It's a stellar mystery: How did this giant star suddenly disappear from the sky?

For ten years, astronomers had been observing a giant star located in a dwarf galaxy that's 75 million light-years away. The luminous blue variable star was one of the largest of the known universe, and about 2.5 million times brighter than the Sun.

However, one day, the star suddenly vanished, leaving no visible trace behind.

What happened? — Astronomers are not sure what happened to their precious stellar giant, or why its exit was not marked by a supernova, but they think it may be one of two scenarios.

The star either had a massive outburst, or it quietly turned into a black hole. Astronomers first began observing the distant star in the year 2001. The star was located in the Kinman Dwarf galaxy in the constellation Aquarius, and it appeared to be in a late stage of its evolution.

Why it's news — The study of this rare event will appear this week in the journal Monthly Notices of the Royal Astronomical Society.

Where is this star in space? — Because the star is located in a galaxy that's quite far, far away, astronomers could not observe the star directly but rather detect its signature. And from the years 2001 to 2011, they were picking up consistent evidence that a large, and rather unstable luminous blue variable star resided in the dwarf galaxy.

Blue variables are unstable stars, displaying dramatic shifts in their brightness and spectra, or thermal radiation emitted by the star. These types of stars leave a distinct mark on the galaxies that host them and are therefore easily detectable.

How they learned of this mystery — The team behind the new study was interested to see how these massive stars end their life. Therefore, in August, 2019, they directed the European Southern Observatory’s (ESO) Very Large Telescope towards the location of the star. However, they could no longer find it, nor any traces of its possible extinction.

They tried again a few months later, and any evidence of the once luminous star was now gone.

"We were surprised to find out that the star had disappeared!” Andrew Allan, a PhD student at Trinity College in Dublin, Ireland, and lead author of the new study, said in a statement.

Why do stars suddenly disappear? — Stars don't just suddenly disappear all of sudden. Instead, when a star nears the end of its life, it goes out in quite a showing known as a supernova. As a star runs out of fuel, it collapses under the weight of its own gravity and sheds its material in an explosive death.

A star as big as the one that was being observed should have left behind a massive explosion that scientists would have been able to see. Instead, there was nothing.

In order to solve this mystery, the team turned to older data collected on the star in the years 2002 and 2009, located in the ESO Science Archive Facility.

“The comparison of the 2002 high-resolution UVES spectra with our observations obtained in 2019 with ESO's newest high-resolution spectrograph ESPRESSO was especially revealing, from both an astronomical and an instrumentation point of view,” Andrea Mehner, a staff astronomer at ESO in Chile, who took part in the study, said in a statement.

Based on their observations, the team of researchers came up with two possible scenarios for the sudden disappearance of the star.

Theory #1: a stellar outburst — Older observations of the star revealed that it had been undergoing a strong period of outbursts that likely ended in 2011.

Stars of this kind go through giant outbursts throughout their existence, which causes them to experience a loss of mass and brightness at a much higher rate. As a result, the researchers believe that the once massive, bright star turned into a smaller, dim star that is partly obscured by dust, making it difficult to spot in the distant galaxy.

Theory #2: a black hole — Another possible scenario is that the star suddenly collapsed into a black hole without leaving behind traces of a stellar explosion, or supernova. However, that would make this an extremely rare event since most stars go out with a bang.

“We may have detected one of the most massive stars of the local Universe going gently into the night,” Jose Groh, a researcher at Trinity College Dublin, and co-author of the study, said in a statement.

What's next — Astronomers will have to investigate this mysterious case further in order to figure out what exactly happened to this once giant star, and they plan on doing it using ESO's upcoming Extremely Large Telescope. The telescope, set to begin observations in 2025, will be able to observe stars in distant galaxies with great detail in order to resolve stellar mysteries.

Abstract: We investigate a suspected very massive star in one of the most metal-poor dwarf galaxies, PHL 293B. Excitingly, we find the sudden disappearance of the stellar signatures from our 2019 spectra, in particular the broad H lines with P Cygni profiles that have been associated with a massive luminous blue variable (LBV) star. Such features are absent from our spectra obtained in 2019 with the ESPRESSO and X-shooter instruments of the ESO’s VLT. We compute radiative transfer models using CMFGEN that fit the observed spectrum of the LBV and are consistent with ground-based and archival Hubble Space Telescope photometry. Our models show that during 2001–2011 the LBV had a luminosity 퐿∗ = 2.5 − 3.5 × 106 퐿, a mass-loss rate 푀¤ = 0.005 − 0.020 푀 yr−1 , a wind velocity of 1000 km s−1 , and effective and stellar temperatures of 푇eff = 6000 − 6800 K and 푇∗ = 9500 − 15000 K. These stellar properties indicate an eruptive state. We consider two main hypotheses for the absence of the broad emission components from the spectra obtained since 2011. One possibility is that we are seeing the end of an LBV eruption of a surviving star, with a mild drop in luminosity, a shift to hotter effective temperatures, and some dust obscuration. Alternatively, the LBV could have collapsed to a massive black hole without the production of a bright supernova.