That occasionally new sources ("Stella Nova") would pop up in the heavens was noted more than a thousand years ago. The earnest study of cosmic explosions began in earnest less than a hundred years ago.

Stella Novae are now divided into two major families, novae and supernovae (with real distinct classes in each). Equally the variable stars have a rich phenomenology. Together, supernovae and variable stars have contributed richly to key problems in modern astrophysics: distances to galaxies, cosmography and build up of elements in the Universe.

Shri Kulkarni, A.D. White Professor-at-Large, the McArthur Professor of Astronomy and Professor of Planetary Sciences at California Institute of Technology and director of the Caltech Optical Observatories and Micheslon Science Center, discusses the Palomar Transient Factory (PTF).

The PTF, an innovative 2-telescope system, was designed explicitly to chart the transient sky with a particular focus on events which lie in the nova-supernova gap. PTF is now finding an extragalactic transient every 20 minutes and a Galactic (strong) variable every 10 minutes.

The results so far: classification of 2000 supernovae, identification of an emerging class of ultra-luminous supernovae, the earliest discovery of a Ia supernovae, discovery luminous red novae, the most comprehensive UV spectroscopy of Ia supernovae, discovery low energy budget supernovae, clarification of sub-classes of core collapse and thermo-nuclear explosions, mapping of the systematics of core collapse supernovae, identification of a trove of eclipsing binaries and the curious AM CVns.