A huge cloud of expanding supernova debris was found hidden in plain sight

When a massive star explodes at the end of its life, it creates one of the most ostentatious displays in the Universe: a supernova.

This is a colossal release of energy that propels the outer layers of the star outward at speeds of an appreciable fraction of the speed of light. For a brief time, this single object can outshine an entire galaxy of billions of stars.

So you'd think hiding the results of one would be tough. But, it turns out, that can in fact be the case. Astronomers just discovered the expanding debris from one that's relatively nearby yet so faint it escaped notice for decades.

The technical name of this supernova remnant is SNR G249.5+24.5, but the astronomers who discovered it dubbed it Hoinga, after the medieval name of the first author's hometown, Bad Hönningen am Rhein.

The Hoinga supernova remnant (upper right) seen in X-rays (converted to colors representing the energies of the X-rays) by eROSITA. The bright blob at the bottom is actually two supernova remnants: Vela and Puppis A. Credit: SRG/eROSITA

They found it in observations taken by a mission called eROSITA (extended Röntgen Survey Imaging Telescope Array; Röntgen is the German word for X-rays, after their discoverer). This is a space-based X-ray observatory on board a German-Russian spacecraft called Spectrum-Roentgen-Gamma, which orbits the Sun about 1.5 million kilometers from the Earth in a spot called the L2 point, a volume of space where a spacecraft can remain semi-stable without too much effort.

eROSITA can survey the entire sky in X-rays every six months, and is planned to complete eight such surveys over its four year mission. X-rays are high-energy forms of light that are emitted from extremely energetic sources like black holes, pulsars, and exploding stars. In this case, X-rays are emitted by the expanding debris from an exploded star as the gas is accelerated by shock waves, or powerful magnetic fields accelerate electrons to nearly the speed of light.

X-ray map of the Hoinga supernova remnant, showing it has an overall circular shape with some of it missing (perhaps absorbed by material in the foreground). Credit: SRG/eROSITA/MPE

The astronomers inspected the images taken by eROSITA in the hopes of finding new supernova remnants, and Hoinga was the first they found. It's big, about 4° across — eight times wider than the full Moon on the sky! — but quite faint. Once they saw it they looked at old X-ray and radio wave surveys (many remnants are bright in radio) and found it in a few… but that was because they knew where to look. The remnant is so large and faint that it was missed before. Also, it's in a part of the sky off the galactic plane, where you don't expect to find supernova remnants as often, and in a direction where there are a lot of background sources confusing the images. It's no wonder no on spotted it before.

It appears to be a classic spherical shell, made as the expanding debris pushes against the gas between the stars, snowplowing it up. It's also filled with hot gas that emits X-rays as well, something that's also commonly seen.

Hoinga seen in radio waves in a survey taken by the Australian Continuum HI Parkes All-Sky Survey. It’s large and very faint, which is in part how it’s been missed for years. Credit: CHIPASS/SPASS/N. Hurley-Walker, ICRAR-Curtin

However, not much else is known about it. They looked for some ways to ID what kind of star exploded and what the gas is made of, but came up empty. They're not even sure it was the result of a massive star exploding; that usually leaves behind a pulsar but none was found. They note that sometimes the huge explosion can give a brobdingnagian kick to the pulsar, sending it flying through space. If the remnant is more than about 17,000 years old then that's enough time for the pulsar to physically leave the remnant, making the search more difficult. They're hoping to follow up on that. But it's possible that this is what's called a Type I supernova, where a white dwarf explodes. That kind of supernova can leave nothing behind, which may be why nothing is seen.

Weirdly they can't even be sure of its physical size and distance. They compare it to the better-studied Vela supernova remnant, and if it's the same size that would put it about twice as far, so roughly 1,500 light years away. That's pretty close; the famous Crab Nebula supernova remnant is 6,500 light years away, and visible in binoculars.

A composite of Hoinga in X-rays (magenta) and radio (blue). Extremely hot gas filling the remnant emits X-rays, while electrons whizzing around magnetic fields at the edge of the shell structure generate radio waves. Credit: eROSITA/MPE (X-rays), CHIPASS/SPASS/N. Hurley-Walker, ICRAR-Curtin (Radio)

The reason Hoinga is so faint and big is that it's old. They aren't sure how old, but it may be at least 20,000 years old. Over time the gas slows and cools, making it fainter and harder to spot.

In fact that's something of an issue. A star explodes in the Milky Way galaxy roughly two or three times per century. We should be able to see them for about 100,000 years, and astronomers calculate that means there should be 1,200 in the galaxy. However, only 300 are known. Some may be too faint to spot, or hidden on the other side of the galaxy, or faded faster than usual. But it's still an open question why so few are seen compared to what's expected.

Hoinga and eROSITA may help there. If more are found in the X-ray data, and Hoinga can be studied at other wavelengths to get more physical data about it, that could solve this mystery of the missing supernova remnants.

There could be up to a thousand such objects in the galaxy, the expanding debris from titanic explosions littering the sky, yet almost totally invisible unless you know exactly what to look for.

Amazing. I keep saying the sky is full of treasures we have yet to find, and here the sky is helpfully proving me right once again.