Starship Asterisk*

http://apod.nasa.gov/apod/ap070116.html

Fantastic picture but I'm confused by the text underneath.

The APOD text says "... more typical of a Type Ia supernova, indicating that the progenitor was a white dwarf star that exploded when it accreted too much material and went over Chandrasekhar's limit".

Whereas the Wikipedia link for Chandrasekhar's limit embedded within says "...If a white dwarf were to exceed the Chandrasekhar limit through accretion, it would begin to collapse under gravity. It was once believed that this mechanism triggered Type Ia supernova explosions, but this idea fell out of favor in the 1960s."

These two statements are contradictory. (Or possibly just out-of-date). Which is correct?

nb: please excuse my ignorance/confusion, I'm no scientist but I am interested!

Can't answer the above, but I followed the same link and found the Wkipedia saying, "For more massive stars, electron degeneracy pressure will not keep the core from collapsing to very great density, leading to formation of a neutron star, black hole, or, speculatively, a quark star."
A "quarkstar"?! Between a neutron star and a BH. Curiouser and curiouser - this seems to be very recent work, that I wot not of before.

John

I think that APOD is right and wikipedia is wrong on this

supernova type 1a ; (let's see if this can be properly plagiarized)


Type Ia

The most commonly accepted theory of this type of supernovae is that they are the result of a carbon-oxygen white dwarf accreting matter from a nearby companion star, typically a red giant.

The progenitor binary system consists of main sequence stars, with the primary possessing more mass than the secondary. Being greater in mass, the primary is the first of the pair to evolve onto the asymptotic giant branch, where the star's envelope expands considerably. If the two stars then share a common envelope, the system can lose considerable mass and the angular momentum, orbital radius and period will all be reduced. Once the primary has evolved into a degenerate white dwarf, the secondary star later evolves into a red giant and the stage is set for mass accretion onto the white dwarf. During this final shared envelope phase, the two stars spiral in closer together as angular momentum is lost. The resulting orbit can have a period of only a few hours.

If the accretion continues long enough, the white dwarf may eventually approach the Chandrasekhar limit (1.44 solar masses), the maximum mass that can be supported by electron degeneracy pressure, beyond which the white dwarf would collapse to form a neutron star (if nothing intervened to stop the process).

The current view is that this limit is never actually attained, so that collapse is never initiated. Instead, the increase in pressure raises the temperature near the center, and a period of convection lasting approximately 1,000 years begins. At some point in this simmering phase, a deflagration flame front powered by carbon fusion is born, although the details of the ignition—the location and number of points where the flame begins—is still unknown. Oxygen fusion is initiated shortly thereafter, but this fuel is not consumed as completely as carbon.

Once fusion has begun, the temperature of the white dwarf starts to rise. Normally a typical main sequence star would expand and cool in order to counter-balance an increase in thermal energy. However, degeneracy pressure is independent of temperature, so the white dwarf is unable to regulate the burning process in the manner of normal stars. The flame accelerates dramatically, through the Rayleigh-Taylor instability and interactions with turbulence. It is still a matter of considerable debate as to whether this flame transitions from a subsonic deflagration into a supersonic detonation.

Regardless of the exact details of nuclear burning, it is generally accepted that a substantial fraction of the carbon and oxygen in the white dwarf is burned into heavier elements within a period of only a few seconds, raising the internal temperature to billions of degrees.

This energy release from thermonuclear burning (≈10^46 joules) is more than enough to unbind the star; that is, the individual particles making up the white dwarf gain enough kinetic energy that they are all able to fly apart from each other. The star explodes violently and releases a shock wave in which matter is typically ejected at speeds on the order of 5-20,000 km/s, or roughly 3% of the speed of light. The energy released in the explosion also causes an extreme increase in luminosity. The typical visual absolute magnitude of Type Ia supernovae is Mv = -19.3 (≈ 5 billion times brighter than Sol), with little variation. Whether or not the supernova remnant remains bound to its companion depends on the amount of mass ejected. As a general rule, the system will remain bound if the remnant is heavier than one half the original total system mass. If not, the companion will evolve into a runaway star.

[perhaps this helps explain tales of black holes traveling at 1000km/sec through our galaxy, their companion star was massive enough the recoil sent both flying apart ?]

The theory of this type of supernovae is similar to that of novae, in which a white dwarf accretes matter more slowly and does not approach the Chandrasekhar limit. In the case of a nova, the infalling matter causes a hydrogen fusion surface explosion that does not disrupt the star.

[a surface thermonuclear explosion]


Unlike the other types of supernovae, Type Ia supernovae generally occur in all types of galaxies, including ellipticals. They show no preference for regions of current stellar formation. As white dwarf stars form at the end of a star's main sequence evolutionary period, such a long-lived star system may have wandered far from the region where it originally formed. Thereafter a close binary system may spend another million years in the mass transfer stage (possibly forming persistent nova outbursts) before the conditions are ripe for a Type Ia supernova to occur.

A second possible, but much less likely, mechanism for triggering a Type Ia supernova is the merger of two white dwarfs. In such a case, the total mass would not be constrained by the Chandrasekhar limit. This is one of several explanations proposed for the anomalously massive (2 solar mass) progenitor of the "Champagne Supernova" (SN 2003fg or SNLS-03D3bb).

Collisions of solitary stars within our galaxy are thought to occur only once every 107–1013 years; far less frequently than the appearance of novae. However, collisions occur with greater frequency in the dense core regions of globular clusters. (C.f. blue stragglers.) A likely scenario is a collision with a binary star system, or between two binary systems containing white dwarfs. This collision can leave behind a close binary system of two white dwarfs. Their orbit decays and they merge together through their shared envelope.

Type Ia supernovae have a characteristic light curve, their graph of luminosity as a function of time after the explosion. Near the time of maximum luminosity, the spectrum contains lines of intermediate-mass elements from oxygen to calcium; these are the main constituents of the outer layers of the star. Months after the explosion, when the outer layers have expanded to the point of transparency, the spectrum is dominated by light emitted by material near the core of the star, heavy elements synthesized during the explosion, most prominently iron-group elements. The radioactive decay of Nickel-56 through Cobalt-56 to Iron-56 produces high-energy photons which dominate the energy output of the ejecta at intermediate to late times.

The similarity in the absolute luminosity profiles of nearly all known Type Ia supernovae has led to their use as a secondary standard candle in extragalactic astronomy. The cause of this uniformity in the luminosity curve is still an open question. In 1998, observations of Type Ia supernovae indicated the unexpected result that the universe seems to undergo an accelerating expansion.

What a complete answer! Thanks a bunch Kovil!
My hunch was to go with APOD over Wiki so I'm glad that turned out right.
Better than that I now understand a whole lot more about this whole supernova stuff! Thanks!!