An Unusually Fast-Evolving Supernova
  Analyses of supernovae (SNe) have revealed two main types of progenitors:
exploding white dwarfs and collapsing massive stars. We present SN2002bj, which
stands out as different from any SN reported to date. Its light curve rises and
declines very rapidly, yet reaches a peak intrinsic brightness greater than -18
mag. A spectrum obtained 7 days after discovery shows the presence of helium
and intermediate-mass elements, yet no clear hydrogen or iron-peak elements.
The spectrum only barely resembles that of a Type Ia supernova, with added
carbon and helium. Its properties suggest that SN2002bj may be representative
of a class of progenitors that previously has been only hypothesized: a helium
detonation on a white dwarf, ejecting a small envelope of material. New surveys
should find many such objects, despite their scarcity.
