When asked what the strangest object in the universe is there will be an almost unanimous answer: a black hole.
Why? Because they’ve captured the imagination of the entire planet. An infinitely dense, infinitely small chunk of matter…on its own, that is enough to conjure up some weird and wonderful thoughts. It boggles the mind. Black holes seem so strange that some scientists believe that they must not exist and have, therefore, come up with other ideas regarding their existence. There are many weird and wonderful ideas… one of these is the Quark Star or Strange Star.
The reason why it goes by the name “strange star” is because it is a massive compact stellar object made up of strange matter (I mention a quark star as well because they are very similar).
So where does a quark star fit into the whole stellar scheme of things? In some respects it replaces both the neutron star and the black hole, but at the same time it doesn’t. Right now we’re entering a very fuzzy area in our knowledge of physics; we don’t fully understand how neutron degeneracy acts, and we know even less about what is really happening inside a black hole. This uncertainty makes our figures a little hazy. The minimum mass of a neutron star is set by the maximum mass of a white dwarf, 1.4 solar masses.
The maximum mass, however, isn’t quite known as physics becomes a *little* complicated in a neutron star (to make an understatement). It has been suggested that the maximum mass of a neutron star is about 2.2 solar masses, which is a little higher than had been previously calculated. However, due to uncertainties, the upper limit may even be as high as 2.9 solar masses. With the largest neutron star discovered to date only being 2 solar masses, it seems far more likely that the conservative number of 2.2 is more likely.
What about a black hole? What is their minimum mass? Well, the smallest found to date is 3.8 solar masses (but black holes are expected to fall somewhere within the 3 solar masses range). You can now see that there is a bit of an obvious gap in here, a bit of a break between the neutron star and the black hole. This is where the quark star fits in. No quark star have been discovered just yet, nor is it a complete replacement of a black hole (there are many other theoretical stellar objects, like the gravastar, for that). It just replaces the need for a neutron star to be larger than what our estimates indicate is possible, or for a black hole to be even stranger than it already is… if that’s even possible.
So, what is a quark star? Well, a neutron star is held up by neutron degeneracy, the repulsive forces of the neutrons themselves not allowing gravity to win. A quark star is when gravity wins and the neutrons are broken down into their constitute parts: one up quark and two down quarks. And then quark degeneracy stops gravity from winning.
Earlier on I mentioned that there were strange stars as well, these are quark stars but a little, well, stranger. What can theoretically happen is, some of those down quarks could change into another form of matter known as “strange quarks,” if these kinds of quarks are mixed in with the quark star then it becomes known as a strange star.
The modelling of neutron stars and quark stars are quite different, neutron stars look far more complicated, but that could also just be that we know more about neutron degeneracy than quark degeneracy.
The theoretical size of quark stars is also interesting to note. When stars degenerate further down the line they get smaller, black holes are smaller than neutron stars, neutron stars are smaller than white dwarfs, white dwarfs are smaller than actual stars. Quark stars are thought to break this mold. They might actually be larger than neutron stars. It is thought that a 2.5 solar mass quark star may actually be larger than a 2 solar mass neutron star, it seems contrary to logic, but we are dealing with strange matter here!
Not having discovered any quark stars yet, doesn’t mean that they’re not out there. Remember, it was only about a decade ago that black holes were still considered theoretical, partially because many scientists believed that they were just too strange to exist. Now they’re an everyday topic. In another decade, quark stars might be the next big thing…and then we’ll have to start searching for Preon Stars!
Notably, quark stars may not exactly be alternatives to black holes in general, only the smaller stellar mass black holes.