A New Type of Supernova

Astronomers have discovered a new supernova class where the star might survive the explosion.

A new type of supernova
An artist's illustration shows a white dwarf pulling matter from a companion star. If it pulls too much, a thermonuclear reaction ignites. The resulting deflagration could destroy the dwarf — or not.
Christine Pulliam (CfA)
Supernovae are typically thought to herald the transformation, if not utter destruction, of a star. But scientists have recently discovered a new, milder type of supernova, one where the star can survive the explosion.

Ryan Foley (Harvard-Smithsonian Center for Astrophysics) and his team collected observations old and new of 25 supernovae that look almost — but not quite — like their Type Ia brethren, explosions in which a white dwarf accretes too much material from a companion star. The added material pushes the white dwarf over its limit of 1.4 solar masses (the so-called Chandrasekhar limit), setting off a chain nuclear reaction that releases light 5 billion times more luminous than the Sun.

Because Type Ia white dwarfs explode when they surpass their mass limit, astronomers can estimate the explosions’ intrinsic luminosities to use the objects as cosmic yardsticks. Observe a Type Ia supernova in a distant galaxy and you know that galaxy’s distance — observe many Type Ia supernovae throughout the universe and you can measure how quickly space is expanding.

But not all Type Ia supernovae look alike. The 25 supernovae gathered by Foley’s team share a dozen or so properties that distinguish them from their normal Type Ia siblings, including lower peak brightnesses and lower ejecta velocities. So Foley and his colleagues place these supernovae in their own class: Type Iax.

Type Iax supernovae also result from thermonuclear fusion deep within a white dwarf that rises to its surface. But the reaction fails to demolish the entire star: instead, only half-a-Sun’s worth of material (on average) is ejected, including ash from the thermonuclear burning. In many cases, some of the white dwarf will survive the deflagration.

“The proposed explanation . . . fits many of the observed properties,” says Craig Wheeler (University of Texas at Austin), who was not involved in the study. “Modeling of these events is nevertheless a new art and probably deserves maturing before firm conclusions are reached,” he cautions.

Foley and his team estimate that the new class is about one-third as common as regular Type Ia supernovae. But are these explosions worthy of the supernova title if they don’t destroy the star? As Wheeler puts it, “I'm not at all sure whether we should call these supernovae or ‘super novae.’”

“My take,” Foley says, “is that (1) some of these objects may in fact completely disrupt their star, and (2) in every way except for the possibility of the remaining star, SNe Iax are more like supernovae than novae. Maybe we need a new word. ‘Pretty good’ novae?”

7 thoughts on “A New Type of Supernova

  1. Mike W. Herberich

    Apart from these newly discovered Semi-Supernovae …: what discerns a regular Supernova from a Nova? And: are there therefore "partial" Novae, like "Semi"-Novae?

  2. Jason Buczyna

    I noticed a common statement in this overall good article: "The added material pushes the white dwarf over its limit of 1.4 solar masses (the so-called Chandrasekhar limit), setting off a chain nuclear reaction that releases light 5 billion times more luminous than the Sun." While this is the explanation that has usually been given for a long time in the intro textbooks for undergraduates, it has always been dissatisfying due to probably being incorrect. A white dwarf generally cannot reach the Chandrasekhar limit because the compression force that is responsible for the limit in the first place (extreme pressure in the relativistic regime adding to the gravity, thus compressing the star further) causes a carbon deflagration at around 1.32 solar masses, resulting in the explosion of the entire star (or, as this article nicely addresses, at least much of the star). After all, if it reached the Chandrasekhar limit, then electron degeneracy pressure would simply cease to support the star, and it would collapse into a neutron star, just as occurs in a massive star with an iron core after nuclear fusion turns off (and thus is no longer able to help support the >Chandrasekhar mass core from collapse), and there would be a degenerate remnant. Instead, the carbon flash at around 1.32 solar masses explodes most of the material out into space in the well-known Type Ia event. I have always preferred this explanation (which fortunately is appearing in some of the newer lower-level books) for the non-merger Type Ia supernova, as it is not only more satisfying in terms of how the event probably occurs, but it better illustrates the various interesting physics involved in this complicated process.

  3. Mike W. Herberich

    I’m still (or even more?) confused: Wikipedia seems to strictly bind the term "Nova" to binary white dwarfs. Isn’t that what the above article refers to, mostly? Yet, it almost exclusively uses the term "Supernova"!? In general, I think in the last 10 years or so, I seem to have heard the simple term "Nova" almost never, as opposed to about 20, 30 years ago. Is that 1. possible, 2. justified by better classification or 3. just due the tendency innate to humans to exaggerate or simplify?

  4. Jason Buczyna

    Mike: The article was discussing Type Ia supernovae. This was a special situation in which a Type Ia supernova might not explode the star completely, but still fuses a pretty decent percentage of its carbon. A nova, on the other hand, basically only produces nuclear fusion among the gas (hydrogen and/or helium) that has been robbed from a companion star: when enough hydrogen and/or helium accumulates on the carbon white dwarf, the pressure/temperature produced by the gravity of such a dense object results in a sudden period of nuclear fusion of this accumulated material, which we call a nova. After this material has been fused, the nova turns off, and we go back to having a normal white dwarf star (which may eventually rob enough material from its companion to repeat the process, or even to take it to the critical point for a Type Ia supernova, depending on the white dwarf’s mass).

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