Astronomers Might Have Seen a Star Just Disappear. Turning Straight to a Black Hole Without a Supernova

Large stars have violent deaths. As they run out of hydrogen to fuse, the star’s weight squeezes its core to make it increasingly hot and dense. The star fuses heavier elements in a last-ditch effort to keep from collapsing. Carbon to Silicon to Iron, each step generating heat and pressure. But soon it’s not enough. The fusion even heavier elements don’t give the star more energy, and the core quickly collapses. The protons and neutrons of nuclei collide so violently that the resulting shock wave rips the star about. The outer layers of the star are thrown outward, becoming a brilliant supernova. For a brief time, the star shines brighter than its entire galaxy, and its core collapses into a neutron star or black hole. It was thought that all large stars end with a supernova, but new research finds that might not be the case.

A multi-wavelength layout of the Crab Nebula. Credit: (Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL/Caltech; Radio: NSF/NRAO/VLA; Ultraviolet: ESA/XMM-Newton).

The evidence comes from a galaxy 75 million light-years away known as the Kinman Dwarf Galaxy. Between 2001 and 2011, several groups of astronomers were studying the spectra of this galaxy because it has a particularly low metallicity. Because the galaxy is small and far away, astronomers can’t see all the individual stars, but the galactic spectra allow them to identify some of the bright stars by particular emission lines. One of these was a luminous blue variable star.

Luminous Blue Variables (LBVs) are very large stars in their elder days. They have active and quiet periods and can shine 2.5 million times brighter than the Sun. The bright emission lines of LBVs are unusual for a blue giant star, so they are easy to identify. Since astronomers saw these lines in the spectra of the Kinman Dwarf Galaxy, they knew the galaxy contained such a star.

Line spectra of a luminous blue variable star. Credit: Campagnolo et al., 2017

But when new spectra measurements of the Kinman Dwarf Galaxy were taken in 2019, the spectral lines of the LBV were gone. They had simply vanished as if the star wasn’t there. It seemed the star had disappeared. But giant blue stars don’t just disappear. They should explode as a supernova before fading away. This particular star didn’t become a supernova, so something strange is going on.

Image of the Kinman Dwarf galaxy, also known as PHL 293B. Credit: NASA, ESA/Hubble, J. Andrews (U. Arizona)

Since astronomers haven’t observed the star directly, it’s possible that the star did become a supernova and we happened to miss it. But that is highly unlikely. The Kinman Dwarf Galaxy is regularly observed, and a supernova would be difficult to miss. It’s more likely that the star did fade from view, and the authors of this latest research have a couple of ideas why.

One idea is that the star has entered a particularly quiet stage at the same time it happens to be obscured by dust in the galaxy. But a second idea is more interesting. Perhaps the star underwent an unusual core collapse, forming a black hole without undergoing a supernova. There has been some debate over whether this is possible, and this new work could be evidence to support the idea.

But this is only one star and not even one that was directly imaged. It will take much more work to determine what really happened.

Reference: Allan, Andrew P., et al. “The possible disappearance of a massive star in the low-metallicity galaxy PHL 293B.” Monthly Notices of the Royal Astronomical Society 496.2 (2020): 1902-1908.