A star is reborn: Helium flash

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  • Published: Oct 1, 2016
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: A star is reborn: Helium flash

Life cycle

Stellar spectroscopic studies have revealed new details about the life cycle of a distant star and shown for the first time a stellar rebirth after three decades of astronomical observations. This artist’s impression shows the evolution of SAO 244567’s rapid evolution.  Credit: ESA/Hubble & NASA

Stellar spectroscopic studies have revealed new details about the life cycle of a distant star and shown for the first time a stellar rebirth after three decades of astronomical observations.

An international team of astronomers has used the Hubble Space Telescope to follow the evolution of a star in real time. It was already known that over the last thirty years there has been a dramatic increase in the temperature of the star SAO 244567, which lies some 2400 light years from Earth. The latest observations suggest, however, that the star is now cooling having been bizarrely "reborn" into an earlier phase of its stellar evolution. This, the astronomers say, makes it the first star to have been observed during both the heating and cooling stages of its rebirth.

Commonly cosmic

Commonly, cosmic events occur on timescales way beyond our every day experience they occur not only in deep space but in deep time. However, some phenomena, the formation of supernovae, the pulsing of pulsars and other events, do take place quickly, although given their great distances, our observations are well after the event, as it were. The warming and cooling of a star leading to what is termed its rebirth is another example of a cosmic event happening on a timescale comprehensible to human beings. "SAO 244567 is one of the rare examples of a star that allows us to witness stellar evolution in real time," explains Nicole Reindl of the University of Leicester, UK, who is lead author on the published study. "Over only twenty years the star has doubled its temperature and it was possible to watch the star ionising its previously ejected envelope, which is now known as the Stingray Nebula."

SAO 244567 is the pivotal, central star of the Stingray Nebula and has been quickly evolving over a period of 45 years or so. Between 1971 and 2002 the surface temperature of the star skyrocketed by almost 40 000 degrees Celsius. Now, new observations made with the Cosmic Origins Spectrograph (COS) on the NASA/ESA Hubble Space Telescope have revealed that SAO 244567 has started to cool and expand. The phenomenon seems bizarre but is not unheard of. Indeed, the rapid heating might be explained easily if one assumes that SAO 244567 had an initial mass of between three and four times that of our Sun. However, this is not the case, the data show that SAO 244567 was originally approximately the same mass as the Sun and such low-mass stars usually evolve on much longer timescales; its rapid heating has therefore remained mysterious for decades.

Astronomical teamwork

In 2014, Reindl and her team came up with a new theory that might explain the phenomenon and resolve the paradox of the low mass of SAO 244567 and its rapid heating. They suggested that the heating effect might be due to a helium-shell flash event, also known as a late thermal pulse; a brief ignition of helium outside the stellar core. The implications of this explanation are that SAO 244567 should have expanded and cooled if it had indeed experienced such a helium flash and that this would make it revert to a prior phase of its stellar evolution. The new observations confirm that this has happened, Reindl explains. "The release of nuclear energy by the flash forces the already very compact star to expand back to giant dimensions - the born-again scenario."

The phenomenon has been observed before, in the star FG Sagittae, but not both the heating and cooling in the same star. The observations of the transformation in the same cosmic entity put the theory on a much firmer footing especially given that no current stellar evolutionary models can fully explain the evolution of SAO 244567. "We [now] need refined calculations to explain some still mysterious details in the behaviour of SAO 244567," Reindl adds. "These could not only help us to better understand the star itself but could also provide a deeper insight in the evolution of central stars of planetary nebulae."

"We hope that we could encourage theorists to calculate more late thermal pulse evolutionary models to explain the evolution in more detail," Reindl told SpectroscopyNOW. "Follow-up observations in the next years, decades and centuries would be great to further follow the evolution of the star. For this new UV-telescopes would be needed, since HST will be in operation only for a few more years and, unfortunatly, there are no other UV telescopes at the moment."

The team comprised Nicole Reindl (University of Leicester, UK), Thomas Rauch and Klaus Werner (Eberhard Karls University, Germany), Marcelo Miguel Miller Bertolami (UNLP-CONICET, Argentina), and Helge Todt (University of Potsdam, Germany).

Related Links

Monthly Notices Royal Astronom Soc 2016, online: "Breaking news from the HST: The central star of the Stingray Nebula is now returning towards the AGB"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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