It’s amazing what can be lurking in space, hidden by the limitations of our eyes, our technology, and our preconceptions.
Astronomers have only found an absolutely colossal supernova remnant, an expanding cloud of dust and gas left over from a stellar explosion, which occupies an area of the sky almost 100 times that of 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 for Extraterrestrial Physics in Germany has named the remnant Hoinga after the medieval name of Becker’s hometown.
How could we miss it? The reason 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: X-rays and Hoinga radio compounds.
“The eROSITA telescope, which is on board the Russian-German SRG satellite, is 25 times more sensitive than its predecessor ROSAT, so we were expecting to discover new traces of supernova in the coming years, but we were pleasantly surprised that one would show up right away, ”he said. astronomer Natasha Hurley-Walker of the Curtin University node at Australia’s International Radio Astronomy Research Center.
“In addition to our excitement, 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 them is the death of a massive star. When the material is finished to melt into the cores, the resulting drop in thermal pressure to the outside means that the pressure is no longer enough to prevent the star from collapsing under the inner pressure of gravity, and it all sinks, collapsing the nucleus into a neutron star or 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 parent star – escapes 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 thrown into space, creating shock fronts where it strikes the interstellar medium. This is the remnant of the supernova.
Most Milky Way stars are low-mass – it is estimated that 90 percent of all stars are main-sequence dwarfs that will not end up in a supernova (stars that are currently “alive” merging nuclei in their nuclei), and another 9 percent are dead white dwarfs.
Thus, although it is estimated that there are 100 billion stars in the Milky Way, supernova explosions are rare; astronomers estimate that it should be fired every 30 to 50 years, leaving behind a bright, energetic cloud that lasts about 100,000 years.
Artist’s impression of a supernova. (ESA / Hubble, CC BY 4.0)
At this rate, there should be about 1,200 remnants of supernovae currently detectable in the Milky Way; but we only know about 300 of them. Which means our calculations are turned off or we just couldn’t detect them, for whatever reason. This is where eROSITA comes in.
Most astronomical objects emit X-ray, invisible to the naked eye. eROSITA, designed to conduct a sky-wide survey, is much more sensitive than its predecessor and has revealed X-ray objects we had never seen before.
EROSITA should have detected remnants of hitherto unknown supernovae, but even so, Hoinga was a surprise, not only because it was found so quickly, but where it was found, far from the galactic plane, where most of it was found. the stars in the Milky Way (and therefore remnants of supernovae) reside.
The team checked their findings against radio astronomy data and found weak evidence that Hoinga dates back a decade. It even appeared weakly in ROSAT data taken 30 years ago.
“Upon examining the archive radio data, we discovered that Hoinga had been sitting there waiting to be discovered in surveys up to ten years old, but as he was well above the Milky Way plane, he he lost, ”Hurley-Walker explained.
“Remains of supernovae at high galactic latitudes are not normally expected to be found, so these areas are not usually the focus of surveys, which means there may be even more of these ignored remains waiting to be discovered. “.
The team calculated, based on these radio data, that the remnant is between 21,000 and 150,000 years old (but probably at the youngest end of this range) and is relatively close to Earth. , between 1,470 and 3,915 light-years away.
Nor could they find the remnant of the parent star, which suggested that the explosion was type Ia. This is also consistent with location, as massive stars tend to concentrate in the galactic plane.
eROSITA will conduct a total of eight sky-wide surveys. The team hopes that data from future surveys will help reveal the nature of Hoinga and find many more of the “missing” supernovae in the Milky Way.
Team investigations should appear in Astronomy and astrophysics, and is available at arXiv.