It is amazing what can be hidden in space, hidden by the limitations of our eyes, our technology and our preconceptions.
Astronomers have just found a remnant of an absolutely colossal supernova, an expanding cloud of dust and gas left over from a stellar explosion, occupying an area of the sky almost 100 times larger than 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 named the remnant Hoinga after the medieval name of Becker’s hometown.
How could we miss him? The reason is that it is visible only in X-rays and only for one of the most powerful X-ray telescopes we have built so far, eROSITA based on space launched in 2019.
(eROSITA / MPE (X-ray) / CHIPASS / SPASS / N. Hurley-Walker, ICRAR-Curtin)
Above: radiograph and composite radio Hoinga.
“The eROSITA telescope, which is on board the Russian-German satellite SRG, is 25 times more sensitive than its predecessor ROSAT, so we expected to discover new supernova remnants in the coming years, but we were pleasantly surprised to appear one immediately, ”said astronomer Natasha Hurley-Walker at the Curtin University node of the International Center for Radio Astronomical Research in Australia.
“In addition to our enthusiasm, 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 is the death of a massive star. When they run out of material to merge into their cores, the resulting decrease in external thermal pressure means that the pressure is no longer enough to prevent the star from collapsing under the pressure of internal gravity, and the whole thing moves, collapsing the core into a neutron star. or a black hole (or completely cancels it).
The other trigger is a type Ia supernova, in which a white dwarf star – the collapsed core of a low-mass parent star – removes so much material from a binary companion that it becomes unstable and achieves the same goal.
In both scenarios, an expanding shell of the star’s outer material is thrown into space, creating shock fronts where it hits the interstellar environment. This is the remnant of the supernova.
Most Milky Way stars have low mass – about 90 percent of all stars are main-sequence dwarfs that will not reach a supernova (stars that are currently “alive” merging into their nuclei) and another 9 percent are dwarfs. dead whites.
So, although there are about 100 billion stars in the Milky Way, supernova explosions are rare; astronomers estimate that it should leave every 30 to 50 years, leaving behind a bright, energetic cloud that lasts about 100,000 years.
The artist’s impression of a supernova. (ESA / Hubble, CC BY 4.0)
At this rate, there should be about 1,200 supernova remnants currently detectable in the Milky Way; but we only know about 300. Which means that either our calculations are disabled or we haven’t been able to detect them, for whatever reason. This is where eROSITA intervenes.
Most astronomical objects emit X-rays, invisible to the naked eye. eROSITA, designed to conduct a full survey, is much more sensitive than its predecessor and has revealed X-ray objects that we have never seen before.
Remains of previously unknown supernovae should be detected by eROSITA, but even so, Hoinga was a surprise, not only because it was found so quickly, but because of where it was found – far from the galactic plane, where most Milky Way stars (and therefore supernova remnants) live.
The team checked its results against radio astronomy data and found weak evidence that Hoinga dates back a decade. It appeared even weaker in the ROSAT data taken 30 years ago.
“Researching the archive radio data found that Hoinga was sitting there waiting to be discovered in surveys up to the age of ten, but because it was high above the Milky Way, it was missed,” Hurley-Walker explained.
“Supernova remnants are not usually expected to be found at high galactic latitudes, so these areas are not usually the focus of investigations, which means there may be more of these remnants overlooked that 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 younger end of that area) and is relatively close to Earth, between 1,470 and 3,915 light-years away.
They also could not find the remnant of the parent star, suggesting that the explosion was type Ia. This is also consistent with location, as massive stars tend to focus in the galactic plane.
eROSITA will conduct a total of eight surveys across the sky. The team hopes that data from future surveys will help reveal the nature of Hoinga – and find many of the “missing” supernovae in the Milky Way.
The team’s research is to appear in Astronomy and astrophysics, and is available on arXiv.