4KM wd_def simulation: High_Res (4000x3000) tiffs here Click on images for 1024x768 jpeg version (Click HERE for verions that do not have the length scale and time embedded in the images.
Image 1 (0.400 s after ignition):
During the first tenths of a second after a slightly off-center ignition in a stellar white dwarf, a hot bubble of nuclear ash grows slowly as the nuclear flame burns slowly through the center of the star. (The rings seen at the surface of the star in all of the images are an artifact of the visualization of the simulation, and are not present in the simulation itself.)

Image 2 (0.973 s after ignition):
The hot bubble of nuclear ash becomes buoyant and starts to rise toward the surface of the star, much like a hot air balloon rises in the Earth's atmosphere. At the same time, an "aneurysm" begins to appear on the top surface of the bubble. This feature originates because the boundary between the hot, tenuous material in the bubble and the cold, dense material above it in the surrounding stellar material is unstable, in much the same way that the familiar example of a layer of oil under water is unstable on Earth.

Image 3 (1.323 s after ignition):
As the bubble rises through the star, its upward velocity increases due to the stronger gravity away from the center of the star, which makes the bubble more buoyant. The complexity of the top surface of the bubble continues to increase, due to the action of the heavy-over-light fluid instability. In this snapshot, the bubble has reached the surface of the star and begins to break through.

Image 4 (1.563 s after ignition):
The hot bubble has broken through the surface of the star, and the top part of it is spreading rapidly (at speeds of 3,000 - 5,000 km s^-1) across the surface. The spreading material sweeps up unburnt fuel in the surface layers of the star in a compressed layer at its head, much as a snowplow sweeps up fallen snow. By about 2 seconds, the spreading material will crash into itself at the opposite point on the surface of the star, further compressing and heating the unburnt material, and possibly initiating a detonation. If the collision does produce a detonation, it will propagate through the star at the sound speed, incinerating it in another ~ 1-2 seconds. The entire nuclear burning phase of the explosion is thus over in ~ 3 seconds. The energy released by nuclear burning is enough to overcome the pull of gravity, so the material in the star freely expands. The nickel produced during the nuclear burning phase decays first to cobalt and then to iron. These decays emit gamma rays, which heat the expanding envelope of the stellar material, making it glow in the ultraviolet, optical, and infrared portions of the electromagnetic spectrum. This emitted light makes the supernova visible to astronomers on Earth.