123 Some stars 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 “star.” 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 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 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 (5,000F). 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 luminosity, 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 planetary nebula. 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 6,000K. 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 30,000K to 100,000K — 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.