost outer layers become visible as a planetary nebula. But this final blaze exhausts what little fuel is left, and the star's remains quickly head for oblivion as a faint white dwarf--an enormously dense stellar cinder that fades like a cooling piece of hot coal. The whole evolution, from supergiant to hot white dwarf, takes from 10,000 to 100,000 years.

Death Throes and Metamorphosis

The pace of evolution sketched above applies to "normal" stars. But what if the star is too tough to die? If the critical mass for ignition of the helium shell is reached one last time on its descent toward the white-dwarf regime, the star can temporarily postpone its final fate. This is what has happened to Sakurai's Object.

The final helium flash renews the star with a vigorous source of energy, which causes both a tremendous brightening and a swelling in size. The object shoots across the H-R diagram in a matter of months and, in a triumphant encore, the star once again joins the AGB as a supergiant. But is the final flash a true fountain of youth? Well, not quite. Within 100 years or so--less than a blink of the eye in cosmic time--the remaining nuclear fuel runs low once more and the star retraces its earlier evolutionary path toward white dwarfdom. This time there's no turning back.

The paucity of observed "born-again" stars is more a consequence of their brevity than their rarity. In theory, up to 20 percent of all stars between one and eight solar masses will experience a Sakurai's Object phase. So, in a sense, this final helium flash is really just a part of normal stellar evolution. Yet it's rarely, if ever, observed. Astronomers think they've caught only one other star in the act: V605 Aquilae in 1919.

Fortunately there is a way to distinguish a giant on its second visit to the AGB from all the other first timers. The best evidence for Sakurai's Object being a born-again star comes from the faint remains of a surrounding planetary nebula, which proves that the star was already approaching the white-dwarf stage. Furthermore, the star's spectrum reveals a severe shortage of hydrogen --a telltale sign of a star that has thrown off its outer envelope of gas after its first phase as a supergiant. To astrophysicists, Sakurai's Object stands out as distinctly as a black sheep.

The most fascinating aspect of Sakurai's Object is the dramatic changes seen in its physical properties on time scales as short as months or even weeks. Right from the start, the spectrum of Sakurai's Object drew attention with its wealth of heavy-element absorption lines. Carbon, nitrogen, and oxygen are plentiful, while hydrogen is strongly underrepresented.

A more detailed analysis indicated something very exciting: the character of the stellar surface was obviously changing. Its already inconspicuous hydrogen lines became weaker, whereas lines from elements such as scandium, rubidium, yttrium, zirconium, barium, and tin started to appear. This is a direct consequence of the final helium flash, which removed hydrogen through convective motion. The cathartic event transported gas on the surface downward and dredged up other heavier elements from the deep interior. Thus Sakurai's Object gave astronomers their first-ever opportunity to study directly the very nuclear reactions that produce many of the chemical elements we find on Earth.

The observed mass loss most likely varies over time and occurs in clouds and clumps rather than in a simple spherical shell. Currently these shed layers have formed opaque dust clouds directly between us and Sakurai's Object, obscuring our view and causing the apparent brightness to dwindle. This behavior is very similar to the repeated fadings observed in a rare class of irregular variables, the R Coronae Borealis (R CrB) stars. These popular targets for amateur astronomers form clouds of dust that, when blocking our line of sight, cause visual fadings of up to eight magnitudes for weeks or months.

Currently the dust and gas are so thick that the star is very faint (less than 20th magnitude), and observations of its surface are all but impossible. Fortunately, astronomers are still monitoring the system's evolution in dust-penetrating infrared wavelengths, where Sakurai's Object remains bright.

Knowledge Gained

Already Sakurai's Object has provided invaluable insight into the physics of a final helium flash. Many details were reported in a special conference last year devoted entirely to the star. The gas in its surrounding nebulosity has preserved information about the properties of the star before the flash, allowing astronomers to perform a type of stellar autopsy. From this they have demonstrated that Sakurai's Object was indeed a highly evolved white-dwarf precursor with a planetary nebula already in place when the flash of rebirth occurred.

We are currently in a much better position to look for other possible "rebirth flash" candidates and to reassess the spectrum and evolution of V605 Aquilae. It's now clear that V605 Aquilae has an inner, hydrogen-poor nebula--just barely resolvable with the Hubble Space Telescope--whose size and expansion rate agree with a 1919 formation. Observations of the star itself are very difficult as it is still deeply buried in dust and is consequently extremely faint. The best results currently indicate that it is already turning into a hot white dwarf--a mere 80 years after being a cool red supergiant. This tells us that it is rapidly retracing its own evolution as theory predicts.

Sakurai's Object gives astronomers a unique opportunity to follow stellar evolution as it unfolds. The combination of timing, human effort, and technical capability has shed light on the physics of a final helium flash--and provided concrete evidence that this interesting theory actually works! Further observations at visible wavelengths will be more difficult as Sakurai's Object lowers its curtain of dust for privacy. But such difficulties have yet to sway astronomers, who continue to observe the peculiar object at other wavelengths with an arsenal of modern instrumentation.

It's possible Sakurai's Object will once again emerge from the ashes and return to its former glory by heating up and destroying the obscuring cocoon of dust that envelops it. The wait might be decades, but then again Sakurai's Object might surprise us. And should the dust curtain be lifted, another vigilant amateur astronomer may well be the first to notice.

FLORIAN KERBER is an instrument scientist at the Space Telescope European Coordinating Facility in Garching, Germany. His favorite research involves planetary nebulae and late stellar evolution. MARTIN ASPLUND, a research astronomer at the University of Uppsala in Sweden, specializes in modeling the atmospheres of cool stars.
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Title Annotation:evolution of Sakurai's Object
Author:Kerber, Florian; Asplund, Martin
Publication:Sky & Telescope
Date:Nov 1, 2001
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