The mystery of the Milky Way’s most bizarre supernova

Across the cosmos, only two main pathways exist for making a supernova.

Many of the cataclysms that occur in space are typical supernovae: either core-collapse from a massive progenitor star or type Ia from an exploding white dwarf. The most massive stars of all have hundreds of times the mass of the Sun and live just 1 or 2 million years, total, before running out of fuel and dying in such a cataclysm.

One is when a very massive star reaches the end of its life.

This image of the Cassiopeia A supernova remnant shows the aftermath of a type II, core-collapse supernova that occurred more than 350 years ago. The supernova remnant glows in a variety of electromagnetic wavelengths, including in various X-ray and infrared bands, with the latter shown here. The color-coding reveals the diversity of elemental signatures found within.

Its core collapses and implodes, leaving a neutron star or black hole remnant.

The anatomy of a very massive star throughout its life, culminating in a type II (core-collapse) supernova when the core runs out of nuclear fuel. The final stage of fusion is typically silicon-burning, producing iron and iron-like elements in the core for only a brief while before a supernova ensues. The most massive core-collapse supernovae typically result in the creation of black holes, while the less massive ones create only neutron stars.

The other is when a white dwarf detonates due to mergers or accretion.

This image shows the remnants of a type Ia supernova. The second most common type of supernova in the Universe behind core-collapse supernovae, we have now observed 1550 of these events through modern telescopes, enabling us to understand the history and composition of our Universe as never before. However, only a small percentage of all the stars that have ever formed have completed their life cycles, dying and resulting in stellar remnants such as white dwarfs, which are needed to create a type Ia supernova event.

Under typical conditions, the progenitor star explodes and is destroyed.

This illustration shows two merging white dwarfs, the preferred theoretical mechanism for the triggering of some, and perhaps most or even nearly all, type Ia supernovae. The double detonation scenario, where a “detonation” event on the surface propagates to the core and causes a detonation that leads to total destruction of the stellar remnant, is one very intriguing theoretical possibility for most type Ia events, although exceptional examples that don’t fit this scenario can be found.

Within our galaxy, only ten supernovae are known across the past 2000 years.

This infographic shows nine of the ten historical supernovae, along with remnants, that have been identified over the past 2000 years within the Milky Way. Not shown is G1.9+0.3, which occurred near the galactic center approximately 155 years ago, but was only discovered posthumously in 1985.

A candidate eleventh, in 1843, turned out to be a supernova impostor.

In 1843, previously modest star Eta Carinae brightened to become the 2nd brightest object in the sky, ahead of Canopus (shown in frame) and trailing only Sirius, the brightest star in Earth’s night sky. Gradually, over the next 13 years or so, it faded until it finally became visible in telescopes only.

Eta Carinae‘s outburst was only a massive star erupting, not exploding.

This image shows the Homunculus Nebula surrounding the massive blue supergiant Eta Carinae. The Eta Carinae Nebula was largely created during an 1843 eruption, where it appeared as a supernova impostor. More distant ejecta indicate a much earlier eruption, while the central star has brightened in recent decades to once again become visible to the naked eye.

Of the known ancient supernovae, only SN 1181 lacked an associated remnant.

This optical image of the suspected location of ancient supernova SN 1181 shows no particularly notable features in this narrow region of sky and is located quite far away from 3C 58. A 2013 discovery of a supernova remnant in infrared wavelengths has its location indicated with the yellow arrow.

The longstanding best remnant candidate, 3C 58, possessed the wrong properties.

The X-ray luminous nebula 3C 58 was long associated as a possible remnant of SN 1181 since its discovery in 1971: the only known such nebula in a similar region of sky to where SN 1181 was spotted. Further analysis of the remnant’s properties, as well as better source localization of SN 1181 from historical sources, have now ruled out 3C 58 as originating from that explosion, instead dating its origin to much earlier: before historical records of astronomical events survive.

Its central pulsar isn’t slowing, and its expanding shell yields a minimum age of ~3500 years.

The X-ray signature of nebula 3C 58 does indeed reveal a supernova remnant, but not one that’s consistent with SN 1181. The central pulsar exhibits no signs of slowing down, whereas the Crab Pulsar, created just 127 years prior, has slowed substantially since its discovery. The size and speed of the nebula indicates an age of 3500-5500 years: much older than the ~840 years that have elapsed since SN 1181 exploded.

Its location also conflicts with the ancient reported observations.

A map of the sky showing traditional eastern and western constellations (gray and colored lines) along with the shaded area of overlap from ancient sources showing the location of SN 1181 (cyan), where it was observed to occur. Supernova remnants 3c 58 and Pa 30 are shown with red crosses, with 3c 58 firmly outside the cyan area but with Pa 30 firmly within it.

However, in 2013, a new candidate remnant was found: Pa 30.

This image of the supernova remnant Pa 30 was first found in data from NASA’s WISE mission by amateur astronomer Dana Patchick in 2013.

Its age, energetics, and expansion properties are consistent with SN 1181.

This image, taken from the 2.4 meter telescope atop Kitt Peak, shows the expanding supernova remnant, Pa 30, with its radial filaments in light emitted by the signature of ionized sulfur. This remnant has now been identified as the aftermath of the ancient supernova SN 1181.

The wind-driven nebula’s filaments look like nothing seen previously.

This dynamical model of the remnant of supernova SN 1181, thought to be a type Iax supernova, shows the remnant of a double degenerate system post-merger, at right, compared with the X-ray data from XMM-Newton at left.

Surprisingly, a central, intact white dwarf remains.

This composite image of supernova SN 1181 showcases the full extent of the nebula in X-rays (blue contours), with optical/infrared and element-specific signatures shown in red and yellow light, respectively.

Its origin: a rare. low-luminosity type Iax explosion.

Back in 2012, a supernova was observed in nearby galaxy NGC 1309: SN 2012Z. The top inset panels show a pre-explosion image from 2005/6, while the lower panels show the post-explosion brightnesses, revealing an extremely faint, new class of supernova explosions, the type Iax class.

Ultimately, a double degenerate merger created this supernova.

Two low-mass white dwarfs, as shown in a simulation here illustrating matter density (top) and the temperature of the surrounding hot disk (bottom) as a function of time. With low enough masses for the original white dwarfs, a surviving hot white dwarf can remain even after the merger concludes.

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