This chapter introduces the different classes of gravitational wave sources targeted by terrestrial and space-based detectors. The possibility and implications of gravitational wave emissions from supernovae and coalescing binary systems of neutron stars and/or black holes are discussed, as well as the possible connection between gravitational wave sources and gamma-ray bursts. The chapter also discusses continuous gravitational wave sources and describes how a stochastic gravitational wave background could be produced from astrophysical sources or from events in the early Universe.
Astrophysics provides us with a variety of candidate systems which should be observable in the spectrum of gravitational waves. However, it is important to remember that our powers of prediction of new phenomena are limited, so any list of sources is almost certain to be incomplete.
Amongst stellar mass systems we expect detectable gravitational radiation from the formation of black holes and neutron stars (Fryer et al., 2002), and from the coalescence of binary neutron stars and final collapse of such binaries to form a black hole (Phinney, 1991). We would expect not only discrete sources, but also continuous stochastic backgrounds created from large numbers of discrete sources. In our Galaxy the very large populations of binary stars create a stochastic background in the 10-3 to 10-5 Hz range (Hils et al., 1990; Cutler, 1998; Nelemans et al., 2001).