May 302012
 

SCIENCE!

The universe is jam-packed with structures, objects and phenomena that are both startling and just plain amazing. One such is the Type Ia supernova.

The concept of a star blowing itself to smithereens is inherently interesting enough. But the Type Ia is especially interesting: because every Type Ia, no matter where in the universe it may occur, is pretty much like every other Type Ia. It is recognizable and consistent.

The Type Ia supernova is also interesting in that it comes not from a single star blowing up, but two stars in a complex dance that results in an inevitable explosion. The first star needs to be a white dwarf… a “dead” star, basically the end state of about 97% of all stars.

After the star has burned through is supply of fusionable hydrogen, the star expands to a diffuse red giant, burning the helium to form carbon and oxygen. If the star is not massive enough to fuse carbon, the fusion process stops, and gravitational collapse crushes the star.

A living star is the result of two opposing forces: gravity trying to pull it all down, and gas pressure trying to blow it up. But once fusion has ceased, the temperature declines, reducing gas pressure… but the mass of the star does not decrease, and thus gravity begins to drag everything towards the core. As the stellar gasses get dragged closer to the core, the gravitational forces only get stronger, making the collapse all the more invincible. For stars of less than 1.38 solar masses, the final diameter is about that of the Earth… the matter of the star having been converted into “degenerate matter.” Here, pressure has collapsed the electron shells around atoms, forming something like a soup of atomic nuclei with electrons just sorta swimming around. This is not neutron star material (neutronium), as the atomic nuclei remain independent, and the electrons and protons have not been mashed together. Nevertheless it is a state of matter not to be found on Earth apart from the cores of exploding nuclear bombs.

Why 1.38 solar masses? Because at higher masses, the gravitational forces are enough to cause carbon to undergo nuclear fusion, continuing the fusion process of the star for a little while longer. So any star less than 1.38 solar masses that comes to the end of its life will end up as a chunk of carbon and oxygen the size, roughly, of the Earth.

Once the white dwarf is formed, it shines quite brightly. This is not due to ongoing nuclear reactions, but due to it simply being a very massive, very hot thing radiating away its stored heat. Over many billions of years it will simply darken and cool and form a dead husk of cold degenerate matter. (No white dwarfs can be old enough yet to be cooler than several thousand degrees)

Unless, that is, the white dwarf is in orbit around another star.

Most stars in the universe appear to be in pairs or triplets. Thus it would not be unusual for a white dwarf to be in orbit around a living star… as is the case with Sirius B in orbit around the much more massive (and vastly brighter) Sirius A. If the two orbit close enough, then gas from the living star may be grabbed by the white dwarf. This is most pronounced when the living star is itself nearing the end of its life as a gas giant. The star expands greatly, becoming vast but diffuse, barely able to hold onto the outer layers of its atmosphere. If the red giant is big enough and the white dwarf close enough, the gas will be simply drawn off the red giant by the dwarf.

The gas will form an accretion disk around the dwarf. It will appear to an outside observer that the dwarf has a ring system that extends all the way to the surface, and which gets brighter and hotter as it does so. An “arm” of gas will reach out from the gas giant to feed the accretion disk. The superheated gas will be deposited onto the surface of the white dwarf, adding its mass to that of the dwarf. And if the gas adds up enough to push the mass of the dwarf over 1.38 solar masses… bang.

What happens at 1.38 solar masses is that the pressure finally becomes enough to cause the carbon in the dwarf to undergo fusion. And it does so rapidly; the “carbon detonation” cycle begins in the core of the white dwarf and forms an outwardly expanding shockwave. The pressure within the shockwave is enough to cause the outer layers of carbon to undergo fusion. So the white dwarf blows itself to bits from within over a period of just a few seconds. The explosion is so powerful that the fragments of the former star are hurled outward at about 6% lightspeed, and the explosion at its peak is about five billion times brighter than the sun.

The Type Ia supernova is not only an interesting phenomenon, it is also a useful one… because every Type Ia supernova is the result of a carbon detonation of a 1.38-solar mass white dwarf. They are all the same thing, with the result that they are all pretty much equally bright over the same timeframe. Thus, if you see a sudden supernova in a  distant galaxy and over a few hours it follows a standard brightness dropoff curve that matches the Type Ia, you can measure with fair accuracy how far away the explosion was by calculating its brightness. This “standard candle” allows astronomers to map how far away distant galaxies are.

The massive explosion of the Type Ia supernova helps to create the heavy elements that float around in the universe.  The peak intensity of the luminocity curve of the Type Ia supernova is given off by emissions from oxygen and calcium that formed the outer atmosphere of the white dwarf. Within a few months of the detonation, the outer envelope has expanded and cooled enough to be transparent to the radiation given off by the materials from the core, and this radiation is driven by the radioactive decay of nickel-56 down through cobalt-56 finally to iron-56.

Almost as interesting as the Type Ia supernova is the death of a white dwarf with no companion star. Given a sufficiency of time, a white dwarf will cool off to become a non-radiating black dwarf. This is a process likely to take astonishing long time (1015 years to cool to 5K under “normal” conditions,  1037 years if proton decay is entered in) before the dwarf equalizes its temperature with that of the surrounding sky… just a few degrees kelvin above absolute zero. Lower mass white dwarfs will cool faster than heavier white dwarfs. And in the process of cooling, an interesting thing occurs.

The degenerate matter of a white dwarf is an extremely dense and hot form of plasma. The electrons have been stripped from the nuclei, and are free to wander. But as the degenerate matter cools, it remains a plasma… even if it’s not even all that hot, or even all that fluid. The massive pressure is enough to keep it in this state. But as it cools, the chaotic plasma forms itself into a crystalline structure, calculated to be a body centered cubic lattice, somewhat akin to salt crystals. The white dwarf star becomes a solid. Since it is composed largely of carbon nuclei… it becomes a diamond, of sorts, the size of a planet.

 Posted by at 4:58 pm
  • MrDakka

    So white dwarf eventually becomes “crystallized plasma”?

    • Anonymous

      Yes. It’s a strange form of matter that could not exist outside of a pressure field vastly beyond normal human comprehension. Science fiction stories often have structures made out of neutronium, but that, like degenerate matter crystals, would simply explode like nuclear weapons if removed from the conditions that created it.

  • Anonymous

    Has the Chandrasekhar limit changed (1.44 solar masses as I learned it back 30 years ago) or am I confabulating two different paths to novae and their remnants?

    • Anonymous

      The Chandrasekhar limit remains 1.44, which is where a collapsing core needs to be to become a neutron star. But it appears that if a white dwarf is *increasing* in mass, it will undergo carbon detonation at 1.38. So it would seem you can’t take a chunk of degenerate carbon and *build* yourself a neutron star, since it’ll go FOOOM on you before you get there, but if you have a bigger mass of that in a stellar core it’ll undergo slightly more stately carbon fusion until it eventually gets to iron, where it all starts to crap out and the core collapses and BLAMMO.

      That said, do a Google search on Chandrasekhar 1.44 and another one on Chandrasekhar 1.38, and you’ll see that the Chandrasekhar limit is defined as both. It’s almost as if the internet isn’t entirely reliable.

      • Anonymous

        Ah, I see I forgot the POW!, BIFF!, and OOF!. That said, I’m not sure that 30 years ago in an intro Astronomy class anyone had yet cottoned on to the accretion method of creating Ia supervovae, which would explain my confuzzlement.

        Oh, and here’s an extraneous SOCK!.

  • It appears they might have found two white dwarfs about 100 light years away that could be over 11 billion years old. http://www.sciencedaily.com/releases/2012/04/120411144326.htm