The death of massive stars, ten or more times the mass of the Sun, is thought to have a predictable -- if violent -- trajectory.
During most of their lifetimes, stars are in a relative balance between two forces. Fusion of hydrogen into helium in the core releases heat energy, which increases the pressure in the core and generates an outward-pointing force. At the same time, the inexorable pull of gravity generates an inward-pointing force. For the majority of the star's life, the two are in equilibrium; if something makes the core cool a little bit, gravity wins for a while and the star shrinks, increasing the pressure and thus the rate of fusion. This heats the core up, increasing the outward force and stopping the collapse.
Nice little example of negative feedback and homeostasis, that. Stars in this long, relatively quiescent phase are on the "Main Sequence" of the famous Hertzsprung-Russell Diagram:
Once the hydrogen fuel starts to deplete, though, the situation shifts. Gravity wins once again, but this time there's not enough hydrogen-to-helium fusion to counteract the collapse. The core shrinks, raising the temperature to hundreds of millions of degrees Kelvin -- enough to fuse helium to carbon. This release of energy causes the outer atmosphere to balloon outward, and the star becomes a red supergiant -- the surface is cool (and thus reddish), but the interior is far hotter than the core of our Sun.
Two famous stars -- Betelgeuse (in Orion) and Antares (in Scorpio) are in this final stage of their lives.
Here's where things get interesting, because the helium fuel doesn't last forever, either. The carbon "ash" left behind needs an even higher temperature to fuse into oxygen, nitrogen, and heavier elements, which happens when the previous process repeats itself -- further core collapse, followed by further heating. But this can't go on indefinitely. When the fusion reaction starts to generate iron, the game is up. Iron represents the turnaround point on the curve of binding energy, where fusion stops being an exothermic (energy-releasing) reaction and becomes endothermic (energy-consuming). At that point, the core can't respond with anything to support the pull of gravity, and the entire star collapses. The outer atmosphere rebounds off the collapsing core, creating a shockwave called a core-collapse (type II) supernova, releasing in a few seconds as much energy as the star did during its entire life on the main sequence. What's left afterward is a super-dense remnant -- either a neutron star or a black hole, depending on its mass.
Well, that's what we thought happened. But now a paper in Science describing the collapse of a supergiant star in the Andromeda Galaxy has suggested there may be a different fate for at least some massive stars -- that they may go out not with a bang, but with a whimper.
The occurrence that spurred this discovery was so underwhelming that it took astronomers a while to realize it had happened. A star began to glow intensely in the infrared region of the spectrum, and then suddenly -- it didn't anymore. It seemed to vanish, leaving behind a faintly glowing shell of dust. Kishalay De, lead author of the paper, says what happened is that we just witnessed a black hole forming without a supernova preceding it. The core ran out of fuel, the outer atmosphere collapsed, and the star itself just kind of... winked out.
"This has probably been the most surprising discovery of my life," De said. "The evidence of the disappearance of the star was lying in public archival data and nobody noticed for years until we picked it out... The dramatic and sustained fading of this star is very unusual, and suggests a supernova failed to occur, leading to the collapse of the star’s core directly into a black hole. Stars with this mass have long been assumed to always explode as supernovae. The fact that it didn’t suggests that stars with the same mass may or may not successfully explode, possibly due to how gravity, gas pressure, and powerful shock waves interact in chaotic ways with each other inside the dying star."
