Weird star theories will attempt to list all the crazy ideas for other possible stellar objects that physicists have come up with. Some are possible, others are mere conjecture. All are weird!
A lot can happen to states of matter between the neutron star stage and the final collapse into a black hole. Indeed, on this site's page on neutron stars, we've already considered two things - quark stars and strange stars.
However, another equilibrium could theoretically be reached, just as the black hole collapse is about to happen. Stars with just the right density might reach sufficiently high temperatures that the electromagnetic force and the weak force would be caused to merge, forming the electroweak force.
This combined force then acts on the quarks within the star and turns them into leptons and these are far less massive than the quarks they started out as. The upshot of this is that the electroweak star gives off huge amounts of energy - maybe even enough to halt further collapse. Thus, such a star could remain in this state for many millions of years, before succumbing to its fate and becoming a black hole.
It wouldn't shine very brightly, most of its energy output being in the form of neutrinos.
This is one of the more sensible postulations. For even more weird star theories, read on...
In the 1970s, some physicists suggested that quarks were not elementary particles at all and proposed the existence of an even smaller particle that constituted them, which they named the "preon". So, in a sense, they were sub-subatomic particles.
The preon star theory considers that matter could be compressed still further than the quark state, resulting in a star made entirely of these things.
Trouble is, the existence of the preon is only possible without the existence of the Higgs boson. Since this seems to have been discovered by the Large Hadron Collider, it would appear that these "point-like particles" don't exist after all.
This, together with the next three examples, is an alternative theory to explain the phenomenon currently identified as a black hole. As a black hole is merely a collapsed star, these still qualify as weird star theories.
The name comes from GRAvitational VAcuum STAR and it's an attempt to incorporate quantum mechanics into the explanation. The theory utilises the "Planck length", which is generally accepted in quantum physics as being the shortest possible length measurable. It is many many times smaller than an electron and it is thought that any lengths shorter than it would have no meaning in physics.
It is known that, as light approaches a black hole, its wavelength becomes shorter - it is "blue shifted". Taking things to their logical conclusion, the light wavelength will eventually become shorter than the Planck length, creating, so the theory goes, a "gravitational vacuum", or an area of "immeasurability". It has also been described as a "void in space-time". Outside of that would be an area of Bose-Einstein condensate, an exotic form of matter, with extremely high density and a temperature about one billionth of a degree above absolute zero.
An even more crazy idea has been proposed that the whole Universe is inside a giant gravastar!
These initials stand for Magnetospheric Eternally Collapsing Object, no less. The theory goes that complete collapse of a star couldn't go all the way to becoming a black hole - the collapse would be brought to a near halt by radiation pressure. Thus the object is "eternally collapsing".
The "magnetospheric" bit refers to the fact that these things have a magnetic field, a feature not displayed by some black holes.
This theory proposing an alternative to black holes utilises superstring theory. It attempts to address the paradox that the centre of a black hole (the "singularity") in conventional theory is said to be of infinite density and of zero dimensions - an impossibility when considered with known physics.
Instead, the proponents of the fuzzball theory suggest that, beyond the event horizon, the centre is a ball of strings. We're not talking about the sort of string you wrap parcels with, of course, but the strings found in string theory.
The logic goes that, as all particles are ultimately made from "strings", oscillating at different rates, then collapsing matter will degenerate past the quark stage ending as these ultimate objects.
The event horizon of such an object has been calculated as only a few Planck lengths across.
When Einstein produced his theory of general relativity, it predicted the formation of black holes. Now, we need to consider what happens at the event horizon. To an outside observer, objects falling into the black hole would appear to become frozen in place at this point - as if time stood still. This was fine - Einstein's theory allows for time to be affected by gravity.
But, with the invention of quantum mechanics, this became a problem, as this scientific discipline relies on there being a "universal time" that is constant Universe-wide.
So, the "dark energy star" theory was proposed, suggesting that, at the event horizon, a "quantum phase transition" of space-time occurs, resulting in the area within the event horizon becoming dark energy. This will be negative energy and any particles falling into it will be bounced back out again after being converted to their anti-particle. They will then annihilate other particles, producing a huge outburst of energy.
This is, by far, the strangest of all the weird star theories. If you've read this site's pages on subatomic particles, you'll know that bosons are force carrying particles and not the sort that actually form matter. So how could a star form from them?
It's not beyond the bounds of possibility that an undiscovered boson could exist that could form matter - and such matter could then clump together to form a star. The boson involved would have to be stable and with low mass and none of the known bosons have these attributes.
However, if these stars did exist, they would most likely reside at the centre of galaxies. It's a sobering thought isn't it that, at the centre of our Milky Way, there might be lurking an object composed of a completely unknown form of matter.