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Hubble finds Mass of White Dwarf
Chris Bradshaw writes "The mass of the nearest white dwarf star to Earth has been measured accurately for the first time. from the article: 'Sirius B is just 12,000 km (7,500 miles) in diameter, similar to Earth, but its mass is 98% that of the Sun. Studying Sirius B has been difficult because of the bright light coming from its neighbour Sirius A, the "Dog Star." The results, published in the Monthly Notices of the Royal Astronomical Society, come from astronomers using the Hubble Space Telescope.'"
additionally, more can be found on the white dwarfs in general Here.
I'm not fat, just big boned...
not mentioned in the article, at http://columbia.thefreedictionary.com/Sirius
Selected excerpt:
"Sirius A is about twice the size of the sun and about 20 times as luminous. It is also one of the nearest stars, lying at a distance of 8.7 light-years, so that it has been studied extensively. From an analysis of its motions, F. W. Bessel concluded (1844) that it had an unseen companion, which was later (1862) confirmed by observation. The companion, Sirius B, is a white-dwarf star and has also been the object of considerable study because it is the first white dwarf whose spectrum was found to exhibit a gravitational red shift, as predicted by the General Theory of Relativity."
Right idea, wrong mechanism.
A white dwarf is not supported by thermal pressure, or by nuclear fusion; it is supported by degeneracy pressure between electrons, a consequence of the exclusion principle in quantum mechanics that forbids two electrons from occupying the same quantum state.
1.4 solar masses is correctly given as the critical point at which gravity prevails over the internal pressure; at this point, the star switches from degeneracy pressure between electrons to degeneracy pressure between neutrons, in the process dropping considerably more than the weight of the Sun from the size of the Earth to something more like the size of Belgium, through an enormously strong gravitational field. This releases an awful lot of energy, and is the main power source for such a supernova.
There's another type of supernova which is driven by fusion, but that's more typical of accretion systems in which the infalling matter has heated the white dwarf sufficiently to reignite fusion processes; then the fusion reaction is an uncontrolled runaway and can wholly disrupt the star.
Real Daleks don't climb stairs - they level the building.
It's all relative. For years, astronomers have known that Sirius B was about 1 solar mass, plus or minus maybe a tenth. They found this by observing the size of its orbit and its period. This time, using spectroscopy, they can estimate the surface gravity of Sirius B, which will give its radius and mass. There's still uncertainty in the measurement, as in all measurements, but that uncertainty is smaller than previous measurements. Who knows, 20 years from now new techniques could give an even more accurate measure of Sirius B's mass, but one could still say "measured accurately for the first time" since it's better than previous measurements. This is not revolutionary, it's evolutionary.
No, white dwarfs do become fusion-powered supernovae, not gravity-powered. IAAA (I am an astrophysicist.)
A white dwarf becomes a Type Ia supernova when, at around 1.4 solar masses, the pressure at the center reaches the point where it can burn by fusion the carbon-nitrogen-oxygen left over from previous rounds of burning. This leads to a fusion-driven explosion that gets no net energy from gravitational collapse, leading to an expanding gas cloud that is largely hot iron-group elements.
There is another class of supernova that is gravitationally driven. Core collapse supernovae are produced when a massive star (>8 solar masses, last I heard) has burned 1.4 solar masses at its center to iron. (The 1.4 solar mass value is semi-coincidental with that in the previous paragraph, based on similar but not identical physics.) This is a gravity-powered supernova that blows the outer parts of the core away, leaving a neutron star or black hole where the core was.
There is no way for a white dwarf to become a core collapse supernova, the fusion kicks in and blows it apart before that happens.
You are right that the gravitational binding energy of a white dwarf is comparable to the fusion energy of its combustion from carbon to iron. However, comparable doesn't mean equal and if you work it out, the fusion energy dominates. The gravitational energy is negative, so it reduces the amount of available energy for the supernova (i.e. the ejecta are moving more slowly because it has to climb out of its own gravity well), but it is still energetically favorable to go from a dense chunk of carbon to an expanding diffuse cloud of iron.