Blue dwarfs- Over 70% of stars in the universe today are what is classified as red dwarf stars. These are much smaller than our Sun and live for much greater periods of time. While our Sun is predicted to expand into a red giant star billions of years from now, before collapsing into a white dwarf in perhaps 8 billion years, red dwarf stars are estimated to live for anywhere from 800 billion to 12 trillion years (!) depending on their size. This staggering difference in longevity is both due to red dwarfs burning their hydrogen supply far slower than large stars, as well as the fully convective nature of red dwarfs allowing them to use their entire hydrogen supply. Larger stars such as our Sun are normally only able to fuse the hydrogen that is inside their core.
The point is that due to the extremely long time that it takes for a red dwarf to die, the universe does not contain any red dwarfs that have lived long enough to allow for the observation of the later stages of their existence. Nonetheless, using models that are based off of observation of the lifecycles of other stars, it has been predicted that red dwarfs with a mass of 1/4th of our Sun or less will evolve into a new type of star called the blue dwarf in the late stages of its life.
All stars become more luminous as they convert more and more of their hydrogen into heavier elements, but unlike larger stars such as our Sun, which expand into red giant stars late in life, smaller red dwarf stars retain their original size and instead become hotter and hotter, shifting its light to the blue side of the electromagnetic spectrum. The blue dwarf star will continue to burn its remaining hydrogen supply for billions of years before completely running out of fuel and contracting into a white dwarf. Unlike larger stars which expand at the end of their lives, blue dwarf stars do not explode or otherwise shed their outer layers.
Black dwarfs- Not to be confused with the theoretical star above, the black dwarf is a frigid stellar remnant that is expected to form when a white dwarf cools down to the background temperature of the universe. They are "black" due to no longer emitting any heat or light whatsoever. The reason that they are considered theoretical is that it is not yet possible to prove that the universe as we know it will continue to exist long enough for such objects to form.
The universe is currently estimated to be approximately 13.7 billion years old (1.37 * 1010 in scientific notation.) It is estimated that it will take approximately 1 quadrillion years (1015) for a white dwarf to completely cool down to the background temperature of the universe. While white dwarfs, unlike stars, are no longer able to generate heat via nuclear fusion, they are extremely hot and radiate away their heat at a very slow pace.
If dark matter is made up of weakly interacting massive particles (WIMPs) as some physicists theorise, then it is estimated to take exponentially longer for black dwarfs to form. In this scenario, it is predicted that white dwarfs will slowly accrete dark matter via gravity before annihilating with it in its core. If dark matter consists of WIMPs, then it only interacts with regular matter via the force of gravity and the weak nuclear force, while not being affected by the strong nuclear force or the electromagnetic force. On smaller timescales, dark matter will very rarely interact with and annihilate with regular matter, but on much larger timescales it is expected that much of the dark matter in the galaxy will wind up being burned up inside white dwarfs. This process will keep black dwarf formation at bay for as long as a septillion years, or in scientific notation: 1024 years. This is especially mindboggling when one considers that almost all stars that will have ever lived will have died by the start of the Degenerate Era of the universe, a comparatively meager 100 trillion years, or 1014 years.
Iron stars- Once a white dwarf cools down to the background temperature of the universe and becomes a black dwarf, one would imagine that this would finally close the book on the dead star. With no more heat left to emit and no exterior objects to ever interact with (a white dwarf that lives long enough to fully cool down will likely have been ejected from its galaxy or else it will have spiraled into the black hole in the center of its galaxy beforehand,) it is left to float in the now almost entirely empty void of space for eternity. It turns out, however, that on the quantum level there are still some weird processes that will continue to give life to the cold, dead star for eons to come.
As time goes on, all of the elements inside of a black dwarf that are heavier than iron will decay into iron by way of processes such as fission and emission of alpha particles. On even longer timescales, quantum tunneling will enable cold fusion to occur between elements lighter than iron, fusing them up into iron. This process is similiar to what goes on inside massive stars, with the most notable difference is that it takes incomprehensibly longer here due to the extremely small probability of individual such events occuring.
While a number of objects on this list cannot form until the universe's age is several orders of magnitude of what it is today, the time that is required for an iron star to form is truly obscene. According to an estimate by Freeman Dyson in his paper "Time Without End: Physics and Biology in an Open Universe", the time that it would take for stellar remnants and other objects to completely transform into iron stars is approximately 101500 years - that's 1 with 1,500 zeros after it! It's worth noting that there is no guarantee that the universe will live long enough to allow for the formation of iron stars to happen and there are a lot of potential cataclysms that could change everything long before this happens, such as proton decay or the collapse of a false vacuum. Check out the "far future of the universe" page on the Observatory section of this site once it is up, for more details on this, as well as the ultimate fate of iron stars if such things do not occur.
Frozen stars- Long before even the first primeval white dwarf finally cools into a black dwarf and begins its slow, weary march towards becoming an iron star, the universe may witness the births of some even more unorthodox stellar objects.
First conjectured in Fred Adams and Gregory Laughlin's famous 1996 paper "A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects," the frozen star is a type of star that may exist many billions of years into the future when the contents of the universe will look very different from today.
Billions of years ago, when the universe had finally cooled down enough from the Big Bang that atoms could exist; hydrogen, helium, and an infinitesimal amount of lithium were the only elements in the universe. As stars began to form, live out their life, and finally explode and die, more and more hydrogen and helium were converted into metals (elements heavier than hydrogen and helium, in astronomical terms.) As dying stars exploded and scattered their elements in the form of nebula, new stars and planets formed from their ashes. As a result of these processes, each "generation" of stars has more metals than the one before.
In the far future when the proportion of metals in the gas clouds of space reaches a value of around several times of the Sun, (the Sun's metallicity is around 0.02, meaning that 2% of its matter is in the form of "metals") it is predicted that frozen stars will be able to form. These will be smaller than any star that can exist today - around 4% of the mass of the Sun or 40 times the mass of Jupiter - and despite being an active star, they will be so cold and dim that their surface temperature will be cold enough for ice clouds to form and cover their surface.
The reason for this star's bizarre nature is that the increased amount of metals inside of a star's core insulates it and prevents it from radiating away the heat that it generates as much as a regular star would. This in turn allows the star to maintain a high enough temperature in its core to allow it to continue fusing hydrogen. Were such an object to form with less metals in its core, it would be doomed to beeome a brown dwarf. Due to the extremely slow rate that they use up their hydrogen supply, these stars will live for far longer than even the smallest red dwarf star.
Quasi-stars- Just as the Moon revolves around the Earth, so too does the Earth revolve around the Sun, and the Sun itself revolve around the supermassive black hole in the center of the Milky Way. The black hole at our galaxy's center, Sagittarius A*, is estimated to have a mass of around 4 million times that of the Sun. Although already incomprehensible in size, this black hole is far from the largest one that scientists have discovered so far. Our neighbour galaxy, Andromeda, hosts a black hole that is estimated to be between 100 to 200 million times the mass of the Sun. Elsewhere, TON-618, the biggest black hole discovered so far is a mind-boggling 66 billion times the mass of the Sun!
It has long been a mystery how and why such supermassive black holes originated in the center of nearly every large galaxy so quickly. The less massive a black hole is, the slower the rate at which it is able to absorb matter. As a black hole's gravity attracts more and more matter, the matter flying towards the black holes at high speeds forms an accretion disc that heats up to blazing hot temperatures due to the interactions between all of the accelerated matter. As a result of the Eddington limit, after the accretion disc grows large enough, the radiation that it emits starts pushing away any additional incoming matter, preventing the black hole from growing any faster.
One interesting theory posits that these behemoth black holes formed from extremely massive stars that could only have existed under the conditions present in the very early universe when virtually all baryonic matter was in the form of hydrogen and helium and metals were nonexistent. Physicists have speculated that massive clouds composed of these two light elements could have formed protostars whose cores would collapse into a black hole without the outer layers of the protostar being blown away in a supernova explosion. These massive stars would then start generating vast amounts of energy from the radiation given off by the collisions occuring around the black hole as it sucks up the star's matter at velocities approaching the speed of light. This radiation would create a push outwards that would counteract the force of gravity and prevent the star from collapsing inward.
Eventually after a maximum lifespan of approximately 7 million years, the quasi-star would finally collapse into a black hole that could weigh as much 10,000 times the mass of the Sun.
Quark stars- Unlike most of the other objects on this list, which are theoretical either because the universe is too young or too old for them to have formed, this one could very well exist right now, but not enough is known to say for sure.
The vast majority of stars in the universe end their lives as white dwarfs - stellar remnants that resist gravitational collapse via electron degeneracy pressure. Basically, the electrons inside a white dwarf are resisting being compressed any further by the force of gravity due to the Pauli exclusion principle not allowing two electrons to occupy the same quantum state. If a star's core exceeds the Chandrasekhar limit of approximately 1.4 times the mass of our Sun, even electron degeneracy pressure is no longer enough to keep the core from undergoing gravitational collapse. The protons and electrons inside of the core are crushed into neutrons and the core winds up stabilising itself via neutron degeneracy pressure. The dead star's core becomes a neutron star - an incomprehensibly dense object that contains a mass greater than the Sun jammed into an area the size of a city.
In even more extreme cases where a star's core exceeds the Tolman-Oppenheimer-Volkoff limit (approximately 2.17 times the mass of the Sun,) even neutron degeneracy pressure is not enough to keep gravitational collapse at bay and the core collapses even further into a black hole. Right now, the smallest black hole that has been detected has a mass of approximately five times that of the Sun, and what lies between the smallest known naturally-formed black hole and the largest possible neutron star is currently just speculation.
In 1965, Russian physicists Dmitri Ivanenko and D.F. Kurdgelaidze proposed the existence of a possible stellar remnant that could result from the collapse of a star core that is too massive to become a neutron star, but is still able to withstand complete gravitational collapse through quark degeneracy pressure. Quarks are an elementary particle that composes the protons and neutrons which (along with electrons) form the atoms that we and everything that we interact with are made up of. In a quark star, the immense pressure of the collapsed star core would crush quarks so tightly that they would no longer even be able to form neutrons. Such an object would be so dense, that despite having a minimum mass of over twice the Sun, it would have a diameter of a small town!
Although there have been no quark stars confirmed to exist as of yet, a number of potential candidates have been observed. Most recently in 2020, scientists announced the detection of gravitational waves emitted from the collision of a black hole and an unknown object estimated to be 2.6 times the mass of the Sun - much smaller than any black hole that has ever been detected.
Electroweak stars- As extreme of an object as the quark star is, there is another stellar remnant other than a black hole that could hypothetically be even heavier and denser. According to some predictions, a star core too massive to be held together by quark degeneracy pressure could still prevent gravitational collapse for a period of time by becoming an electroweak star.
In such an object, the severe pressure and temperature would lead to conditions similiar to that of the universe moments after the Big Bang. Under such extreme conditions, the distinction between the electromagnetic and weak forces would break down and the quarks inside the star would start being converted into leptons through the process of electroweak burning. This process would also generate neutrinos and light/heat, which would create pressure outwards and keep the star from collapsing. Eventually however, after burning for millions of years, the electroweak star would run out of quarks to burn and would finally collapse into a black hole under its own gravity.