Object 76: Sakurai's Object
Podcast release date: 27 June 2022
Right ascension: 17:52:32.7
Corresponding Earth location: About 1700 km southwest of Lima, Peru, in the Pacific Ocean
Located in the constellation Sagittarius, Sakurai's Object was unsurprisingly discovered by a person named Sakurai, or, more specificially, a Japanese amateur astronomer named Yukio Sakurai [1, 2, 3]. Sakurai is one of many amateur astronomers who have spent a lot of time scanning the sky for comets. On February 21st, 1996, while he was inspecting his photos of the night sky, he spotted a relatively bright star that simply had not been there a couple of years earlier. He then took a closer look at his collection of photos of this region and discovered that the star had slowly been increasing in brightness since the beginning of 1995 . This initially looked like some sort of nova-like event. In some sense, this was kind of a disappointment because the new object clearly was not a comet, but I am fairly certain that Sakurai took some solace from later learning that he had actually witnessed a once-in-a-lifetime event.
Within days, the news of the discovery was circulated within the professional astronomy community, and observatories immediately began observing the object to try to understand it. They quickly determined that this object, which they referred to as Sakurai's Object in the first scientific papers on it, was the core of a small dying star that had undergone a brief period of fusion in an event called a helium flash .
So, to begin with, let's start with a description of what happens when stars like the Sun die. At the moment, our Sun and other stars like it are currently fusing hydrogen into helium in their cores, but their cores will eventually fill up with helium. When that happens to any small Sun-like star, the star will expand to become a red giant. The fusion of hydrogen into helium will continue in a shell around the helium core, but the core will be relatively inert until enough helium builds up to trigger the fusion of helium into carbon and oxygen, at which point the star will become brighter and yellower again but also shrink in size. However, the core will eventually fill up with carbon and oxygen, and the star will expand and become red again. (It will look a lot like a red giant, but technically, it isn't. It's instead called an asymptotic giant branch star for reasons that I discussed in Episode 72.) A small star like the Sun does not have enough mass to fuse carbon and oxygen, so the carbon and oxygen that builds up in the star's core will just sit there. After a while, the star will expel its outer gas layers in the form of a planetary nebula. What's left is that dense core of carbon and oxygen, which will go on to be known as a white dwarf.
Sakurai's Object is almost at the stage where it could be considered a white dwarf. However, it still had a thin shell of helium around it, and the temperature and pressure of this helium built up near the surface of the star to levels sufficient for briefly triggering the fusion of helium into carbon and oxygen, and this produced the flash of light that was seen by Yukio Sakurai in 1996 . As you could imagine, this type of explosive event would basically blow away that thin outer helium shell, and the star began to fade away a few months after this helium flash .
Despite the star fading in brightness, astronomers have been able to perform quite a few follow-up observations to understand Sakurai's Object better. First of all, aside from figuring out that Sakurai's Object was indeed on the verge of becoming a white dwarf, astronomers also determined that it has a mass somewhere around 0.6 times the mass of the Sun . The distance to Sakurai's Object has been estimated to be about 12.4 thousand light years (3.8 kpc), although that number is still somewhat uncertain . Multiple images have been made of the planetary nebula surrounding the white-dwarf-like object , which, to be clear, would have formed long before the helium flash and which will continue to exist for thousands of years after the flash, and in addition, infrared observations have revealed that the star is surrounded by a disc of dust that was also present before the helium flash and that also seemed to have formed at about the same time as the planetary nebula . However, the helium flash also led to the creation of a lot of new dust. Astronomers observing Sakurai's Object with adaptive optics, which involves using deformable mirrors to correct for blurring caused by the Earth's atmosphere, have actually identified blobs of dust that were ejected from the star during the helium flash, and they have even been able to watch these blobs move away from the star since the year 2010 . This is rather interesting, at least to me, because,even though this ejected material will be dispersed into the interstellar medium, it could later become part of the material from which new stars or planets form.
As I indicated before, the helium flash from Sakurai's Object was a once-in-a-lifetime event. The only other star known to have undergone such a helium flash was V605 Aquilae, and that was in 1919 [10, 11]. Given the time interval between the helium flahses seen in V605 Aquilae and in Sakurai's Object, we might expect to see the next such helium flash sometime around the year 2070. Given how rare these events are, you can understand why astronomers spent so much time looking at Sakurai's Object specifically.
As a final note, I just want to point out that Yukio Sakurai has joined Charles Messier in the small group of comet hunters who have discovered objects in the sky which are truly scientifically important and that will bear their names forevermore but that aren't comets. In a way, these people have failed at what they set out to do, but on the other hand, they have truly left their marks on astronomy.
 Nakano, S. et al., Novalike Variable in Sagittarius, 1996, International Astronomical Union Circular, 6322, 1
 Duerbeck, Hilmar W. and Benetti, Stefano, Sakurai's Object---A Possible Final Helium Flash in a Planetary Nebula Nucleus, 1996, Astrophysical Journal Letters, 468, L111
 Tyne, V. H. et al., The continuing saga of Sakurai's object (V4334 Sgr): dust production and helium line emission, 2000, Monthly Notices of the Royal Astronomical Society, 315, 595
 Herwig, Falk, The Evolutionary Timescale of Sakurai's Object: A Test of Convection Theory?, 2001, Astrophysical Journal Letters, 554, L71
 Evans, A. et al., The infrared view of dust and molecules around V4334 Sgr (Sakurai's object): a 20-yr retrospective, 2020, Monthly Notices of the Royal Astronomical Society, 493, 1277
 Chesneau, O. et al., A dense disk of dust around the born-again Sakurai's object, 2009, Astronomy & Astrophysics, 493, L17
 Hinkle, Kenneth H. et al., The Spatially Resolved Bipolar Nebula of Sakurai's Object. II. Mapping the Planetary Nebula Expansion, 2020, Astrophysical Journal, 904, 34
 Wolf, M., Variabilis oder Nova 7.1920 Aquila, 1920, Astronomische Nachrichten, 211, 119
 Clayton, Geoffrey C. et al., V605 Aquilae: The Older Twin of Sakurai's Object, 2006, Astrophysical Journal Letters, 646, L69