The building blocks of atoms, protons and neutrons are composed of a collection of particles called quarks and gluons. Shortly after the Big Bang, however, the Universe was too energetic and too dense for the quarks and gluons to form stable interactions. Instead, the universe was filled with a form of matter called a quark-gluon plasma, where the particles could interact promiscuously.
Billions of years later, a bunch of primates discovered how to recreate a quark-gluon plasma by banging heavy atoms together. It was the first time the material is known to have existed since the first moments of the universe. But a group of astrophysicists now suggest that the largest stars in the universe also form something like a quark-gluon plasma as they explode, and these researchers are using this to explain why we see so many bright-looking supernovae.
It’s going boom
Physical models of stellar explosions have done remarkably well in explaining what we see in the universe. They have helped us understand the amount of mass needed for a star to explode (as opposed to forming a white dwarf) and can describe the differences between a number of classes of supernovae. But something rather embarrassing happens as we move on to bigger stars. For blue supergiants, with tens of times the mass of the sun, the models stop exploding.
(Many of these stars also experience periodic outbursts and explosions that don’t destroy them, although we’ve discovered them recently.)
It’s not that nothing happens to the stars. It’s just that they go out whimpering, forming a black hole fast enough that the shock wave from the star’s collapse never gets anywhere. While there is some evidence that this is happening, we have also seen some very large stars explode. We’ve even seen some types of supernovae likely caused by the death of massive stars. Which tells us the problem is with our models.
The new paper suggests the problem is how the models deal with the moment of a star’s destruction, where major events happen within a few fractions of a second when a star’s fusion reactions stop producing enough energy to force gravity into it. to bring balance. At this point, the star’s iron-rich core collapses under gravity, crushing the atoms to an extremely hot, dense state. Hot enough to produce a quark-gluon plasma?
It’s hard to know for sure. In general, we made the plasma by hitting gold or lead atoms together in particle accelerators with enormous energies. During the collision, the boundaries between individual protons and neutrons dissolve for a brief moment before the collision disappears into surrounding space, creating a nebula of well-defined particles.
During a supernova, different solar masses of material can reach density greater than 2.6 × 1014 grams per cubic centimeter (for context, lead is 11 grams/cm3). We’re not quite sure what happens then, although theoretical work has suggested that the pressure creates something called quark matter, which is related to the quark-gluon plasma. The exact energies and pressures required for the phase transition are not clear, and it is possible that a mixture of phases may exist under certain circumstances.
So, how do you model these terms if you don’t know what they look like? In this case, the researchers built a more general quark-gluon plasma model and recorded a phase transition between that and normal matter. By itself, this model can adopt a wide range of behaviors depending on the values chosen for some of its parameters. So the researchers found ways to reject large sets of parameters. For example, we have not observed neutron stars with much more than two solar masses of material, so all versions of the model that produced them were discarded. Same if the model allowed a speed of sound during the phase transition that was higher than the speed of light.
The behavior of the model also had to be consistent with the details of things like heavy ion collisions and the behavior of the two neutron stars that LIGO/VIRGO saw collide.
With that in place, the team modeled a star with a composition like that of the Sun, but 50 times its mass. As the amount of iron in the core approaches two solar masses, the core begins to collapse and then some of it bounces out in a shock wave as a neutron star forms. However, that shock wave stops when the neutron star’s gravity pulls more of the core inward. This is where supernova models normally stop exploding.
But in the new model, the neutron star’s core undergoes a phase transition from individual neutrons to a quark-gluon material. The neutron star suddenly shrinks, causing a second shock wave that comes out at nearly the speed of light and blows the rest of the star apart.
All of this takes place in a matter of about 10 seconds, astonishingly fast in astronomical terms. And the end result is a neutron star just below two solar masses, with most of that material being an exotic form of matter of unbound quarks and gluons.
Better yet, the model can explain multiple types of supernovae. If the star was unstable and shed many of its outer layers before exploding, the material ejected from the supernova will collide with it and create an extremely bright event. If the star is mostly intact when it explodes, it will create a relatively faint event. And under the right conditions, matter can collapse on the neutron star fast enough to overcome both shock waves and cause it to collapse into a black hole (the model produced this with a 25-solar star).
All this makes this a viable explanation for how the largest stars explode. But ideally, we want our models to make testable predictions. In this case, the authors identify one: two different bursts of neutrinos, one from the synthesis of new elements as the nucleus collapses and a second from the phase transition to quark matter. Although these would be only about a second apart, they would have a clear balance between antineutrinos and muon neutrinos. The authors calculate that the difference could be visible in existing detectors.
However, this would require a supernova close enough to detect a significant neutrino burst. Separately, it’s also possible that the study of neutron stars, including the gravitational waves from neutron star mergers, would provide an additional hint of the presence of quark matter. And if people examine this model in more detail, they may find other features of the phase transition to quark matter.
Natural Astronomy2018. DOI: 10.1038/s41550-018-0583-0 (About DOIs).