Just to clarify to the uninitiated: Stellar remnants are what is left of a star after it dies. These come in many forms you have probably already heard of, from white dwarfs (the fate awaiting our own sun) to black holes (the impossible and mathematically stupefying singularities that plague both the dreams and nightmares of physicists). However, there are a few types of stellar remnant you probably haven’t heard of, mainly because they haven’t been confirmed to exist yet. This week, I will be going through just one type in detail.
Strange stars are… well, strange. Literally.
When a massive star goes supernova (with a core below 2.5 solar masses but above about 1.4), its outer layers crash inwards due to both the massive gravity and the sudden loss of electrostatic repulsion. These layers smack into the core and explode outwards in a shockwave. The core also collapses under its own gravity, turning from a mere ball of elements into an incredibly dense neutron “soup”, made up of, you guessed it, neutrons. This is an incredibly dense form of matter. Whereas regular matter is overwhelmingly just empty space, neutronium is completely packed with particles, with no free movement at all. The weight of just one cubic centimeter of this stuff is about 1,000,000,000 tonnes, so this kind of stellar remnant is already weird. Combined with other properties, such as its intense magnetic fields, neutron stars are veritably scary.
But, what happens if a star is very close to the threshold of becoming a black hole, but not quite massive enough? Is there some other intermediate stage? Well, I’m glad you asked, convenient reader, because I was just about to go into that…
You see, stellar remnants exist in a state of semi-equilibrium, just as their progenitor stars did. Before a star goes supernova, a delicate balance exists between the force of gravity pushing in on the star and the energy created by fusion reactions (plus electrostatic repulsion) pushing out on the star. This balance must be maintained in order to keep the star from imploding under its own gravity. After a star has gone supernova, the stellar remnant must also exist in equilibrium, with the forces of gravity being balanced by forces pushing outwards from the inside of the core. These forces are various forms of degeneracy pressure (and I don’t mean degeneracy the way your grandparents might). This degeneracy pressure is explained by the Pauli exclusion principle, which states that two identical fermions (a type of subatomic particle with a half-integer spin, and includes quarks and leptons) don’t like to occupy the same quantum states. This means that a pressure is pushing outwards to keep the remnant in equilibrium with gravity.
The degeneracy pressure keeping a white dwarf from imploding is electron degeneracy, as electrons are leptons. The degeneracy pressure supporting neutron stars is the next layer down, neutron degeneracy pressure, as neutrons are baryons (particles made up of quarks) so they don’t want to occupy the same quantum states. In neutron stars, the mass of the star and resulting gravity overwhelm the electron degeneracy pressure, but the collapse of the core is stopped by neutron degeneracy pressure. Most textbooks or resources will tell you that this is the last stop for our stellar core before it becomes a black hole, but there is actually one more theoretical limit…
The final layer of degeneracy pressure between a neutron star and a black hole is called quark degeneracy pressure. Essentially, if you take neutronium, a substance made up of neutrons packed together so tight there is no space between them, and subject it to even more intense pressures, the neutrons are expected to break down even further into their constituent quarks, forming an even denser form of matter known as quark-gluon plasma or “quark matter”, with neutron degeneracy having been overcome and quark degeneracy being the last degenerative force keeping the star from imploding before it totally collapses into a black hole.
However, this quark matter is very unstable, as it has an extremely high “Fermi energy”, and so can only exist under very high temperatures and pressures.
The Most Stable State of Matter?
This high fermi energy can be lowered significantly if you transform enough of the down quarks into strange quarks, as strange quarks have a high mass compared with other quarks. In fact, since neutrons are made of one up quark and two down quarks, it is theorized that under high enough pressures, half of the down quarks will turn into strange quarks, as as strangeness value of -1 is energetically favoured at these pressures. The inclusion of three types of quarks rather than two allows more quarks to be placed at lower energy levels, which is why this form of matter is so stable. Essentially, the quarks “trick” their way around the Pauli exclusion principle by transforming into different kinds of quarks, to allow more of them to occupy lower quantum states.
The binding energy of these “strangelet” particles per nucleon is higher than that of iron-56, the most stable nucleus, where iron’s is 930.4MeV and strange matters’ is 939MeV. This makes strange matter hypothetically the most stable form of matter in the universe, as each constituent part is bound with a higher amount of energy than the nucleons in iron. This kind of star would last for a very, very long time, essentially.
A strange star would be smaller than a neutron star, but not by a huge amount. If we were trying to find strange stars, the most telltale feature would be a slightly higher mass than is to be expected for its volume, because strange stars are denser than neutron stars. They would also be colder than they should be, as they would have used some of their energy to turn down quarks into (more massive) strange quarks. As of March 2018, we have several plausible candidates for quark stars, such as 3C58, a remnant previously thought to be a neutron star from a supernova in the 11th century, but appears to be too cool (only a million kelvin or so) and slightly too dense to be just a neutron star. It is unknown whether this is a quark star definitively, and if it is, we don’t know what kind of quark star it could be, as it could be made of mostly neutrons, but with a quark core, although the temperature of the star indicates that it may be a strange star.