Sakurai's Object: A Once-in-a-Llifetime Experience
The Story of a Rarely Seen Stellar Explosion
7 March 1996
A bright 'new' star was discovered by Japanese amateur astronomer Yukio Sakurai in late February 1996. It is located in the star-rich, southern constellation of Sagittarius (The Archer) and qualifies to join an extremely select class of stars. In fact, we know only one additional object of this type and the remains of two - possibly three - others. Compared to the 6000 stars in the sky seen with the naked eye, the several millions so far catalogued, and the billions of stars photographed, it is a very special class indeed. Nevertheless, Sakurai's star holds unique information about a dramatic evolutionary state, which all stars must to pass through whose masses are more than a few times that of the Sun, but still too small to produce a supernova explosion. This happens just before they end their active life and cool down into visual oblivion.
When Yukio Sakurai inspected his sky photographs, taken in the early morning of February 21 (Japanese time), he discovered a comparatively bright 'new' star in Sagittarius. Checking his earlier photographs, he found no trace of this star before January 1995, when it first appeared at a magnitude somewhat fainter than 12.5 (about 400 times fainter than what can be perceived with the naked eye). The star was also present on all later photos, throughout 1995 and the beginning of 1996. The most recent observations show that it continues to brighten, although at a much slower rate; the current visual magnitude is 11.2.
Such a discovery merits an announcement in the Circulars of the International Astronomical Union (IAU), the world-wide fast information service for observers. The communication about Sakurai's new star reached the European Southern Observatory at La Silla on February 23. Here the first spectra of the possible nova (the technical term for a star that has suddenly brightened considerably) were immediately taken at the ESO 3.6-metre telescope. This observation and the following investigations at La Silla were undertaken by a small team of ESO astronomers, including Hilmar Duerbeck, Waltraut Seitter and Stefano Benetti.
Which type of nova ?
Before the first spectra from ESO became available, the object was suspected to be a very slow nova, that is a small and compact 'white dwarf' star in a binary system which experiences a hydrogen nuclear explosion below its surface. During a nova outburst of this type, the spectrum of the exploding star contains bright emission lines of hydrogen and other chemical elements, superimposed on a rapidly weakening, almost featureless spectrum.
But Sakurai's object showed nothing the like. Instead, the spectrum displayed a multitude of narrow absorption lines. The otherwise almost omnipresent lines of hydrogen, the most abundant chemical element in the Universe, were comparatively weak. Spectra of higher resolution, taken at the ESO 1.5-metre telescope the following night, revealed the characteristics of a fairly cool, chemically peculiar star with absorption lines of neutral helium, carbon, nitrogen, and oxygen and singly ionized lines of carbon and silicon. This spectrum is reproduced as eso9619b, accompanying this Press Release.
Another immediate action relating to the new bright star was to search for its pre-outburst state. For this, the ESO/SERC Atlas of the Southern Sky was used; this is the most detailed photographic atlas in the south and was produced in the 1970's during a joint project with the ESO (La Silla) and UK (Siding Spring, Australia) Schmidt telescopes. At the location of Sakurai's object, three very faint stars were found as well as a minute trace of a possible nebulosity.
The combination of the long phase of maximum light, the hydrogen-poor and carbon-rich outburst spectrum, and the hint of a nebulosity confirmed the suspicion of the ESO astronomers that this star had experienced its 'Final Helium Flash' , the explosive, very last phase of nuclear burning in a star of medium mass.
Similarity with Nova Aquilae in 1919
Sakurai's object is only the second case of an observed Final Helium Flash. The first one was the 'nova' of 1919 in the northern constellation of Aquila (The Eagle), now known as the variable star V605 Aquilae and located at the center of a conspicuous nebulosity, the planetary nebula A58 . A very low dispersion spectrum was taken two years later of this star. It showed the molecular bands of the C2 carbon molecule which are characteristic for a hydrogen-poor carbon star .
The spectrum of Sakurai's object is too warm to display molecular lines and bands, but the numerous lines of singly ionized carbon atoms seen in the ESO spectra give strong support to the assumption that the stars of 1919 and 1996 are in fact of the same nature.
The birth of a planetary nebula
Modern theoretical studies of stellar evolution are able to explain in quite some detail the various phases a star must pass during its life. In particular, it has been established that this evolution is critically dependent on the star's total mass.
Normal stars with masses like that of our Sun draw most of their energy from the transformation of hydrogen into helium, often referred to as 'hydrogen nuclear burning'. But at some moment, the hydrogen fuel will run out and the hydrogen burning comes to an end. This phase - still many billions of years into the future for the Sun - signals the beginning of profound, increasingly rapid changes in the star which will ultimately lead to its death.
When this happens for a star that is a few times heavier than the Sun - and which is bound to experience the above-mentioned final helium flash - it next evolves to a cool and bright, giant star with a very extended atmosphere. Deep inside such a star, energy is now generated by nuclear burning of helium to carbon. During this process, the star builds up what will eventually become an incompressible ('degenerate') core of carbon. Further out, above the helium-burning shell around this core, there is a layer where hydrogen still burns to helium.
Eventually and repeatedly, a sequence of intricate processes of energy generation, as well as mixing and transport of the stellar material in different layers, produces a multitude of chemical elements and isotopes and moves them into the outer regions of the giant star. From here, strong stellar winds carry the matter into interstellar space. During its further evolution, the giant star blows off its outer layers altogether, thus exposing the very dense, very hot, small and almost 'naked' nucleus of the star. Its freely escaping radiation excites spectral line emission in the ejected matter: in this way a surrounding, shining planetary nebula is born.
The Helium Flash and thereafter
The stellar nucleus of this planetary nebula experiences a comparatively short phase as a very compact, 'pre-white dwarf star' during which some burning of hydrogen to helium still takes place near the stellar surface. But then, when this nuclear burning ceases due to lack of hydrogen, the layer with the newly created helium begins to contract. The compression proceeds rapidly until the helium reaches the maximum possible density ('becomes degenerate'). It heats up and soon acquires the high temperature of the carbon core. It is at this moment that the helium suddenly ignites in a spectacular Final Helium Flash.
In this new phase, the outward appearance of the star rapidly returns to its former, bright giant appearance, but this time it is a deceptive one. What looks to the distant observer as a 'sturdy', bright giant atmosphere is nothing but the temporarily blown-up, carbon-rich layer produced at the time of the helium flash. After years or decades it will gradually become transparent and reveal the very hot and compact stellar nucleus at the center of the small, hydrogen-poor secondary planetary nebula which was created during the Final Helium Flash episode.
Thereafter, the stellar nucleus slowly cools down, this time to its final state of an inactive, cooling white dwarf. Its brightness decreases and at some moment it drops out of sight.
A planetary nebula around Sakurai's Object
On the basis of the above description of stellar evolution, the ESO astronomers decided to look for the expected planetary nebula at the location of Sakurai's new star, which should have been ejected during a former phase. And they found it !
Direct images were obtained at the Dutch 0.9-metre telescope at La Silla through narrow-band filters, which transmit only the red light of the hydrogen H-alpha line or the green 'forbidden' lines of doubly ionized oxygen, characteristic for normal planetary nebulae. As can be seen on the accompanying eso9619a, the observations did reveal an old planetary nebula with a diameter of 32 arcseconds, intensively radiating in the mentioned emission lines.
This finding strongly supports the proposition that Sakurai's object is a star now experiencing its Final Helium Flash.
A very rare event
Two comparatively bright planetary nebulae, A30 and A78, have central structures which from spectral observations are known to contain only small amounts of hydrogen. They were the first objects found which are believed to be the left-over nebulae after helium flashes. The measured sizes of the central nebulae inside these planetary nebulae suggest that they are only a few thousand years old.
As far as the current theory is concerned, the Final Helium Flash - a decisive episode in the evolution of a medium mass star - provides an excellent explanation for the behaviour of the two outburst objects observed in 1919 and 1996. Nevertheless, there is a problem relating to the time scales of the stellar models. According to the theoretical calculations of the helium flash, the star ought to brighten over a period of hundreds of years and then decline over tens of thousands of years. The observations, however, tell us that the brightness maximum is reached in a matter of years only and that the dispersion of the ejected nebula reveals the central white dwarf already after another several tens of years - this is documented by the evolution of the nucleus of V605 Aql.
The full chain of events during the late rise and the long decline of the Final Helium Flash has never been seen. In the years to come, a hitherto un-observed evolutionary path of fundamental importance for our understanding the late stages of stars of medium mass will be followed by ESO astronomers and throughout the world.
Sakurai's Object should become a favorite target for astronomers well into the 21st century - an event of a lifetime, indeed.
 The term 'planetary nebula' is historical and does not refer to any physical relations to planets, but rather to the extended appearance and green colour of some of these nebulae to the eyes of telescopic observers in the last century.