Since very little, if any, star formation occurs today in elliptical galaxies, it is thought that Type Ia supernovae arise from older, less massive stars. Today, these models involve sophisticated and complex computer simulations of the explosions. These are young, relatively massive stars, which form in spiral and irregular galaxies. They also find that the Type Ia supernovae can best be explained by the explosion of somewhat exotic low-mass stars known as white dwarfs. Stellar evolution is the study of how stars evolve and change, both internally and externally, throughout their lives.
Stars generate their own energy during their lives by the process of nuclear fusion.
The nuclei of lighter elements, such as hydrogen and helium, are forced to fuse, or combine, under the tremendous pressures and temperatures at or near the centers of stars, into the nuclei of heavier elements. When nuclear reactions occur inside stars, these reactions liberate huge amounts of energy, which inevitably trickles out from the star's interior to its surface, resulting in the light we see from the stars, their starshine. In massive stars, those more massive than about 8 solar masses, the sequence of nuclear fusion progresses from the very simplest reaction of hydrogen nuclei to form helium nuclei, to more complicated reactions, involving the synthesis, as it is known, of silicon nuclei into iron nuclei. The iron nucleus is the most stable nucleus in Nature, and it resists fusing into any heavier nuclei, unless it is forced to do so with the input of truly formidable amounts of energy. As a result, when the central core, as it is known, of a star becomes pure iron nuclei, the core, which is generally the site of most of a star's energy production, is no longer able to produce energy and therefore support the star.