Failed Supernovae

Stellar evolution is a fascinating topic and what happens in the last phases of a star’s life is mind boggling. Some stars are so big that their collapsing core turns straight into a black hole that suppresses the supernova.

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Supernovae are one of the universe’s most impressive displays of fireworks, with its last gasp, a supernova can outshine its host galaxy. They have been recorded throughout history as temporary stars or harbingers of doom, for the more dramatically inclined. The forces involved in blowing a star apart are staggering and the resultant object that remains is even more mind boggling. We’ve recently looked at magnetars and neutron stars and covered the incredible events that happen when two neutron stars collide. These massive displays of energy are well observed, especially supernovae, and more recently with our ability to detect gravitational waves we are building a better understanding of what happens when neutron stars collide with themselves or with black holes.

Yet, there is a more sinister process that might be out there in the universe, and I don’t mean star munching Dyson spheres built by some alien civilisation. What I meaning here, is that some stars don’t quite get to make an impressive explosion that shakes the neighbours before the core collapses into a black hole. These types of star deaths are called failed supernovae.

Pismis 24 stars, some 300 solar masses (Credit: NASA)

To describe how these failed stars work, it’s worth considering what is happening in the final moments of a star that’s well on its way to supernova. Massive stars have a lot of fuel to burn but burn through it very quickly. The core is where most of the action happens with the star burning through hydrogen until there’s not much left. There’s always a tussle happening between gravity, which wants to squash the star, and the pressure created by the fusion reactions occurring in the core, which wants to blow the star apart. At different times in the star’s life this reaches an equilibrium, called hydrostatic equilibrium, where the force of gravity matches the thermonuclear force. The star is happily burning hydrogen in the core for a while until there’s not much left. Then the outward force of the fusion reduces and gravity begins to win the competition and starts squashing the star’s core. As it squashes the core, the temperature heats up and helium starts fusing, which pushes back against gravity and the star reaches a new equilibrium, and becomes just a bit hotter.

After a while, the star burns through the helium and starts marching up the periodic table through elements such as carbon, oxygen, calcium, nickel and iron. Stars can quite happily burn carbon and oxygen but run into difficulties when it comes to trying to fuse nickel and iron. Fusing these elements doesn’t produce more energy than goes into the reaction so the core is unable to fuse them. Because the pressure of fusing fuel eases, gravity starts to win the battle again and the core gets squashed down to what is known as the degeneracy pressure of electrons. This is basically where the atoms are being squashed so much that they simply cannot be squashed any further or the electrons would breach Pauli’s exclusion principle as the electrons would be so tightly packed that they’d be forced to occupy the same energy states, which they’re not allowed to do. This limit creates a pressure which holds gravity back from squashing the core anymore. What happens is that the nuclear fire still burns around this inert core, that has been squashed as much as it can be, so more and more iron and nickel gets aded to the core.

Supernova 1987A, before on the left, supernova on the right (Credit: NASA)

When the mass of the core gets to 1.4 times the mass of our Sun the degenerative pressure of the electrons no longer can hold back gravity (you really do not want a lump of iron this big). This magic number of 1.4 times the mass of the Sun was worked out by Subrahmanyan Chandrasekhar in 1930, and is called the Chandrasekhar Limit. So now that the electrons are not able to hold things back there is a lot of different processing occurring to make room for more squishing. One of these is that the iron nuclei start to fall apart into free neutrons and helium atoms as they are bombarded with gamma rays. Also the the protons and electrons that are now swimming around in close proximity to each other start to combine and form neutrons – during this combining they spit out a neutrino which leaves the core. You can see what’s happening, there’s extra room being created as the electrons are disappearing with the protons into neutrons and the neutrinos are going away. All of this is happening very very quickly, in milliseconds. The core is now in a state of collapse. This mechanism is so fast that particles on the outer edge of the core can reach speeds as high as 23% of the speed of light. The core is now becoming a big mass of neutrons all getting much closer to each other and as they approach the density of the nucleus of an atom they can’t squash anymore. So the rapid collapse all of a sudden hits a wall of super squished neutrons. This bounces back in the form of a shockwave and ripping the core of the star apart releases loads more neutrinos, which essentially blows the star apart.

Of course there’s there’s still the bit in the middle which got incredibly squashed and for stars under about 20 masses of the Sun this remnant core becomes a neutron star. For stars larger than this, the degenerative pressure of neutrons cannot hold back the pressure from gravity and the core continues to collapse into a black hole. It is thought that for stars that are bigger than about 40-50 solar masses the force of gravity from the collapsing core sucks the shockwave and the neutrinos into a black hole. So the indicators of this would be seeing the beginnings of a supernova only to have it rapidly disappear and leave no remnant indication. Adams, Kochanek, Gerke, Stanek and Dai talk about a similar process in their paper last year wher they observed a massive star suddenly flare up and slowly fade away into nothing. They determined is was likely a failed supernova showing a star undergo core collapse and then swallow up the supernova before it could blast the stellar remnant apart.

A star that disappeared, probably as a black hole (Credit: NASA/HST)

The new instruments such as LIGO are really helping scientists understand stellar evolution. The two black holes that were observed to emit gravitational waves last year led a lot work to understand how black holes of those masses could form. It was determined that was just not enough supernovae to explain the number of massive blackholes being observed per galaxy so some other mechanism must be causing them, hence the failed supernovae concept. It will be really exciting to see how this field develops as more observations are made.