Using the NASA/ESA Hubble Space Telescope, astronomers have been able to study stellar evolution in real time.
This image of the Stingray Nebula was taken with Hubble’s Wide Field and Planetary Camera 2 in 1998. In the center of the nebula the fast evolving star SAO 244567 is located. Observations made within the last 45 years showed that the surface temperature of the star increased by almost 40,000 degree Celsius. Now new observations of the spectra of the star have revealed that SAO 244567 has started to cool again. Image credit: NASA / ESA / Hubble.
The star in question is called SAO 244567. It is the central star of one of the youngest known planetary nebula, the Stingray Nebula.
Located approximately 2,700 light-years away, this star has been visibly evolving between observations made over the last 45 years.
Between 1971 and 2002 the surface temperature of SAO 244567 skyrocketed by almost 40,000 degrees Celsius.
Now new observations made with Hubble’s Cosmic Origins Spectrograph have revealed that SAO 244567 has started to cool and expand.
This is unusual, though not unheard-of, and the rapid heating could easily be explained if one assumed that SAO 244567 had an initial mass of 3 to 4 times the mass of the Sun.
However, the data show that the star must have had an original mass similar to that of our Sun.
Such low-mass stars usually evolve on much longer timescales, so the rapid heating has been a mystery for decades.
“SAO 244567 is one of the rare examples of a star that allows us to witness stellar evolution in real time”, said Dr. Nicole Reindl, from the University of Leicester, UK.
“Over only twenty years the star has doubled its temperature and it was possible to watch the star ionizing its previously ejected envelope, which is now known as the Stingray Nebula.”
Back in 2014 Dr. Reindl and co-authors proposed a theory that resolved the issue of both SAO 244567’s rapid increase in temperature as well as the low mass of the star.
The team suggested that the heating was due to what is known as a helium-shell flash event: a brief ignition of helium outside the stellar core.
This theory has very clear implications for SAO 244567’s future: if it has indeed experienced such a flash, then this would force the central star to begin to expand and cool again — it would return back to the previous phase of its evolution. This is exactly what the new observations confirmed.
“The release of nuclear energy by the flash forces the already very compact star to expand back to giant dimensions — the born-again scenario,” Dr. Reindl said.
It is not the only example of such a star, but it is the first time ever that a star has been observed during both the heating and cooling stages of such a transformation. Yet no current stellar evolutionary models can fully explain SAO 244567’s behavior.
“We need refined calculations to explain some still mysterious details in the behavior of SAO 244567,” Dr. Reindl said.
“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.”
The team’s findings were published online September 12, 2016 in the Monthly Notices of the Royal Astronomical Society.
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Nicole Reindl et al. Breaking news from the HST: The central star of the Stingray Nebula is now returning towards the AGB. MNRAS, published online September 12, 2016; doi: 10.1093/mnrasl/slw175
