A star recently went missing. This is one of those stories where I took the time to read the research paper in detail because the press release headline just seemed way too fantastic – stars shouldn’t just disappear. But this one did.
About 75 million light-years away there is a dwarf galaxy called PHL 293B. This particular dwarf galaxy is very metal-poor, with stars having just one-tenth the heavy elements as we see in our Sun. This metal-poor system, however, underwent star formation in the not too distant past, and in 2001 astronomers were able to identify the light of what looks like a luminous blue variable in this system. This bright blue star is the end stage of a massive star’s life, and what happens during this phase in a star’s evolution will determine how the star dies. Luminous blue variables are characterized by massive winds and hydrogen emission lines.
Characterizing any one star in PHL 293B is tricky. This system is so far away that individual stars can’t be resolved, and the studies have to look at the aggregate light from the population. That said, while it’s not possible to point at an image and say “that’s the luminous blue variable”, it is possible to look at the spectra and say, “These lines indicate something consistent with a luminous blue variable is present.” Spectra taken in 2001, 2002, 2009, and 2011 all show these kinds of blue variable lines, but in data from 2016 and 2019, these features are just gone.
The easiest way to interpret these data is to say there was a luminous blue variable, and now its light has either been obscured or the star has stopped generating light. This could happen if the massive outflows produced a cloud of obscuring dust and gas that is blocking the star’s light from us, the way squid ink can obscure a squid from its enemies. This could happen if the star somehow quietly collapsed into a black hole without a supernova. Again, we don’t totally understand how low metallicity stars evolve, and without data – data this team was trying to get when this star went missing – we can’t know what kinds of things to expect.
In addition to these “it was a luminous blue variable that disappeared” theories, there is also the possibility that the original observers misunderstood what they were looking at, and that the 2001-2011 set of observations were of an expanding supernova remnant from a supernova that occurred between 1995 and 1998, during a period of time when this field wasn’t being observed.
This is the kind of science that can drive an observer crazy. We have a single example of this kind of star, and something happened, and the data isn’t quite good enough to know exactly what happened. The observations in this paper required the full power of all four of the Very Large Telescopes working together; getting better data isn’t possible until the next generation of tens-of-meter telescopes is built. Until then, all we know is there was something bright and blue with outflows consistent with being a metal-poor luminous blue variable, and that something is now gone.
“The Possible Disappearance of a Massive Star in the Low Metallicity Galaxy PHL 293B,” A. Allan et al., 2020 June 30, Monthly Notices of the Royal Astronomical Society (Preprint)