There are no supernovae in our sky. Stars live for millions or billions of years. However, given the sheer number of stars in the Milky Way, we should expect these catastrophic star deaths every 30-50 years. Few of these explosions will be on Earth with the naked eye. Nova comes from Latin and means “new”. In the past 2000 years, humans have seen about seven “new” stars in the sky – some bright enough to be seen during the day – until they faded after the first explosion. While we haven’t seen a new star in the sky in over 400 years, we can see the consequences with telescopes – supernova remnants (SNRs) – the hot expanding gases from stellar explosions. SNRs are visible up to 150,000 years before they transition into the galaxy. So after doing the math, there should be around 1200 visible SNRs in our sky, but we only managed to find around 300. That was until “Hoinga” was recently discovered. Named after the hometown of the first author scientist Werner Becker, whose research team found the SNR using the eROSITA all-sky X-ray examination, Hoinga is one of the largest SNRs ever seen.
Composed of the X-ray image (pink) and the radio image (blue) from Hoinga. The X-rays discovered by eROSITA are emitted from the hot debris of the exploded precursor star. Radio antennas on earth record the radiation emission from electrons in the outer shell of the supernova
Photo credits: eROSITA / MPE (Röntgen), CHIPASS / SPASS / N. Hurley-Walker, ICRAR-Curtin (radio)
Hide giant
Hoinga is great. Very large. The SNR extends over 4 degrees of the sky – eight times wider than the full moon. The obvious question – how could astronomers not have found something this huge already? In Hoinga we don’t usually look for supernovae. Most of our SNR searches focus on the plane of the galaxy towards the core of the Milky Way, where we would expect the densest concentration of older and exploded stars. But Hoinga was found at high latitudes outside the plane of the galaxy.
Also, Hoinga is hiding in the sky because it is so big. On this scale, the SNR is difficult to distinguish from the other large dust and gas structures that make up the galaxy known as the “Galactic Cirrus”. It’s like trying to see a single cloud in a cloudy sky. The Galactic Cirrus also outshines Hoinga in the radio light, which is often used to search for SNRs, forcing Hoinga to hide in the background. Regarding cross-references to older Radio Sky surveys, the research team found that Hoinga had been previously observed but was never identified as an SNR due to its comparatively faint glow on the radio. Here eROSITA has the advantage of seeing X-rays. In the X-ray light, Hoinga glows brighter than the Galactic Cirrus, which makes it stand out from the galaxy to be discovered.
Color-coded image of the first x-ray all-sky x-ray examination by eROSITA over a period of six months (red: 0.3-0.6 keV, green: 0.6-1 keV, blue: 1-2.3 keV). (Note 0.1 keV corresponds to a gas temperature of about 1.1 million degrees.) These central bubbles, which rise from the center of the Milky Way (the blue stripe through the middle), were also important discoveries by eROSITA about previous activities at the center of ours Galaxy millennia ago. Credit eROSITA Twitter account
The ancestor
As stars age and burn their hydrogen fuel supply, they will end their lives in different ways depending on their mass. Lower mass stars like our sun swell into red giants and eventually throw their outer layers into space. The star’s spent core resides beneath the cuticle – a highly compressed, hot glowing carbon sphere the size of Earth known as a white dwarf. It’s basically a planet-sized hot space diamond. Not a dramatic explosion. You will cool down over eons and become a “black dwarf”. Amazingly, the universe itself is not old enough for a white dwarf to have completely cooled down to a black dwarf. 99% of the stars will end their lives this way. However, with one push, white dwarf stars can sometimes still generate supernova.
White dwarfs do not generate new energy, but rather bleed residual heat into space. However, if it is entangled in a binary pair with another star by gravity, material can be drawn from the companion star onto the white dwarf. When the white dwarf collects enough material to exceed a critical threshold of 1.44 solar masses (the mass of our sun), a “runaway” reaction occurs, in which a large part of the superdense star is simultaneously undergoing nuclear fusion … in just a few seconds. Temperatures rise to billions of degrees (the core of our own sun is a cool 15 million in comparison) and the star achieves what astronomers calmly call “non-binding energy” – BOOM! White dwarf supernovas are classified as Type Ia supernovas.
The G299 Supernova Remnant is also the product of an exploding white dwarf star. This exploded about 4,500 years ago. Credit NASA Chandra X-ray Observatory
In contrast, the top 1% of the most massive stars become supernova on their own without a companion having to extract material from them. These stars explode, creating exotic objects in the form of black holes or pulsars – a super-super-dense object with multiple solar masses crammed into a 15 km long sphere. Explosions of massive stars are known as “core collapse supernovae” or Type II. Pulsars and black holes are sources of x-rays in the surrounding SNR. However, Hoinga does not have a central X-ray object. There are 11 x-ray point sources (no diffuse gas, but concentrated energy points) that are visibly “inside” the Hoinga SNR and can be pulsars or black holes. However, these sources appear to be in the foreground or background. With no central X-ray source, Hoinga’s ancestor star was likely a white dwarf. Unlike a massive star that explodes, leaving the core that becomes a pulsar or black hole, a white dwarf was the remaining core of a star. If it explodes, the point source will be destroyed.
Different Types of Supernova – Universe Today Video by Fraser Cain
Into the light
Determining the other properties of Hoinga is difficult because the SNR is outside the galactic plane, away from other objects that we can use for reference. When SNRs are in the galactic plane, they are surrounded by pulsars, the distance between which is easier to measure than diffuse gas clouds. There are no known pulsars within 20 degrees of Hoinga in the sky. The research team then offers a distance measurement compared to other known SNRs.
In regions of space like the Magellanic Clouds – satellite galaxies of the Milky Way with massive star-forming regions – we see SNRs of similar brightness and shape as Hoinga with known distances. The researchers draw contrasts and similarities and conclude that the distance to Hoinga must be at least 450 parsecs (about 1470 light years). We also know that most of the Hoinga-shaped SNRs observed are no more than 100 pc (326 light-years) in diameter. Knowing how wide the SNR is, we also get clues as to its distance, which suggests that Hoinga is at most 1200 parsecs (3900 light years) away. So now we have a maximum and a minimum distance.
The region around Hoinga after contamination of sources of distant background objects, closer foreground objects and galactic “cirrus” is filtered out of the image. Hoinga is the crescent-shaped object in the right picture. The bright yellow dot in the upper right is the distant galaxy cluster Hydra A, which is nearly a billion light years away. C. Becker et al. 2021
Researchers can also deduce the distance from observations on another very well-known supernova called Vela. Vela exploded about 12,000 years ago, creating a pulsar. The resulting SNR is one of the most incredible images of space I’ve ever seen. Once we know how bright Vela is, we can compare the two remains as another data point to narrow our range from 450 to 1200 pieces and determine that Hoinga is likely 500 pieces (1630 light years) from Earth.
Energy in the dark
eROSITA made the Hoinga discovery with just one pass of its all-sky x-ray and gave hope that more hidden SNRs could be found. The device scans the entire sky at a speed of 0.025 degrees per second and performs a scan every six months. The first scan was started in July 2019 and completed on June 12, 2020. A total of eight surveys are planned over a period of four years. eROSITA is itself the main instrument on board the Russian-German mission “Spectrum-Roentgen-Gamma” or “SGR”, which was launched from Baikonur Kazakhstan. While several missions carry out all-sky measurements, SGR was the first to carry out an all-sky measurement in the X-ray area.
Outside of the supernova chase, SGR observes the movement of galaxy clusters to gain insight into “dark energy,” the poorly understood force believed to be the cause of the universe’s expansion. Like the upcoming James Webb Space Telescope, SGR is not orbiting the earth, but is parked at “L2” or Lagrange 2, a kind of gravitational pocket created by the interaction of the earth, sun and moon (think how these swirl eddies in the water who follow you in a boat. If foam or dirt gets caught in the vortex, come with you for the ride. Since X-rays are absorbed by the Earth’s atmosphere, you’d better take your X-ray telescope into space, where SGR lives.
Excerpt from the larger SRG / eROSITA all-sky survey image from above. The Hoinga supernova remnant is highlighted. The large bright source in the lower quadrant of the image comes from the supernova remnant “Vela”. The image colors correlate with the energies of the detected X-ray photons. Red stands for the energy range from 0.3 to 0.6 keV, green for 0.6 to 1.0 keV and blue for 1.0 to 2.3 keV. Image and text credit. SRG / eROSITA
With each pass of the All-Sky survey, more details about objects like Hoinga are revealed. Combined with other ongoing all-sky surveys and huge new telescope projects, we are collecting more data about the sky than ever before. We’ll likely find a lot more SNRs and cooler things about the universe that will ultimately help us understand ourselves. Supernova made us! Stars cook elements that are as heavy as nickel and iron, but everything heavier than them in the periodic table is created by these star explosions, which then sow the raw material of our existence through the cosmos. We have evidence that our own solar system was enriched with supernova debris as early as 4.567 billion years ago. Seeing the remains of those explosions means knowing better the forces that made us know.
eROSITA Mission Overview Animation – Credit eROSITA
More to discover:
Press release article: Hoinga SNR | Max Planck Institute for Extraterrestrial Physics (mpg.de)
Original research publication (Open Access)
Astronomy without a telescope – alchemy of supernova – universe today
eROSITA (@eROSITA_SRG) / Twitter
eROSITA | Max Planck Institute for Extraterrestrial Physics (mpg.de)
Vela Supernova residual mosaic | Science Mission Directorate (nasa.gov)
Bad astronomy | The remnant of the Vela supernova is an area of chaos in the sky (syfy.com)
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