Prior to becoming a supernova, an evolved massive star resembles an onion in that it contains many layers of different elements, each undergoing fusion. The outermost layer consists of hydrogen, followed by helium, carbon, oxygen, and so forth. Thus when the outer envelope of hydrogen is shed, this exposes the next layer that consists primarily of helium (mixed with other elements). This can occur when a very hot, massive star reaches a point in its evolution when significant mass loss is occurring from its
stellar wind. Highly massive stars (with 25 or more times the mass of the
Sun) can lose up to 10−5
solar masses () each year—the equivalent of every 100,000 years. Type Ib and Ic supernovae are hypothesized to have been produced by core collapse of massive stars that have lost their outer layer of hydrogen and helium, either via winds or mass transfer to a companion. There is some evidence that a small fraction of the Type Ic supernovae may be the progenitors of
gamma ray bursts (GRBs); in particular, Type Ic supernovae that have broad spectral lines corresponding to high-velocity outflows are thought to be strongly associated with GRBs. However, it is also hypothesized that any hydrogen-stripped Type Ib or Ic supernova could be a GRB, dependent upon the geometry of the explosion. In any case, astronomers believe that most Type Ib, and probably Type Ic as well, result from core collapse in stripped, massive stars, rather than from the thermonuclear runaway of
white dwarfs. They normally occur in regions of new star formation, and are extremely rare in
elliptical galaxies. Because they share a similar operating mechanism, Type Ibc and the various Type II supernovae are collectively called core-collapse supernovae. In particular, Type Ibc may be referred to as
stripped core-collapse supernovae. ==Light curves==