It’s amazing what can hide in space, hidden by the limitations of our eyes, our technology and our prejudices.
Astronomers have just found an absolutely colossal supernova remnant, a growing cloud of dust and gas left over from a stellar explosion, which covers an area of the sky almost 100 times larger than the full moon (from our perspective) , at a maximum distance of 4,000 light-years from Earth.
A team of astronomers led by Werner Becker of the Max Planck Institute of Extraterrestrial Physics in Germany named the remnant Hoinga after the medieval name of Becker’s hometown.
How can we miss it? The reason for this is that it is only visible in X-rays, and only for one of the most powerful X-ray telescopes we have built so far, the space-based eROSITA launched in 2019.
(eROSITA / MPE (X-ray) / CHIPASS / SPASS / N. Hurley-Walker, ICRAR-Curtin)
Above: Hoinga X-ray and radio composite.
“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.
“If we add excitement, Hoinga is the largest supernova remnant ever discovered by X-rays in terms of apparent size: about 90 times the full moon.”
Supernovae have two main triggers. One is the death of a massive star. When the material is up to fuse their core, the consequent drop in the outward thermal pressure means that the pressure is no longer sufficient to lower the star under the inward pressure of gravity, and the whole thing goes awry. to collapse the core. a neutron star or black hole (or completely erased).
The other trigger is a type of Ia supernova, in which a white dwarf star – the folded core of a low-mass precursor star – swallows so much material from a binary companion that it becomes unstable and comes to the same end.
In both scenarios, an expanding shell of the star’s outer material is blown into space, creating shock fronts where it falls into the interstellar medium. This is the supernova remnant.
Most stars in the Milky Way have a low mass – it is estimated that 90 percent of all stars are dwarfs that do not end up in a supernova (stars that are currently ‘alive’ melting from nuclei in their nucleus), and another 9 percent are dead white dwarfs.
Although there are an estimated 100 billion stars in the Milky Way, supernova explosions are rare; astronomers estimate that one has to leave 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 found in the Milky Way; but we only know of about 300. Which means that our calculations are out or that we could not locate them for whatever reason. This is where eROSITA comes in.
Most astronomical objects emit X-rays, invisible to the naked eye. eROSITA, designed to do a sky survey, is far more sensitive than its predecessor and reveals X-ray objects we’ve never seen before.
Previously unknown supernova remnants must be traced 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 checked their findings with radio astronomical data and found that there was vague evidence that Hoinga was dating for a decade. It even appears faintly in ROSAT data taken 30 years ago.
“By searching the archive of radio data, we discover that Hoinga was sitting there waiting to grow up to ten years old, but because it was high above the level of the Milky Way, it was missed,” Hurley-Walker explained .
‘Supernova remnants are not usually expected to be found at high galactic latitudes. Therefore, these areas are usually not the focus of surveys, which means that even more of these can be overlooked waiting to be discovered. ‘
The team calculated, based on radio data, that the remnant is between 21,000 and 150,000 years old (but probably at the younger end of the range), and that it is relatively close to Earth, between 1,470 and 3,915 light-years away.
They also could not find the remnant of the progenitor star, indicating that the explosion was type Ia. It also corresponds to the location, as massive stars tend to concentrate in the galactic plane.
eROSITA will conduct a total of eight overall surveys. The team hopes that data from future surveys will help uncover the nature of Hoinga – and find many more of the ‘missing’ supernovae of the Milky Way.
The research of the team should appear in Astronomy & Astrophysics, and is available on arXiv.