Strange star survived a supernova explosion, but shines even brighter than before

(ORDO NEWS) — When it comes to leaving in style, nothing beats the passing of a white dwarf. Their thermonuclear self-destruction is among the most powerful explosions in space, causing a star to go out in rays of glory.

At least that’s the idea. The discovery confirms that some white dwarfs fake their deaths, showing nothing in particular, but continuing to shine even brighter than before.

A decade ago, supernova SN 2012Z was spotted in the nearby spiral galaxy NGC 1309, briefly glowing, singing a swan song that was supposed to herald its annihilation.

Pictures of the home galaxy had been taken years before, so finding out which star exploded was simply a matter of examining subsequent pictures to find empty spaces.

“We expected to see one of two things when we received the latest Hubble data. Either the star will completely disappear, or it will still exist, that is, the star that we saw in the pictures before the explosion was not the one that exploded,” says the University astronomer Santa Barbara Curtis McCully.

“No one expected to see a surviving star that would be brighter. It was a real mystery.”

As unexpected as this observation was, it was not completely devoid of precedent, contributing to a growing body of evidence that life after death may not be such a strange thing for white dwarf stars.

When a star with the mass of our Sun squeezes the last remaining helium into carbon and oxygen, it turns into a dense, hot ball the size of our Earth. Lacking the mass to form larger elements, it simmers, cooling down over centuries until it eventually turns into a cold black lump.

If such a depleted stellar core has a generous companion star orbiting nearby, life may last a little longer as it sucks out some extra gas.

However, at a critical moment, all of this extra mass can push the carbon into fusion, triggering a runaway reaction that releases massive amounts of energy in the blink of an eye and rips the star apart in a so-called Type Ia supernova.

Normally, nothing of note remains in the space once occupied by a white dwarf, just an expanding cloud of stellar guts drifting through space, glowing dimly with residual radiation.

These specific explosions are so finely tuned that they all burn at about the same brightness, making them convenient for measuring distances in the universe.

However, not all explosions are so standard. The more common Type Iax supernovae look less like fireworks and more like raw lumps, flaring up slowly with a comparatively dull howl.

They may not even be that devastating: the aftermath of several of these less impressive supernovas has been seen to show signs of high-density matter with signs of a thick photosphere.

Strange star survived a supernova explosion but shines even brighter than before 2

Color images of NGC 1309 before and after SN 2012Z. The left panel shows an image of NGC 1309 taken by the Hubble Legacy Program (before the explosion). The top-middle panel shows the magnification of the position of the supernova in the image before the explosion.

Top right shows SN~2012Z from the 2013 visit. The middle-bottom panel shows the position of SN~2012Z according to the latest observations in 2016. The bottom right panel shows the difference between pre-explosion images and 2016 observations.

The detection of SN 2012Z’s violent radiation after its own supernova leaves little doubt that in some, if not many, cases, white dwarfs can remain intact even after a fusion explosion.

Why this particular star not only did not break apart, but also returned even brighter, remains a mystery. The researchers who made the discovery speculate that the explosion simply agitated the matter, allowing it to return to a less dense, more bloated form.

With more volume, the cooling remains of a white dwarf would look even more radiant than before.

“The implications for Type Ia supernovae are very profound,” says McCully.

“We found that supernovae can at least grow to their limits and explode. However, explosions are weak, at least in some cases. Now we need to understand what causes a supernova to fail and become an Iax-type star, and what causes it to become a successful Type Ia star.”

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