123 Some cool down n the night of February 20, 1996, Japa- like a lump of hot nese amateur astronomer Yukio Sakurai was searching for comets when he coal. Others die hard. chanced upon something much more re- markable. In a crowded region of Sagit- tarius, Sakurai photographedO a new 12th-magnitude “.” Although it wasn’t a BY FLORIAN KERBER comet, he duly reported the oddity to the International Astronomical Union’s Cen- AND MARTIN ASPLUND tral 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 — an explosion on the surface of a . 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 , 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 re- actions, and perhaps to catch a glimpse of the fate of our Sun.

48 November 2001 Sky & Telescope 456

HUBBLE EUROPEAN OUTREACH CENTER

The Lives of Stars counteract gravity, the depleted core compresses and heats In order to interpret the strange behavior of Sakurai’s Object, up. This causes the outer layers of the star to expand, and its it’s important to understand the life cycle of a normal star. This stellar surface cools to about 3,000 Kelvin (5,000 F). At this is best represented by the Hertzsprung-Russell (H-R) diagram point, the star becomes a red giant — dozens of times larger (see page 51, bottom). A star’s position on the graph depends than its earlier main-sequence size. upon its total energy output, or , and its surface tem- Meanwhile, the core continues to shrink and heat up. When perature. Bodies migrate in characteristic ways across the H-R it reaches 100 million degrees, helium, the “ash” from hydro- diagram. Normally, significant motion occurs only on time gen fusion, spontaneously converts to carbon and oxygen. This scales of millions or billions of years — thus it’s impossible to ignition of helium occurs quickly and “fluffs up” the star’s watch a star travel across the diagram in a human lifetime. outer layers. The expansion lowers the gas density, and without Let’s focus on stars with no more than eight solar masses, ample pressure, the hydrogen shell extinguishes. which will not end up as supernovae. Such a sun begins its life Helium fusion within the core does provide a stable source of somewhere along the H-R diagram’s main sequence. Where energy, but compared to hydrogen it’s only about 1⁄10 as efficient. that point is depends entirely on the star’s mass. But once Therefore, the fresh supply of helium fuel vanishes 10 times there, it stays put, quietly spending the bulk of its life turning faster, again shifting energy production outward. Once more the hydrogen into helium through nuclear fusion. The Sun is no star swells and brightens — becoming a red supergiant several exception. It has spent the past 4.6 bil- lion years burning hydrogen and will do so for 7 billion more. Stars more mas- Above: The evolution of Sakurai’s Object has been remarkably rapid. Before its discovery (1), sive than the Sun shine brighter because Sakurai’s Object was a faint but hot white-dwarf precursor surrounded by its . they consume their nuclear fuel more Shortly after its final helium flash (2), the star has ballooned to gigantic dimensions and cooled rapidly and therefore evolve faster. to about 6,000 K. Over the next year (3) the star cooled further, creating a cocoon of hot dust. In time, nearly all the hydrogen in Here the signature of carbon within the star’s spectra is evident, but the photosphere is still the center becomes helium. However, visible. By 1998 (4), dust between us and Sakurai’s Object had thickened and begun to block our fusion continues in a shell surrounding view. Eventually the dust completely obscured the star’s photosphere, making it a very faint the core, where there is still an ample object visually. Decades from now (5), Sakurai’s Object will end its “born-again” giant phase supply of hydrogen. No longer able to and resume its transformation into a white dwarf. (This stage resembles how a close cousin,V605 Aquilae, looks now.) Thousands of years from now (6) Sakurai’s Object will have freed itself from its dust shroud and will be visible as a faint star at the center of a second, inner planetary nebula.

Sky & Telescope November 2001 49 5¢ SEIICHIRO KIYOTA SEIICHIRO KIYOTA AND YUKIO SAKURAI

Yukio Sakurai (bottom right) discovered the object named after him on February 20, 1996. Located at right ascension 17h 52m 33.7s, declination –17 41¢08†(equinox 2000.0), Sakurai’s Object is just northwest of M23 in Sagittarius. In the discovery image (top right) Sakurai first identified the ob- ject as a nova. In 1995 the star was at magnitude 12.5.When Sakurai found it a year later, the object’s brightness had jumped to 11.4.Today Sakurai’s Object is far less impressive, barely resolvable with even the best visual telescopes. When photographed by the 8-meter Very Large Telescope, Kueyen, in April 2001 (left), it had dimmed below magnitude 20 — nearly 4,000 times fainter than the 1996 discovery image shows. For compari- son, the brightest star in the VLT field is magnitude 10.7.VLT Kueyen image courtesy Florian Kerber and the Hubble European Outreach Center.

hundred times larger than its main-sequence youthful figure. also introduces instabilities and causes the star to pulsate. The Now this sun has moved onto the H-R diagram’s asymptot- helium flash brings heavier elements from the stellar interior to ic giant branch (AGB), and its days as an active star are num- the surface. As a second consequence, the star loses between 30 bered. As the end nears, the star’s life becomes more and more and 80 percent of its mass in a series of shock waves induced by hectic as it rapidly exhausts its remaining gas. During the later the helium flashes. Therefore, only one or two dozen flashes evolution along the AGB, the hydrogen and helium shells sur- occur before the outer layers are completely blown away. Left rounding the core (which now consists mainly of carbon and over is a stellar remnant — a carbon and oxygen core sur- oxygen) take turns providing stellar energy. The hydrogen- rounded by a layer of helium and a skin of hydrogen. fusion shell produces helium, which accumulates in a helium- What’s left of the star then leaves the AGB, getting hotter rich layer below. When the temperature and density in the he- while shrinking in size. Eventually it becomes hot enough — lium shell again climb high enough to convert helium to 30,000 K to 100,000 K — to ionize any gas in its vicinity, and carbon and oxygen, another “helium flash” occurs. This explo- the lost outer layers become visible as a planetary nebula. But sive event is just a normal part of the aging process. After this final blaze exhausts what little fuel is left, and the star’s re- some 1,000 years, hydrogen burning resumes until the next mains quickly head for oblivion as a faint white dwarf — an helium flash 100,000 years later. Therefore, at any given time enormously dense stellar cinder that fades like a cooling piece either hydrogen or helium fusion dominates. of hot coal. The whole evolution, from supergiant to hot white While this trick prolongs the energy production of the star, it dwarf, takes from 10,000 to 100,000 years.

50 November 2001 Sky & Telescope In this image of Sakurai’s Object from 1996, starlight was subtracted to reveal the faint planetary nebula surrounding the curious star. By N analyzing the remnant gas, astronomers hope to better understand E 5 arcseconds why the central star underwent a late helium flash.

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 ig- nition of the helium shell is reached one last time on its descent toward the white-dwarf regime, the star can temporarily post- pone 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 dia- gram 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. EUROPEAN SOUTHERN OBSERVATORY This time there’s no turning back. The paucity of observed “born-again” stars is more a conse- that the star was already approaching the white-dwarf stage. Fur- quence of their brevity than their rarity. In theory, up to 20 thermore, the star’s spectrum reveals a severe shortage of hydro- percent of all stars between one and eight solar masses will ex- gen — a telltale sign of a star that has thrown off its outer enve- perience a Sakurai’s Object phase. So, in a sense, this final heli- lope of gas after its first phase as a supergiant. To astrophysicists, um flash is really just a part of normal stellar evolution. Yet it’s Sakurai’s Object stands out as distinctly as a black sheep. rarely, if ever, observed. Astronomers think they’ve caught The most fascinating aspect of Sakurai’s Object is the dra- only one other star in the act: V605 Aquilae in 1919. matic changes seen in its physical properties on time scales as Fortunately there is a way to distinguish a giant on its second short as months or even weeks. Right from the start, the spec- visit to the AGB from all the other first timers. The best evidence trum of Sakurai’s Object drew attention with its wealth of for Sakurai’s Object being a born-again star comes from the heavy-element absorption lines. Carbon, nitrogen, and oxygen faint remains of a surrounding planetary nebula, which proves 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 Surface temperature (Kelvin) inconspicuous hydrogen lines became weaker, whereas lines from 100,000° 10,000° 2,000° elements such as scandium, rubidium, yttrium, zirconium, bar- 100,000 ium, and tin started to appear. This is a direct consequence of Red the final helium flash, which removed hydrogen through convec- Planetary nebula supergiant 10,000 10,000 – 100,000 years tive motion. The cathartic event transported gas on the surface downward and dredged up other heavier elements from the deep Final Born-again giant 1 – 20 years interior. Thus Sakurai’s Object gave astronomers their first- helium s 1,000 r flash a AGB ever opportunity to study directly the very nuclear reactions e y 7 (Helium M that produce many of the chemical elements we find on Earth. 0 ain sequ 1 flashes) – The observed mass loss most likely varies over time and oc- 100 6

0 1 Red giant curs in clouds and clumps rather than in a simple spherical 7 8 ence 10 –10 shell. Currently these shed layers have formed opaque dust years 10 1 10 clouds directly between us and Sakurai’s Object, obscuring our 0 5 8 6 9 – –10 10 –10 1 view and causing the apparent brightness to dwindle. This be- 0 9 uminosity (Suns) 9 years

L y 1 e yea a r s r s

0.1 The evolution of a normal solar-type star can be represented by the White path it travels along the Hertzsprung-Russell diagram. Stars spend 0.01 dwarf billions of years on the main sequence before expanding to become red giants. From there, they shed gas in a series of helium flashes, form planetary nebulae, and eventually cool to become white dwarfs. 0.001 DIAGRAM But Sakurai’s Object took a different path: before becoming a white S&T OGKMB A F dwarf, the star erupted in a final helium flash. It postponed death, Spectral type grew to red-giant size again, and is currently shedding more gas.

Sky & Telescope November 2001 51 Sakurai’s Object (April 1996) Visible-light spectra of Sakurai’s Object taken in March 1996 (top) and April 1997 (middle) are matched with a 1921 spectrum of its best- known analog, V605 Aquilae. The similarities between the middle and bottom panels are as striking as the dramatic change in Sakurai’s Object that occurred in less than one year. The rapid formation of dust in the surrounding nebula first became apparent in early 1997

Relative intensity Relative in the form of excess infrared radiation. In fact, the orbiting Infrared 380 400 420 440 460 480 500 520 Space Observatory recorded a tenfold increase in infrared brightness Wavelength (nanometers) from February 1997 to February 1998. Soon thereafter, the visual Sakurai’s Object (March 1997) brightness of Sakurai’s Object faded dramatically — only to recover and then decline again.

with the — whose size and expansion Relative intensity Relative 380 400 420 440 460 480 500 520 rate agree with a 1919 formation. Observations of the star itself Wavelength (nanometers) are very difficult as it is still deeply buried in dust and is conse- quently extremely faint. The best results currently indicate that V605 Aquilae it is already turning into a hot white dwarf — a mere 80 years (1921) 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, Relative intensity Relative

V605 AQUILAE SPECTRA COURTESY GEOFF CLAYTON; SAKURAI’S OBJECT FLORIAN KERBER. human effort, and technical capability has shed light on the 380 400 420 440 460 480 500 520 Wavelength (nanometers) 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 havior is very similar to the repeated fadings observed in a rare lowers its curtain of dust for privacy. But such difficulties have yet class of irregular variables, the R Coronae Borealis (R CrB) stars. to sway astronomers, who continue to observe the peculiar object These popular targets for amateur astronomers form clouds of at other wavelengths with an arsenal of modern instrumentation. dust that, when blocking our line of sight, cause visual fadings It’s possible Sakurai’s Object will once again emerge from of up to eight magnitudes for weeks or months. the ashes and return to its former glory by heating up and de- Currently the dust and gas are so thick that the star is very stroying the obscuring cocoon of dust that envelops it. The faint (less than 20th magnitude), and observations of its sur- wait might be decades, but then again Sakurai’s Object might face are all but impossible. Fortunately, astronomers are still surprise us. And should the dust curtain be lifted, another vig- monitoring the system’s evolution in dust-penetrating infrared ilant amateur astronomer may well be the first to notice. wavelengths, where Sakurai’s Object remains bright. Florian Kerber is an instrument scientist at the Space Telescope Eu- Knowledge Gained ropean Coordinating Facility in Garching, Germany. His favorite re- Already Sakurai’s Object has provided invaluable insight into search involves planetary nebulae and late stellar evolution. Martin the physics of a final helium flash. Many details were reported Asplund, a research astronomer at the University of Uppsala in Swe- in a special conference last year devoted entirely to the star. den, specializes in modeling the atmospheres of cool stars. The gas in its surrounding nebulosity has preserved informa- tion 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

The planetary nebula surrounding Sakurai’s Object, shown here in hydrogen-alpha light, has a diameter of 32 arcseconds. The central star is heavily saturated (note diffraction spikes) in this image obtained in March 1997 with the 2.5-meter du Pont telescope at Las Campanas Observatory in Chile. The abundant hydrogen in the nebula was

blown off the central star. FLORIAN KERBER

52 November 2001 Sky & Telescope