Massive stars with more than ~8 times the Sun’s mass are known to end their lives in energetic core-collapse supernovae explosions. During the final evolutionary phases of a massive star, silicon in the star’s core undergoes fusion and produces an iron-rich core. When the iron-rich core grows above a certain mass, it can no longer support itself against the crushing force of gravity and undergoes catastrophic collapse. The collapsing core eventually comes to a halt, causing still infalling matter to rebound. This process launches a powerful outward propagating shock wave. By itself, the shock wave does not have sufficient energy to destroy the star in a supernova explosion. Nevertheless, as the core collapses to form a protoneutron star, copious amounts of neutrinos are produced. Neutrinos rarely interact with normal matter and most of the neutrinos that are produced simply escape out of the dying star at the speed of light. However, a small proportion of the neutrinos interacts with and reenergizes the shock wave, providing it with enough energy to cause the star to explode as a supernova.
For stars with more than ~20 times the Sun’s mass, a fraction of them do not explode as supernovae. Instead, the protoneutron star produced from core-collapse becomes massive enough to collapse further and form a black hole. This causes a portion of the neutrinos that would have been emitted to end up inside the black hole. As a result, there are insufficient neutrinos to produce a supernova. The rest of the star collapses directly into the black hole and simply “disappears” without a supernova. Even without a supernova, the star’s demise might still produce an observable signature. This is because the protoneutron star emits a considerable fraction of its mass-energy as neutrinos before eventually collapsing to form a black hole. As the neutrinos escape the dying star at the speed of light, the energy carried away by them is seen as an abrupt loss of gravitational mass which can amount to 0.2 - 0.5 times the Sun’s mass. In response to this, a shock is formed which propagates towards the outer layers of the dying star.
Although the shock is nowhere as energetic as the shock driving a core-collapse supernova, it is still expected to be energetic enough to eject the hydrogen envelop of the dying star to produce a transient event signalling the star’s demise. The hydrogen envelop is ejected with a low velocity of ~100 km/s and the ejecta temperature is likely to be very cool, on the order of ~3000 K. Most of the emitted energy is from the recombination of hydrogen where electrons and protons come together to form electrically neutral hydrogen atoms. This transient event is basically a massive hydrogen envelope being ejected at low energies. In a way, such a transient event is like a very weak supernova that is orders of magnitude less luminous and less energetic than a core-collapse supernova.
During their final stellar evolutionary phases, massive stars that are more than ~10 times the Sun’s mass will expand to become red supergiants. These stars are the largest known stars in the universe by volume. A survey that monitors red supergiants might catch such anomalous transient events. The transient event would appear as a sudden brightening of the red supergiant. The brightening will last for a year or so before dimming and eventually disappearing entirely. A transient event like this signals the birth of a black hole. If the shock is not sufficiently energetic to eject the hydrogen envelop, a transient event may still be produced. In this case, after the rest of the star has collapsed to form a black hole, the hydrogen envelop eventually falls back towards the black hole. This infalling matter forms an accretion disk around the black hole and powers a long-duration gamma-ray transient event.
Elizabeth Lovegrove and Stan Woosley (2013), “Very Low Energy Supernovae from Neutrino Mass Loss”, arXiv:1303.5055 [astro-ph.HE]