When a giant star dies, it can collapse into a black hole or implode into an ultra-dense neutron star. But there are even stranger possibilities. Here are some theorized stars that make black holes look downright boring.
Before we get to all the weird stars, we should probably review the stars we already know about. Ordinary stars are just gigantic bodies of luminous plasma, most made up of hydrogen but also containing varying amounts of other elements. The mass of stars can vary quite a bit, from tiny red dwarfs to blue super-giants to average-sized yellow spheres like our Sun, but the internal makeup is all more or less the same.
When these stars reach the end of their lives, one of two things can happen. A medium or low mass star goes through a brief period of expansion, and then it contracts back into a white dwarf, a pale husk of the old star that can exist for incalculable eons. But if the star is massive enough, it will explode in a massive supernova, then collapse in on itself to form a neutron star, which can then ultimately become a black hole.
Our Friend the Neutron Star
To understand the stars we're about to talk about, a word needs to be said about neutron stars. The name for these stars is more apt than people generally realize - these stars are composed mostly of neutrons, although other particles are involved as well. Our current models suggest there's an outer layer of regular atoms surrounded by free electrons, then further inside there are nuclei of protons and neutrons, which generally have far more neutrons in them than protons. Still further is a mix of free neutrons, nuclei, and free electrons, and then at the core of the neutron star...well, we don't actually know that bit yet.
The key thing is that neutron stars are what happens when the force of gravity is strong enough to cram a star's worth of matter into a sphere only about twenty miles across. At such absurd densities, the matter takes on a new degenerate form, which is the neutron matter that we see in neutron stars. And if the densities are even more intense than can be supported by neutron matter, the star completely collapses into itself to form the singularity of a black hole.
That's all simple enough, right? But what if there are other kinds of compact stars between neutron stars and black holes? After all, a compact star has to be at least 10 times the mass of the Sun (or 10 solar masses) to become a black hole, but neutron stars are only the definite fate for compact stars between 1.5 and 3 solar masses. What about all the compact stars with masses between 3 and 10 solar masses? Well now, here's where things get weird...or, to be more precise, things get strange.
Let's Get Strange: Quark Stars
The reason why neutron stars can withstand the tremendous gravitational forces that come with being so dense is a quantum property known as degeneracy pressure. Basically, this is where matter reaches such an incredibly high density that the only thing keep its component particles separate is that the laws of quantum mechanics forbid them from occupying the same quantum states. Since individual neutrons are much smaller than atoms, it's possible to cram them much closer together in a neutron star than would be with even the most tightly packed array of atoms.
Now what if you reached a point where the neutron stars could be pressed together no further? Then - and this only a theory - the neutrons might start breaking down into their own component parts, the quarks. Neutrons are composed of one up quark and two down quarks (if you need a refresher on quarks, check out our guide). Some of those down quarks might then turn into their heavier siblings, the strange quarks, and the resultant soup of quarks is known as strange matter after those particles.
So then, if this hypothetical star has just up and down quarks, it is a quark star, and if there are strange stars mixed in as well, then it's a strange star. But do either of these stars exist? The problem from a theoretical standpoint is we just don't know enough about the equations that govern the behavior of neutron-degenerate matter and quark-degenerate matter, so we can't really know for sure.
But don't abandon hope for quark stars - there may not be any theoretical proof, but there's a decent amount of empirical evidence for them. We may not know if quark stars exist, but we at least have a reasonable idea of what they would look like if they did, and two rather odd neutron stars look an awful lot like quark stars. There's RX J1856.5-3754, the closest neutron star to Earth at just 150 light-years away. It has a diameter of just seven miles across, which is too small for the standard models of neutron star formation to explain. Then there's 3C58, another neutron star that has a very high rate of cooling, again outside the range of temperature change of which neutron stars are seemingly capable.
Both of these stars have been put forward as quark star candidates, although most scientists aren't yet ready to accept the existence of such bodies based on the apparent irregularities of these two stars. Again, the problem is one of a lack of knowledge - we haven't yet precisely defined the boundaries of what a neutron star can be, so we don't know for sure that these definitely aren't neutron stars. Based on the current evidence, it's far more logical to allow for a couple of extreme neutron stars than to invent an entirely new type of star to account for them.
That said, there are a few other possible signs of the existence of quark stars. If neutron stars do collapse into quark stars, that incident would not go unnoticed - in fact, it would likely trigger the most violent explosion in the universe, releasing an unimaginable 10^47 joules worth of energy. Indeed, there's some thought these stellar conversions are responsible for the some of the most intense gamma ray bursts we've observed.
The collapse of neutron star into a quark star is known as a quark-nova, and a number of recent supernova explosions that might have really been quark-novae. There's the supernova SN 2006gy, in which a star 150 times the mass of our Sun exploded 238 million light-years away, releasing 10^45 joules of energy. It's possible this explosion was not actually of the star itself, but rather its resultant neutron star undergoing the sudden change into a quark star.
Then there's SN1987A, which was another unusually bright supernova that intriguingly left no neutron star behind despite the fact that all our current models suggest it should have. One possibility is that the stellar core collapsed into a quark star, although this only just one theory. A pair of incredibly bright supernovae, SN2005gj and SN2005ap, were both so luminous that astronomers suggested they might actually be quark-novae.
None of these represents conclusive evidence, and quark stars remain mostly theoretical. Still, there's a slowly growing body of evidence for their existence, and it's possible that quark stars might be what black holes were a generation ago - much mooted objects that lacked enough definitive evidence to be widely accepted. Black holes eventually reached a tipping point of supporting evidence and became readily accepted, and so too might quark stars one of these days. There's also a bit of practical bonus to accept the existence of such objects - quark stars could go some way to accounting for all the missing dark matter.
Going Deeper: Preon Stars
We might as well take the degeneracy idea a step further and ask just what happens to quark matter when it becomes too compact for the individual particles to remain separate. Well, there's two possibilities. The generally accepted answer is that gravity completely overwhelms everything and crushes the particles into a singularity of infinite density, which forms a black hole.
But there is another, wilder idea. Neutrons are composite particles that are made up of quarks...what if quarks are also composite particles, made up of even more fundamental particles? If that were so, then a sufficiently dense star might go from a quark star to a preon star composed of sub-sub-subatomic preons. Preons are proposed point-like particles that are the real constituent parts of all other particles, and different combinations of preons will eventually give you every single type of particle and explain all the seemingly arbitrary values and properties of larger particles.
It's a neat theory, but it's in direct opposition to the Standard Model of Physics, which so far has done an excellent job of explaining the nature of the universe, albeit with a few glaring holes left to figure out. The preon model is generally pretty unpopular with particle physicists, and it's generally considered an unlikely candidate to explain the nature of the universe. It does have a slim chance at gaining a theoretical foothold if the Higgs boson remains undiscovered, as a key feature of the preon model is the non-existence of the Higgs.
Still, let's grant for the sake of argument that preons exist, and at sufficiently ludicrous densities it's possible to form preon matter and, in turn, a preon star. These stars would still have diameters greater than four or so miles long - anything less and a black hole would have to form. Preon stars would pack 10^23 kilograms into every cubic meter, and a preon star with the mass of Earth would be the size of a tennis ball.
Currently, we haven't found any possible preon stars of preon-novae, and the evidence from particle accelerator research weighs heavily against the existence of preons. It's not impossible, exactly, but it is extremely improbable, and probably best left to the domain of science fiction unless astronomers or particle physicists make a truly shocking discovery that supports their existence.
And, in case you're wondering, if preons do exist, they almost certainly would be the end of the line and have no further constituent particles. A preon star that was crushed together long enough would go straight to a black hole.
Something in the Middle: Electroweak Stars
For the sake of argument, let's forget about quarks and preons and just go back to the accepted model, massive stars can either become neutron stars or black holes. Now, let's say a neutron star has reached a gravitational tipping point and is about to collapse into a black hole. The temperatures kicked up by such a process are beyond intense, and it's actually possible that conditions inside the star get hot enough for forces to merge, as electromagnetism and the weak nuclear force fall into one another and become the electroweak force.
The electroweak force likely hasn't existed since the early universe dropped below the critical temperature and forced it to split into two separate forces. But neutron stars with just the right density might be able to rekindle this force and become electroweak stars. So what does that mean? Well, one of the features of the electroweak force is that it converts quarks into leptons, far lighter particles like the electron or neutrino.
Losing so much mass in the conversion would release gargantuan amounts of energy, and it might be enough to actually stop the final collapse of the star. An electroweak star could then persist for another ten million years before the collapse begins again and it finally becomes a black hole. The size and mass of these stars wouldn't be so different from their neutron star cousins, but their energy signatures would be completely different.
Most of the energy emitted by electroweak stars would come in the form of neutrinos, which are notoriously difficult to detect. A small amount of the star's energy would be emitted as visible light, and so we'd need to find a small light signature that doesn't match the expected energy emissions of its star. How we'd actually go about doing that is beyond our current understanding, but we might well figure out a way one of these days.
And Now For Something Completely Different: Boson Stars
Up to this point, all the stars we've discussed were composed - at least initially - of fermions, the particle family to which electrons, protons, neutrons, and quarks all belong. But what if the other major particle group, the bosons, could form stars? The best-known bosons are the ones that carry the forces of the universe, like the electromagnetism carrying photons or the strong force carrying gluons. (Again, for more on the differences between fermions and bosons, check out our guide.)
Still, it's conceivable that there's a type of boson that could form its own type of matter. Such a boson would have to be low mass and stable, and none of the bosons currently known quite fit the bill, but it's within the realm of theoretical possibility. If such bosons existed, enough of them could come together to form a boson star. We don't have any evidence for this, but that hasn't stopped physicists from speculating, and there are a couple rather intriguing reasons not to give up on their existence just yet.
You see, if there are any boson stars out there, the most likely place they're hiding is at the center of galaxies. In particular, galaxies with so-called active galactic nuclei, which are galactic centers that are far brighter than we would have expected, might well have boson stars at their center. Admittedly, although the presence of boson stars would do a good job of explaining the properties of active galactic nuclei, they wouldn't do a markedly better job than super-massive black holes, and the latter doesn't require invoking a whole new form of matter.
Boson stars most likely would have formed during the extreme gravitational crucible of the earliest universe. Since most of the active galactic cores are observed in the most distant, and thus also most ancient galaxies, this might tally together rather well. Still, this is all just some nifty theoretical guesswork, and there's not nearly enough evidence to support their existence. That said, if there's one type of star in this guide that I'm rooting for to be real, I'd have to say it's the boson star. There's just something kind of awesome about an entirely unknown kind of star lurking at the center of galaxies.
So What's Left?
Quite a bit, actually. We've looked at some possible fates for giant stars that don't involve becoming black holes, but all of these at least assume black holes exist. There are other cosmologies, however, that argue black holes don't actually exist, and there are objects that are weirder still waiting out there to take their place. Objects like gravastars and fuzzballs and Magnetospheric Eternally Collapsing Objects...but these, I think, we'll save for another time. If you want to know more about the stars we've already talked about, here's some great further reading:
Second Supernovae Point To Quark Stars (Astronomy Now)
Could the compact remnant of SN 1987A be a quark star? (arXiv)
Preon stars: a new class of cosmic compact objects (arXiv)
Theorists propose a new way to shine - and a new kind of star (Astronomy News)
Electroweak stars: how nature may capitalize on the standard model's ultimate fuel (arXiv)
Supermassive boson star at the galactic center? (Physical Review D)
The cosmological formation of boson stars (Physics Letters B)