This Weird Star Survived a Supernova Only to Shine Even More Brightly Than Before

Nothing comes close to the end of a white dwarf when it comes to going out in style. One of the universe's most potent explosions, their thermonuclear self-destruction forces the star to blink out of existence in a blaze of splendor.

That is, at least, the concept. A finding reveals that certain white dwarfs put on a poor performance to simulate their demise before continuing to shine considerably brighter than before.

Supernova SN 2012Z, which shone briefly in a swan song that should have signaled its extinction, was discovered in the neighboring spiral galaxy NGC 1309 ten years ago.

It was easy to determine which star went supernova by looking at subsequent photographs and identifying the now-empty gaps since images of its parent galaxy had been taken years earlier.

"We were expecting to see one of two things when we got the most recent Hubble data. Either the star would have completely gone away, or maybe it would have still been there, meaning the star we saw in the pre-explosion images wasn't the one that blew up," according to Curtis McCully, an astronomer from UC Santa Barbara.

"Nobody was expecting to see a surviving star that was brighter. That was a real puzzle."  

Even though it was unexpected, the finding wasn't wholly new, adding to the increasing body of proof that white dwarf stars might have life after they die.

When a star with the mass of our Sun converts all of its remaining helium into carbon and oxygen, it condenses into a small, compact sphere that is very hot and dense. It simmers away for ages, cooling because it lacks the bulk to create larger components, and ultimately fading into a cold, black ball.

Life could continue for a little while longer while it siphons out a little additional gas if a generous companion star is circling close by to such a depleted stellar core.

However, at a certain point, all that extra mass runs the danger of driving the carbon into fusion and setting off a runaway reaction that releases a massive amount of energy in a flash, rupturing the star into a Type Ia supernova and destroying it.

Normally, nothing significant remains in the white dwarf's former location; instead, a spreading cloud of star remnants drifts into space, dimly glowing from leftover radiation.

These precise blasts are useful for measuring distances throughout the Universe since they all burn at nearly the same brightness due to their precision.

However, not all explosions are created equal. The more frequent Type Iax supernovae erupt slowly and with a relatively quiet whimper rather than with a bang like fireworks.

With evidence of high-density materials and the telltale characteristics of a thick photosphere found in the aftermath of a few of these less striking supernovae, they might not even be all that destructive.

Color images of NGC 1309 both before and after SN 2012Z. The left panel shows the Hubble Heritage (pre-explosion) image of NGC 1309. The top-middle panel shows a zoom-in on the position of the supernova from the pre-explosion image. The top-right shows SN~2012Z from the 2013 visit. The middle-bottom panel shows the location of SN~2012Z in the latest observations in 2016. The bottom-right panel shows the difference image between the pre-explosion images and the observations from 2016.

There is no doubt that in some, if not many cases, white dwarfs may remain intact even after turning thermonuclear, given the discovery of SN 2012Z glowing ferociously after its own supernova.

It's unclear why this specific star not only stopped short of destroying itself but also managed to come back considerably brighter. The discovery's researchers hypothesize that the blast just shook things up, allowing the material to eventually settle back into a less dense, more inflated state.
The cooling remnants of the white dwarf would appear considerably more luminous than before with a bigger volume.

"The implications for Type Ia supernovae are profound," states McCully.

"We've found that supernovae at least can grow to the limit and explode. Yet the explosions are weak, at least some of the time. Now we need to understand what makes a supernova fail and become a Type Iax, and what makes one successful as a Type Ia."

This research was published in The Astrophysical Journal.