A neutron star is a type of stellar remnant that is left behind after the supernova explosion of a massive star and it consists almost entirely of neutrons. With roughly the mass of the Sun packed into an object measuring just several kilometres across, a neutron star is so dense that a cubic centimetre of its material contains an average mass of a few hundred million metric tons. Such a star is supported against further gravitational collapse by quantum degeneracy pressure where no two neutrons can occupy the same quantum state simultaneously. Between a neutron star and a black hole, another possible stable state known as a quark star can exist. A quark star is even denser than a neutron star and it is made up of quarks instead of neutrons. Similar to a neutron star, quantum degeneracy pressure prevents a quark star from gravitationally collapsing into a black hole. Above roughly 2 to 3 times the mass of the Sun, gravity eventually prevails and a neutron star or quark star is expected to collapse completely to form a back hole.
This image illustrates the size of a typical neutron star in comparison with the size of Manhattan Island.
During the gravitational collapse of a compact star, it is possible that the ever increasing densities and temperatures will eventually cause the distinction between electromagnetic and weak nuclear forces to break down. When this happens, quarks are able to convert into leptons in a process known as electroweak burning which is estimated to last for several million years. The energy produced during electroweak burning can be sufficient to stall the gravitational collapse of the compact star. Throughout this period of electroweak burning, the compact star is known as an electroweak star. Electroweak burning occurs within the core of the star, in a small and incredible dense volume measuring just several centimetres across and containing about twice the mass of Earth. Within this volume, the burning of quarks produces neutrinos which flow out of the central core by diffusion. Neutrinos cannot flow freely out of the electroweak star because the density within the core is so high that matter is opaque even to neutrinos and the mean free paths of all particles are small in relation to the size of the star.
As the neutrinos travel away from the core of the electroweak star, both the local matter density and the energy of the neutrinos will decrease, causing the mean free path of the neutrino particles to increase. The decrease in neutrino energy as a neutrino travel towards the surface of the star is due to gravitational redshift and the opacity of the high density medium through which the neutrino is travelling through. At a certain distance from the centre of the electroweak star, the neutrino’s mean free path will exceed the thickness of the star’s overlying matter. This distance denotes the position of the neutrinosphere and neutrinos crossing this boundary will freely leave the star. As such, there is no backward flow of neutrinos beyond the neutrinosphere. The radial position of the neutrinosphere from the centre of the electroweak star is directly proportional to the initial energy of the neutrinos that are produced from electroweak burning in the star’s core.
A model by De-Chang Dai et al. (2011) consists of an electroweak star with 1.3 times the mass of the Sun and a radius of 8.2 kilometres. If the initial neutrino energy is 300GeV, the radius of the neutrinosphere will be 8.1 kilometres, which places it not far under the surface of the star. This is consistent with the electroweak burning process since it produces neutrinos with energies around 300GeV. For the modelled electroweak star, its minimum lifespan is estimated to be on the order of 10 million years. Electroweak stars are an interesting new class of exotic astrophysical bodies. However, a lot more investigation is still needed to see if such objects can indeed be created from the natural processes of stellar evolution and if they can burn stably for extended periods of time.