With the discovery of gravitational waves, I keep hearing that we now have a "totally new way to explore the universe." But as I understand it, gravitational waves are only detectable when they come from super massive objects (like two black holes colliding). So are gravitational waves just a new way to observe black hole mergers? Could we observe a single black hole? Could stars ever be observed through gravitational waves?
There are two search channels in LIGO, the first is for CBCs (Compact Binary Coalescence). These are binary systems of very dense objects, either BH-BH, BH-NS or NS-NS mergers (BH: Black Hole, NS: Neutron Star). They produce well known signals that increase in frequency over time, an example of this is shown here:
The second search channel is looking for "Bursts", this involves more unpredictable signals such as the one shown below, which are harder to model and know the characteristics about, but such a signal is expected from supernovae explosions and possibly Gamma Ray Bursts.
Gravitational waves are produced by all binary systems to a certain extent, but the current sensitivity of LIGO means it is only sensitive to the most extreme events, like a black hole merger. So individual stars themselves cannot produce them, only quadruople sources can produce gravitational waves, however if most stars have planets orbiting them then in theory they are producing very tiny gravitational waves. The same goes for a single black hole, it alone cannot be a source of gravitational waves.