It’s amazing what’s lurking in space, hidden by the limitations of our eyes, our technology, and our prejudices.
Astronomers have only just found an absolutely colossal supernova remnant, an expanding cloud of dust and gas left over from a stellar explosion, occupying an area of the sky nearly 100 times that of the full moon (from our perspective), at a maximum distance. from 4,000 light years from Earth.
A team of astronomers led by Werner Becker of the Max Planck Institute for Extraterrestrial Physics in Germany named the remnant Hoinga, after the medieval name of Becker’s birthplace.
How can we miss it? The reason is that it’s only visible in X-rays, and only for one of the most powerful X-ray telescopes we’ve built to date, the space-based eROSITA launched in 2019.
(eROSITA / MPE (X-ray) / CHIPASS / SPASS / N. Hurley-Walker, ICRAR-Curtin)
Above: X-ray and radio composite by Hoinga.
“The eROSITA telescope, which is aboard the Russian-German SRG satellite, is 25 times more sensitive than its predecessor ROSAT, so we expected to discover new supernova remnants in the coming years, but were pleasantly surprised that one would appear immediately, said astronomer Natasha Hurley-Walker of the Curtin University node of the International Center for Radio Astronomy Research in Australia.
“Adding to our excitement is that Hoinga is the largest supernova remnant ever discovered by X-rays, in terms of apparent size – about 90 times larger than the full moon.”
Supernovae have two main triggers. One of these is the death of a massive star. When they run out of material to melt into their cores, the resulting drop in thermal pressure outward means that the pressure is no longer enough to keep the star from collapsing under the inward pressure of gravity, and the whole thing goes kaboom, where the core collapses. a neutron star or a black hole (or obliterating it completely).
The other trigger is a type Ia supernova, in which a white dwarf star – the collapsed core of a low-mass ancestor star – gulps down so much material from a binary companion that it becomes unstable and reaches the same end.
In both scenarios, an expanding shell of the star’s outer material is blown into space, creating shock fronts where it collides with the interstellar medium. That’s the remnant of the supernova.
Most of the stars in the Milky Way are low in mass – an estimated 90 percent of all stars are main sequence dwarfs that will not enter a supernova (stars that are currently ‘alive’ and nuclei fusing in their cores), and another 9 percent are dead white dwarfs.
So while there are an estimated 100 billion stars in the Milky Way, supernova explosions are rare; astronomers estimate that one should disappear every 30 to 50 years, leaving behind a glowing, energetic cloud that lasts about 100,000 years.
Artist’s impression of a supernova. (ESA / Hubble, CC BY 4.0)
At that rate, there should be about 1,200 supernova remnants currently detectable in the Milky Way; but we only know about 300 or so. Which means that our calculations are wrong, or that we just couldn’t detect them for some reason. This is where eROSITA comes into play.
Most astronomical objects emit X-rays, invisible to the naked eye. Designed to conduct a full-sky survey, eROSITA is much more sensitive than its predecessor and has revealed X-ray objects we’ve never seen before.
Previously unknown supernova remnants should be detected by eROSITA, yet Hoinga was a surprise not only because it was found so quickly, but also because of where it was found – far from the galactic plane, where most of the stars of the Milky Way (and therefore remain supernova remnants).
The team double-checked their findings against radio astronomy data and found vague evidence of Hoinga dating back ten years. It even appeared faintly in ROSAT data taken 30 years ago.
“By searching archive radio data, we found that Hoinga was there waiting to be discovered in studies up to ten years old, but being high above the plane of the Milky Way, it was missed,” explains Hurley-Walker.
“Supernova remnants are not expected to be found at high Galactic latitudes, so these areas are usually not the focus of studies, meaning there may be more of these overlooked remains waiting to be discovered.”
The team calculated from that radio data that the remnant is between 21,000 and 150,000 years old (but probably on the younger side of that range) and that it is relatively close to Earth, between 1,470 and 3,915 light-years away.
They also couldn’t find the remnant of the ancestor star, suggesting the explosion was Type Ia. This also matches the location, as massive stars tend to concentrate in the galactic plane.
eROSITA will conduct a total of eight star surveys. The team hopes data from future studies will help reveal Hoinga’s nature – and find many more of the Milky Way’s ‘missing’ supernovae.
The team’s investigation will appear in Astronomy and Astrophysics, and is available on arXiv.