Observing a stellar chameleon
Astronomers have been watching an unusual star for over 20 years
We’re so used to the astronomical world being static and unchanging that even small alterations, like this year’s fading of Betelgeuse in Orion, can be both surprising and somewhat unsettling. For some objects, though, change is just a way of life, and that’s certainly true of Sakurai’s Object, the subject of this month’s paper.
It’s now more than 20 years since the Japanese amateur astronomer Yukio Sakurai noticed what he thought was a nova in the constellation of Sagittarius. Follow-up observations soon showed it to be something rather more interesting. It appeared to be a normal supergiant star, but its spectrum kept changing and, in 1997, Sakurai’s star began producing a shell of carbon dust.
Gathering dust
By 1999, less than three years after that initial discovery, this dusty carapace completely blocked the view of the star itself, a situation which persists to the present day. This dust is produced in the star itself, and in prodigious quantities; early observations suggest more than three Earth masses of the stuff was produced each year.
The explanation for this behaviour is that Sakurai’s Object is in a very particular stage of its life, having exhausted the hydrogen and helium fuel available at its core. Instead, fusion is taking place in a shell of material around the star’s core, and this inherently unstable situation leads to the star’s wandering in brightness. The object is currently in a state in which it is brightening, but the disruption has probably dredged up heavy elements such as carbon from the inner layers.
It’s from these heavy elements, the by-products of previous epochs of star formation, that the dust that obscures the star is being assembled.
Putting together more than 20 years’ worth of observations, the authors of this month’s paper give themselves a grandstand seat from which to watch this part of a star’s life. Along with the dust, observations of molecules such as hydrogen cyanide (made up of a single atom each of hydrogen, carbon and nitrogen) help them understand the physical conditions around the object itself.
These observations show that things are still changing around the star. Between 1998 and 2016, the
dust has cooled from over 900˚C to –90˚C or so. In the
years to come, it will cool further and the light from the star itself should become visible once again. At some point, the source will reach its maximum brightness, before fading again. Such cycles will continue until so much mass is lost from the star that fusion is no longer possible, leaving behind a fading white
dwarf and a briefly beautiful planetary nebula.
Such a nebula has already been seen, centred on Sakurai’s Object, indicating that this cycle of mass loss goes back more than 10,000 years. This unusual object shows us the
complex lives that small stars – perhaps no more massive than the Sun – undergo at the end of their
lives. During a single lifetime it’s possible to watch such stars alter and change and this fascinating, albeit dusty, stellar chameleon is worth keeping an eye on.
“It’s more than 20 years since amateur astronomer Yukio Sakurai noticed what he thought was a nova in Sagittarius”