Illustration of Merger Trigger for Supernova
These illustrations shows the main way that new Chandra results indicate Type Ia supernova are triggered in elliptical galaxies. Two white dwarf stars orbit each other and lose energy via gravitational radiation, eventually resulting in a merger between the two stars. Because the total mass of this merger exceeds the weight limit for a white dwarf, the merged star is unstable and explodes as a Type Ia supernova.
View Animation
(Illustration: NASA/CXC/M.Weiss)

Illustration of Alternate Trigger Mechanism
The top panel shows an artist's illustration of gas being pulled from a Sun-like star onto a white dwarf via a red disk. When the amount of material accreted onto the white dwarf causes the weight limit for this star to be exceeded, it explodes as a Type Ia supernova, as shown in the bottom panel. The latest results show that this is not the dominant mechanism for Type Ia supernovas in the galaxies Chandra observed.
(Illustration: NASA/CXC/M.Weiss)

X-ray, Optical & IR Images of M31
This composite image of M31 (Andromeda) shows Chandra X-ray data in gold, optical data from the Digitized Sky Survey in light blue and infrared data from the Spitzer Space Telescope in red. A zoom into the central region of M31 dissolves into the Chandra field of view. Scientists have calculated that the Chandra image would be about 40 times brighter than observed if Type Ia supernova in the bulge of this galaxy were triggered by accretion of material from a normal star onto a white dwarf star. This implies that the merger of two white dwarfs is the main trigger for Type Ia supernovas in this galaxy.
(Credit: X-ray (NASA/CXC/MPA/M.Gilfanov & A.Bogdan), Infrared (NASA/JPL-Caltech/SSC), Optical (DSS))