Real-time death of a star

Astronomers around the world are witnessing in real time the death of a star named BELLS1 in the Triangulum galaxy
The bright and hot Wolf-Rayet star 124 (WR 124) seen by the James Webb Space Telescope. Credit: NASA

Even stars age and die. But astronomers rarely manage to witness the death of a star in real time. Securing a front-row seat, they have discovered that the unusually chaotic star is heading towards an extraordinarily violent death.

Located in the nearby Triangulum Galaxy (M33), it is in the midst of a transition towards a class of highly unstable stars called Wolf-Rayet stars. Recent observations show that the star has started emitting a new signal that was not seen when the star was first spotted in 2018.

Fate of BELLS1

The new signal was detected through spectral analysis of the star. The wavelengths of the emitted electromagnetic radiation indicate that the star, 25 times the mass of the Sun, is producing carbon and iron through nuclear fusion. This is an unequivocal sign that it is nearing its final destiny: to explode in a spectacular supernova.

“It is really interesting that we were able to see an actual change in the spectrum in just four years” said Olivia Gaunt, a graduate student at Tufts University who is part of the new research. The results shared at the 242nd meeting of the American Astronomical Society are extraordinary. “We believe this could be the first observation of a Wolf-Rayet star evolving in real time”.

Gaunt’s team has named the star BELLS1, an acronym that stands for “broad emission-lined bright sources” the type of wide range of emissions emitted by Wolf-Rayet stars. BELLS1 likely originated as a hot and massive star that quickly depleted its hydrogen reservoir by converting lighter elements into heavier ones through nuclear fusion.

Spectral analysis

The rich spectra detected by Gaunt’s team originate from the furious winds of BELLS1. These winds are expelled at speeds ranging from 3.5 million to 8.7 million km/h and release about 10 solar masses of stellar matter every million years. The discarded stellar material is pumped back into the nearby universe, triggering the formation of future generations of stars and enriching them with recycled elements.

When the team first observed BELLS1 in 2018 using the Keck Observatory atop Mauna Kea volcano in Hawaii, the star exhibited three emission lines. During their observations in 2022, BELLS1 showed a new emission line. This suggests that it has taken another step forward in its short and energetic evolution. BELLS1 is now closer to the end of its 10 million-year lifespan. Once the star has completely depleted its fuel, it will explode in what astronomers call a Type I supernova.

The new observations are exciting but not entirely surprising. “We know that time is short. We expect to see rapid changes” Gaunt said at the press conference on Tuesday. According to NASA, Wolf-Rayet stars like BELLS1 live fast and die hard (ref.). Therefore, observing the evolution and death of a star in real time is a rare and precious opportunity for astronomers. Currently, we only know about 200 stars of this type in the Milky Way. Astronomers suspect that another 1,000 or 2,000 may be out there, but they are obscured by thick layers of dust.

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