If you were to find yourself falling into a black hole, it would be quite an uncomfortable experience – not when you actually cross the event horizon of the black hole, but on the outside, just before crossing. The reason is that the gravitational force just outside the event horizon is so strong, the mere difference between the forces on the parts of you closest and farthest away from the horizon would be strong enough to tear you apart. The same is true for a star that got close enough, even if it was on a path that could actually avoid falling into the black hole.
Cosmologists have calculated that, on occasion, a star’s orbit will be disturbed in such a way that it passes very near the super-massive black hole at the center of its galaxy—but not so close that it is captured whole. Such a star will be torn apart by the extreme tidal forces it experiences: the force of gravity on the near side of the star is so much stronger than that on the far side that the gravitational force holding the star together is overwhelmed, causing the star to simply come apart. While some of the star’s matter falls into the black hole, much of it continues in chaotic orbits, crashing into itself and producing intense radiation lasting days to months. These phenomena are called stellar tidal disruption flares, or TDFs.
In spite of the large number of stars that typically orbit near a galaxy’s central black hole, TDF events are estimated to be very infrequent – about once per 100,000 years per galaxy. The present research involved a ground-based search involving observations of more than 2 million galaxies over a period of 10 years. Although 342 intense, well-measured flares were observed, it was necessary to exclude events that could have been supernovae or flares in active galactic nuclei instead of TDFs. (Supernovae are thought to occur 1000 times as frequently as TDFs.) Out of all the detected events, in only 2 cases was it possible to determine that the probability was extremely small the event was not a TDF.
Interestingly enough, two other recent research reports have shown good evidence for TDFs, using space-based equipment. The first suggested that repeated TDF events right in our own galaxy are responsible for gamma-ray bubbles extending 25,000 light-years above and below the galactic plane. The model used assumed that a TDF would occur once every 10,000 to 100,000 years.
The other recent case involved a flare that was bright enough to be a gamma-ray burst, except it lasted much longer. Instead, that event was more likely the result of a TDF that produced a very energetic jet of gamma rays and relativistic particles aimed straight in our direction.