Stellar evolution observed in real time

Astronomers using the Hubble Space Telescope have been able to observe a key stage in the evolution of a star in real time.

This image of the Stingray nebula, a planetary nebula 2700 light-years from Earth, was taken with the Wide Field and Planetary Camera 2 (WFPC2) in 1998. In the centre 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.

This image of the Stingray nebula, a planetary nebula 2700 light-years from Earth, was taken with the Wide Field and Planetary Camera 2 (WFPC2) in 1998. In the centre of the nebula the fast evolving star SAO 244567 is located. Image Credit: ESA/Hubble & NASA

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The Hubble Space Telescope has been used to observe the evolution of a star in real time.

Star SAO 244567 has been increasing in temperature dramatically over the past 30 years and is now cooling, following a nuclear flash that has placed it in an earlier phase of stellar evolution.

Most cosmic occurrences happen on too vast a scale for humans to observe within one lifespan, but this latest Hubble study is an exception

SAO 244567 is 2,700 lightyears from Earth and is the central star of the Stingray Nebula.

Between 1971 and 2002, its surface temperature rose by nearly 40,000°C.

New observations have revealed the star has begun to cool and expand.

Animation depicting the evolution of star SAO 244567 as it heats, goes through a nuclear flash, then expands and cools.NB: this video has no audio. Credit: ESA/Hubble, L. Calçada

The star’s rapid rise in temperature could be explained, says the study, if it is assumed that it had an initial mass of three to four times that of the Sun.

But astronomers have concluded that the star had an original mass similar to that of the Sun.

Low-mass stars like our Sun normally evolve over longer timescales, so the fact that SAO 244567 heated up so dramatically remained a mystery.

The team behind the study suggested in 2014 that the rise in temperature was due to a helium-shell flash event, in which helium just outside the stellar core is ignited, causing a nuclear flash.

The study has confirmed that this is the case and that the flash is causing the star to expand and cool again, bringing it back to its previous phase of stellar evolution.

“SAO 244567 is one of the rare examples of a star that allows us to witness stellar evolution in real time”, says lead author of the study Nicole Reindl of the University of Leicester.

“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.

The release of nuclear energy by the flash forces the already very compact star to expand back to giant dimensions: the born-again scenario.”

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Astronomers were aware of this process in theory, but Hubble has enabled them to observe it in real time for the first time ever.