The stars in the Alpha Centauri system include a pair called “A” and “B,” (AB for short) which orbit relatively close to each other. Alpha Cen A is a near twin of our Sun in almost every way, including age, while Alpha Cen B is somewhat smaller and dimmer but still quite similar to the Sun. The third member, Alpha Cen C (also known as Proxima), is a much smaller red dwarf star that travels around the AB pair in a much larger orbit that takes it more than 10 thousand times farther from the AB pair than the Earth-Sun distance. Proxima currently holds the title of the nearest star to Earth, although AB is a very close second.

The Chandra data reveal that the prospects for life in terms of current X-ray bombardment are actually better around Alpha Cen A than for the Sun, and Alpha Cen B fares only slightly worse. Proxima, on the other hand, is a type of active red dwarf star known to frequently send out dangerous flares of X-ray radiation, and is likely hostile to life. Planets in the habitable zone around Proxima receive an average dose of X-rays about 500 times larger than the Earth, and 50,000 times larger during a big flare.  

Age: 13.51 billion years

Escape velocity: 550 km/s

Number of stars: 250–500 billion

Constellations: Orion, Sagittarius, Scorpius, Carina, Ara

Sagittarius A* is a bright and very compact supermassive black hole and astronomical radio source at the Galactic Center of the Milky Way, near the border of the constellations Sagittarius and Scorpius, about 5.6° south of the ecliptic. 

Distance to Earth: 25,640 light years

Radius: 13.67 million mi

Discovered: February 13, 1974

Coordinates: RA 17h 45m 40s | Dec -29° 0′ 28″

Distance: 8178±13 pc

The Andromeda–Milky Way collision is a galactic collision predicted to occur in about 4.5 billion years between the two largest galaxies in the Local Group—the Milky Way and the Andromeda Galaxy. The stars involved are sufficiently far apart that it is improbable that any of them will individually collide. The galaxies will pass through each other, get snapped back together by gravity, and eventually merge cores.

Distance to Earth: 251.1 million light years

Number of galaxies: 100000–150000

Binding mass: 1×10¹⁷ M_☉

Coordinates: RA 10h 32m 0s | Dec -46° 0′ 0″

Brightest member: Milky Way (mag –5.0)

Distance to Earth: 2.537 million light years

Radius: 110,000 light years

Age: 10.01 billion years

Apparent size (V): 3.167° × 1°

Coordinates: RA 0h 42m 44s | Dec +41° 16′ 9″

Four billion years from now, our galaxy, the Milky Way, will collide with our large spiraled neighbor, Andromeda. The galaxies as we know them will not survive. In fact, our solar system is going to outlive our galaxy. ... Currently, Andromeda and the Milky Way are about 2.5 million light-years apart.

Distance to Earth: 2.723 million light years

Radius: 30,000 light years

Magnitude: 5.72

Coordinates: RA 1h 33m 50s | Dec +30° 39′ 37″

Apparent size (V): 70.8 × 41.7 moa

Constellation: Triangulum

Say hello to the much-photographed Triangulum galaxy – aka Messier 33 – a face-on pinwheel of swarming suns and the second-nearest spiral galaxy to our Milky Way. This galaxy is only about 2.7 million light-years away. It’s huge, with a diameter about half that of our Milky Way. But it’s turned face on to us and thus has a low surface brightness in our sky. Although theoretically visible to the unaided eye under dark sky conditions, it’s still not easy to spot in binoculars or even a telescope. Keep reading to learn more about this nearby, face-on, very beautiful spiral galaxy.

The Andromeda galaxy shines eight to nine times more brightly than the Triangulum galaxy, which is the most distant object that you can easily see with the unaided eye. Fortunately, the Triangulum and Andromeda galaxies are a relatively close 15 degrees apart (for reference, a fist-width at an arm length approximates 10 degrees).

Star-hop to the Andromeda galaxy to orient yourself to the Triangulum galaxy. As seen on the sky chart, the star Mirach stands about midway between the two galaxies. Once you find Mirach and the Andromeda galaxy, you can draw a line between them to point in the general direction of the Triangulum galaxy.

Now for a word of warning: even if you’re staring right at the Triangulum galaxy, it’s still possible to miss it. You won’t see the galaxy’s stars at all. Sometimes, this galaxy looks almost transparent, like a water spot on a window. The small blob in your binocular field might resemble an unwashed spot on an otherwise clean window. If you’ve never seen this deep-sky object before, it’s hard to know what to look for.

Radius: 7,000 light years

Apparent mass: ~10 billion M☉

Coordinates: RA 5h 23m 34s | Dec -69° 45′ 22″

Apparent size (V): 10.75° × 9.17°

Constellations: Dorado, Mensa

he Large Magellanic Cloud (LMC), which is visible to the unaided human eye, is a familiar sight to observers in Earth’s Southern Hemisphere. Along with the Small Magellanic Cloud (SMC), not far from it on our sky’s dome, it looks like nothing so much as a small, faint bit of the Milky Way that has broken off. And yet it is not part of our Milky Way galaxy. It is a separate small galaxy, thought to be orbiting our larger Milky Way.

How to find the Large Magellanic Cloud. For observers south of about 20 degrees south latitude, the LMC is circumpolar, meaning that it can be seen (at least in part) all night every night of the year, weather permitting.

In the Northern Hemisphere, only observers south of about 20 degrees north latitude can ever see it at all. This excludes North America (except southern Mexico), Europe, northern Africa and northern Asia.

The LMC is located about 22 degrees from the South Celestial Pole, approximately on the border between the constellations Dorado and Mensa in a region of faint stars. It covers an area of the sky about 9 by 11 degrees, and shines with a total integrated magnitude of approximately zero. If all of its light were concentrated in a starlike pinpoint, it would be one of the brightest stars in the heavens. However, since the light is spread over nearly 100 square degrees, it appears only as a faint smudge.

From tropical latitudes in the Northern Hemisphere, where it still can be observed, the LMC is best seen in the evening from December to April. When the constellation Orion reaches its highest point in the sky, so does the Large Magellanic Cloud. But even at 15 degrees north latitude (the latitude of Central America), the LMC never gets far above the southern horizon.

However, it’s fairly easy to star-hop to this southern treasure by using the two brightest stars in the nighttime sky: Sirius and Canopus. Draw a line from Sirius and past the right side of Canopus to descend to the LMC. Our sky chart is designed for about 15 degrees north. Farther south, the LMC sits higher in the southern sky.

Radius: 1.9569 million light years

Distance to Earth: 1.045 billion light years

Age: 12.31 billion years

Apparent mass: ~2,500,000 billion M☉

Apparent size (V): 1'.2 × 0'.6

Number of stars: 100 trillion (10¹⁴)

-Wiki-
The galaxy is classified as a supergiant elliptical (E) to lenticular (S0)^[3] and is the brightest galaxy in A2029 (hence its other designation A2029-BCG; BCG meaning brightest cluster galaxy).^[5][6] The galaxy's morphological type is debated due to it possibly being shaped like a flat disc but only visible from Earth at its broadest dimensions. However, most lenticulars have sizes ranging from 15 to 37 kpc (50 to 120 thousand ly).^[7][specify]

IC 1101 is among the largest known galaxies, but there is debate in the astronomical literature about how to define the size of such a galaxy. Photographic plates of blue light from the galaxy (sampling stars excluding the diffuse halo) yield an effective radius (the radius within which half the light is emitted) of 65 ± 12 kpc (212 ± 39 thousand ly).^[4] The galaxy has a very large halo of much lower intensity "diffuse light" extending to a radius of 600 kpc (2 million ly).^[8] The authors of the study identifying the halo conclude that IC 1101 is "possibly one of the largest and most luminous galaxies in the universe".^[8]

Like most large galaxies, IC 1101 is populated by a number of metal-rich stars, some of which are seven billion years older than the Sun, making it appear golden yellow in color. It has a bright radio source at the center, which is likely associated with an ultramassive black hole in the mass range of 40–100 billion 

_☉, one of the largest known black holes in the universe.^[9][10]

The galaxy was discovered on 19 June 1790, by the British astronomer Frederick William Herschel I.^[11] It was catalogued in 1895 by John Louis Emil Dreyer as the 1,101st object of the Index Catalogue of Nebulae and Star Clusters (IC). At its discovery, it was identified as a nebulous feature. Following Edwin Hubble's 1932 discovery that some of the "nebulous features" were actually independent galaxies, subsequent analysis of objects in the sky were conducted and IC 1101 was therefore found to be one of the independent galaxies.

The object can not be seen by the naked eye from Earth, you need a telescope to see it. IC 1101 has a radius of 1,956,900 light years or to put it another way, it has a diameter of 3,913,800 light years. It would take a space ship 3,913,800 years travelling at the speed of light to get from one side to the other.
^{We sure do look tiny. Now multiply our one star by 200-400 billion – that's the number of stars in the Milky Way – and consider that many of them likely harbor planets. Impressive place this Milky Way … until you park it alongside IC 1101 with its 100 trillion stars.}

1.045 billion light years

IC 1101/Distance to Earth

-futuristic.com-
The largest galaxy in the known universe is simply staggering in size. But in order to understand its breadth, you need to understand a little something about our own corner of the universe. For starters, the Milky Way galaxy (our little home) is about 100,000 light-years across. Astronomers estimate that we have somewhere around 200 billion stars. Our closest neighbor, the Andromeda galaxy, is 2.5 million light-years away. It is about twice our size, coming in at 200,000 light-years.

The largest known galaxy is IC 1101. And it is nearly 6 million light-years across in total. Take a moment to try and fathom that. This galaxy could easily swallow the Milky Way, Andromeda, and all of the space in between. It’s not just larger than our galaxy; it’s larger than our corner of the universe.

The Milky Way is a mere 100 thousand light-years. This beast is 6 million.

To help you understand just how immense this is, let’s talk about how long it might take to travel from one side of the galaxy to the other.

The fastest spacecrafts that we’ve ever built are the Helios probes. They were able to reach 157,000 mph (253,000 km/hr). This is pretty fast, especially when you consider that the average commercial airplane only travels some 600 mph (960 km/hr). In fact, these probes traveled 9 times faster than the International Space Station (ISS). Yet, even at this speed, it would still take more than 4,300 years to travel just 1 light-year. That means that it would take you 430 million years to travel across the Milky Way (a long time, to be sure). But it would take you 25.8 billion years to travel across IC 1101. You would literally need to start before the dawn of time in order to complete the journey.

“But,” some may say, “technology has advanced a lot since the Helios probes.” Very true. So let’s give ourselves the benefit of the doubt. Let’s say that we could theoretically travel 670 million miles an hour (1 billion km/hr), which is roughly the speed of light. Would that shorten our journey much? Sure. But not as much as you might hope. It would still take 6 million years to cross this galaxy, even at light-speed.

Essentially, since its inception, the cosmos has been growing at an ever increasing rate. Cosmologists estimate that the oldest photons that we can observe have traveled a distance of 45-47 billion light-years since the Big Bang. That means that our observable universe is some 93 billion light-years wide (give or take a few light-years). These 93 some-odd billion light-years contain all of the quarks, quasars, stars, planets, nebulae, black holes…and everything else that we could possibly observe; however, the observable universe only contains the light that has had time to reach us. How can the universe be 93 billion light-years across if it is only 13.8 billion years old? Light hasn’t had enough time to travel that far…? (⊙_⊙;) Ultimately, understanding this facet of physics is the key to understanding what lies beyond the edge of the observable universe and whether we could ever get there.

To break this down, according to special relativity, objects that are close together cannot move faster than the speed of light with respect to one another; however, there is no such law for objects that are extremely distant from one another when the space between them is, itself, expanding. In short, it’s not that objects are traveling faster than the speed of light, but that the space between objects is expanding, causing them to fly away from each other at amazing speeds.

Ultimately, this means that we could only reach the edge of the observable universe if we develop a method of transport that allows us to either 1) Travel faster than the speed of light (something which most physicists think is impossible) 2) Transcend spacetime (by using wormholes or warp drive, which most physicists also think is impossible).

According to the theory of cosmic inflation, the entire universe’s size is at least 10^23 times larger than the size of the observable universe. That’s a lot of universe that we are missing. So, what *exactly* are we missing? What is outside the observable universe? Unfortunately, since we can’t see it or measure it, we don’t know what lies beyond the bounds of the observable universe. However, we have several theories regarding what exists in the great unknown.

 Despite its strangeness, this first idea is one of the easiest to digest. Astronomers think space outside of the observable universe might be an infinite expanse of what we see in the cosmos around us, distributed pretty much the same as it is in the observable universe. This seems logical. After all, it doesn’t make sense that one section of the universe would be different than what we see around us. And honestly, who can envision a universe that has an end—a huge brick wall lurking at its edge? So, in some ways, infinity makes sense. But “infinity” means that, beyond the observable universe, you won’t just find more planets and stars and other forms of material…you will eventually find every possible thing. Every. Possible. Thing. That means that, if this holds true and we follow it to its logical conclusion, somewhere out there, there is another person who is identical to you in every possible way, and there is also a you who is only slightly different from you in every possible way (one is an inch shorter; one got hit by a bus 5 years ago and died; one has a missing finger etc.). In fact, this “other you” may be reading this article right now; the only difference is that they just picked their nose while you didn’t (or did you?). This notion seems inconceivable. But then, infinity is rather inconceivable. Another theory deals with something called “dark flow.” In 2008, astronomers discovered something very strange and unexpected—galactic clusters were all streaming in the same direction at immense speed, over two million miles per hour. One possible cause: Massive structures outside the observable universe exerting gravitational influence. As for the structures themselves, they could be literally anything: Amazingly huge accumulations of matter and energy (on scales we can hardly imagine) or even bizarre warps in space-time that are funneling gravitational forces from other universes. We simply don’t know what these massive objects could be. Notably, recent analyses have claimed to debunk the dark flow model, but this debunking is still being disputed.

Another option involves a universe of universes. Some believe that the whole of our universe could exist in a small “bubble” in the midst of a vast array of other bubbles. Theorists call this a “multiverse.”

Interestingly, the idea asserts that these universes can come into contact with one another—gravity can flow between these parallel universes, and when they connect, a Big Bang like the one that created our universe may occur.

How might the universe end?
by Wikipedia

The fate of the universe is determined by its density. The preponderance of evidence to date, based on measurements of the rate of expansion and the mass density, favors a universe that will continue to expand indefinitely, resulting in the "Big Freeze" scenario below.^[1] However, observations are not conclusive, and alternative models are still possible.^[2]

Big Freeze or heat death

Main articles: Future of an expanding universe and Heat death of the universe

The Big Freeze (or Big Chill) is a scenario under which continued expansion results in a universe that asymptotically approaches absolute zero temperature.^[3] This scenario, in combination with the Big Rip scenario, is gaining ground as the most important hypothesis.^[4] It could, in the absence of dark energy, occur only under a flat or hyperbolic geometry. With a positive cosmological constant, it could also occur in a closed universe. In this scenario, stars are expected to form normally for 10¹² to 10¹⁴ (1–100 trillion) years, but eventually the supply of gas needed for star formation will be exhausted. As existing stars run out of fuel and cease to shine, the universe will slowly and inexorably grow darker. Eventually black holes will dominate the universe, which themselves will disappear over time as they emit Hawking radiation.^[5] Over infinite time, there would be a spontaneous entropy decrease by the Poincaré recurrence theorem, thermal fluctuations,^[6][7] and the fluctuation theorem.^[8][9]

A related scenario is heat death, which states that the universe goes to a state of maximum entropy in which everything is evenly distributed and there are no gradients—which are needed to sustain information processing, one form of which is life. The heat death scenario is compatible with any of the three spatial models, but requires that the universe reach an eventual temperature minimum.^[10]

Big Rip

Main article: Big Rip

The current Hubble constant defines a rate of acceleration of the universe not large enough to destroy local structures like galaxies, which are held together by gravity, but large enough to increase the space between them. A steady increase in the Hubble constant to infinity would result in all material objects in the universe, starting with galaxies and eventually (in a finite time) all forms, no matter how small, disintegrating into unbound elementary particles, radiation and beyond. As the energy density, scale factor and expansion rate become infinite the universe ends as what is effectively a singularity.

In the special case of phantom dark energy, which has supposed negative kinetic energy that would result in a higher rate of acceleration than other cosmological constants predict, a more sudden big rip could occur.

Big Crunch

Main article: Big Crunch

The Big Crunch. The vertical axis can be considered as expansion or contraction with time.

The Big Crunch hypothesis is a symmetric view of the ultimate fate of the universe. Just as the Big Bang started as a cosmological expansion, this theory assumes that the average density of the universe will be enough to stop its expansion and the universe will begin contracting. The end result is unknown; a simple estimation would have all the matter and space-time in the universe collapse into a dimensionless singularity back into how the universe started with the Big Bang, but at these scales unknown quantum effects need to be considered (see Quantum gravity). Recent evidence suggests that this scenario is unlikely but has not been ruled out, as measurements have been available only over a short period of time, relatively speaking, and could reverse in the future.^[4]

This scenario allows the Big Bang to occur immediately after the Big Crunch of a preceding universe. If this happens repeatedly, it creates a cyclic model, which is also known as an oscillatory universe. The universe could then consist of an infinite sequence of finite universes, with each finite universe ending with a Big Crunch that is also the Big Bang of the next universe. A problem with the cyclic universe is that it does not reconcile with the second law of thermodynamics, as entropy would build up from oscillation to oscillation and cause the eventual heat death of the universe. Current evidence also indicates the universe is not closed. This has caused cosmologists to abandon the oscillating universe model. A somewhat similar idea is embraced by the cyclic model, but this idea evades heat death because of an expansion of the branes that dilutes entropy accumulated in the previous cycle. ^{[citation needed]}

Big Bounce

Main article: Big Bounce

The Big Bounce is a theorized scientific model related to the beginning of the known universe. It derives from the oscillatory universe or cyclic repetition interpretation of the Big Bang where the first cosmological event was the result of the collapse of a previous universe.

According to one version of the Big Bang theory of cosmology, in the beginning the universe was infinitely dense. Such a description seems to be at odds with other more widely accepted theories, especially quantum mechanics and its uncertainty principle. ^{[citation needed]} It is not surprising, therefore, that quantum mechanics has given rise to an alternative version of the Big Bang theory. Also, if the universe is closed, this theory would predict that once this universe collapses it will spawn another universe in an event similar to the Big Bang after a universal singularity is reached or a repulsive quantum force causes re-expansion.

In simple terms, this theory states that the universe will continuously repeat the cycle of a Big Bang, followed up with a Big Crunch.

Big Slurp

Main article: False vacuum

This theory posits that the universe currently exists in a false vacuum and that it could become a true vacuum at any moment.

In order to best understand the false vacuum collapse theory, one must first understand the Higgs field which permeates the universe. Much like an electromagnetic field, it varies in strength based upon its potential. A true vacuum exists so long as the universe exists in its lowest energy state, in which case the false vacuum theory is irrelevant. However, if the vacuum is not in its lowest energy state (a false vacuum), it could tunnel into a lower energy state.^[11] This is called vacuum decay. This has the potential to fundamentally alter our universe; in more audacious scenarios even the various physical constants could have different values, severely affecting the foundations of matter, energy, and spacetime. It is also possible that all structures will be destroyed instantaneously, without any forewarning.^[12]

Cosmic uncertainty.

what is a singularity?

thanks again, wiki.
So a gravitational singularity, spacetime singularity or simply singularity is a location in spacetime where the gravitational field of a celestial body is predicted to become infinite by general relativity in a way that does not depend on the coordinate system. The quantities used to measure gravitational field strength are the scalar invariant curvatures of spacetime, which includes a measure of the density of matter. Since such quantities become infinite at the singularity, the laws of normal spacetime break down.^[1][2]

Gravitational singularities are mainly considered in the context of general relativity, where density apparently becomes infinite at the center of a black hole, and within astrophysics and cosmology as the earliest state of the universe during the Big Bang. Physicists are undecided whether the prediction of singularities means that they actually exist (or existed at the start of the Big Bang), or that current knowledge is insufficient to describe what happens at such extreme densities.

General relativity predicts that any object collapsing beyond a certain point (for stars this is the Schwarzschild radius) would form a black hole, inside which a singularity (covered by an event horizon) would be formed.^[3] The Penrose–Hawking singularity theorems define a singularity to have geodesics that cannot be extended in a smooth manner.^[4] The termination of such a geodesic is considered to be the singularity.

The initial state of the universe, at the beginning of the Big Bang, is also predicted by modern theories to have been a singularity.^[5] In this case the universe did not collapse into a black hole, because currently-known calculations and density limits for gravitational collapse are usually based upon objects of relatively constant size, such as stars, and do not necessarily apply in the same way to rapidly expanding space such as the Big Bang. Neither general relativity nor quantum mechanics can currently describe the earliest moments of the Big Bang,^[6] but in general, quantum mechanics does not permit particles to inhabit a space smaller than their wavelengths.^[7]

Many theories in physics have mathematical singularities of one kind or another. Equations for these physical theories predict that the ball of mass of some quantity becomes infinite or increases without limit. This is generally a sign for a missing piece in the theory, as in the ultraviolet catastrophe, re-normalization, and instability of a hydrogen atom predicted by the Larmor formula.
A conical singularity occurs when there is a point where the limit of every diffeomorphism invariant quantity is finite, in which case spacetime is not smooth at the point of the limit itself. Thus, spacetime looks like a cone around this point, where the singularity is located at the tip of the cone. The metric can be finite everywhere coordinate system is used.

An example of such a conical singularity is a cosmic string and a Schwarzschild black hole.^[9]

Solutions to the equations of general relativity or another theory of gravity (such as supergravity) often result in encountering points where the metric blows up to infinity. However, many of these points are completely regular, and the infinities are merely a result of using an inappropriate coordinate system at this point. In order to test whether there is a singularity at a certain point, one must check whether at this point diffeomorphism invariant quantities (i.e. scalars) become infinite. Such quantities are the same in every coordinate system, so these infinities will not "go away" by a change of coordinates.

An example is the Schwarzschild solution that describes a non-rotating, uncharged black hole. In coordinate systems convenient for working in regions far away from the black hole, a part of the metric becomes infinite at the event horizon. However, spacetime at the event horizon is regular. The regularity becomes evident when changing to another coordinate system (such as the Kruskal coordinates), where the metric is perfectly smooth. On the other hand, in the center of the black hole, where the metric becomes infinite as well, the solutions suggest a singularity exists. The existence of the singularity can be verified by noting that the Kretschmann scalar, being the square of the Riemann tensor i.e. {\displaystyle R_{\mu \nu \rho \sigma }R^{\mu \nu \rho \sigma }}, which is diffeomorphism invariant, is infinite.

While in a non-rotating black hole the singularity occurs at a single point in the model coordinates, called a "point singularity", in a rotating black hole, also known as a Kerr black hole, the singularity occurs on a ring (a circular line), known as a "ring singularity". Such a singularity may also theoretically become a wormhole.^[10]

More generally, a spacetime is considered singular if it is geodesically incomplete, meaning that there are freely-falling particles whose motion cannot be determined beyond a finite time, being after the point of reaching the singularity. For example, any observer inside the event horizon of a non-rotating black hole would fall into its center within a finite period of time. The classical version of the Big Bang cosmological model of the universe contains a causal singularity at the start of time (t=0), where all time-like geodesics have no extensions into the past. Extrapolating backward to this hypothetical time 0 results in a universe with all spatial dimensions of size zero, infinite density, infinite temperature, and infinite spacetime curvature.

Some theories, such as the theory of loop quantum gravity, suggest that singularities may not exist.^[8] This is also true for such classical unified field theories as the Einstein–Maxwell–Dirac equations. The idea can be stated in the form that due to quantum gravity effects, there is a minimum distance beyond which the force of gravity no longer continues to increase as the distance between the masses becomes shorter, or alternatively that interpenetrating particle waves mask gravitational effects that would be felt at a distance.


Until the early 1990s, it was widely believed that general relativity hides every singularity behind an event horizon, making naked singularities impossible. This is referred to as the cosmic censorship hypothesis. However, in 1991, physicists Stuart Shapiro and Saul Teukolsky performed computer simulations of a rotating plane of dust that indicated that general relativity might allow for "naked" singularities. What these objects would actually look like in such a model is unknown. Nor is it known whether singularities would still arise if the simplifying assumptions used to make the simulation were removed. However, it is hypothesized that light entering a singularity would similarly have its geodesics terminated, thus making the naked singularity look like a black hole.^[11][12][13]

Disappearing event horizons exist in the Kerr metric, which is a spinning black hole in a vacuum, if the angular momentum ({\displaystyle J}) is high enough. Transforming the Kerr metric to Boyer–Lindquist coordinates, it can be shown^[14] that the coordinate (which is not the radius) of the event horizon is, {\displaystyle r_{\pm }=\mu \pm (\mu ^{2}-a^{2})^{1/2}}, where {\displaystyle \mu =GM/c^{2}}, and {\displaystyle a=J/Mc}. In this case, "event horizons disappear" means when the solutions are complex for {\displaystyle r_{\pm }}, or {\displaystyle \mu ^{2}<a^{2}}. However, this corresponds to a case where {\displaystyle J} exceeds {\displaystyle GM^{2}/c} (or in Planck units, {\displaystyle J>M^{2}}), i.e. the spin exceeds what is normally viewed as the upper limit of its physically possible values.

Similarly, disappearing event horizons can also be seen with the Reissner–Nordström geometry of a charged black hole if the charge (sorry fot the displacement) ({\displaystyle Q}) is high enough. In this metric, it can be shown^[15] that the singularities occur at {\displaystyle r_{\pm }=\mu \pm (\mu ^{2}-q^{2})^{1/2}}, where {\displaystyle \mu =GM/c^{2}}, and {\displaystyle q^{2}=GQ^{2}/(4\pi \epsilon _{0}c^{4})}. Of the three possible cases for the relative values of {\displaystyle \mu } and {\displaystyle q}, the case where {\displaystyle \mu ^{2}<q^{2}} causes both {\displaystyle r_{\pm }} to be complex. This means the metric is regular for all positive values of {\displaystyle r}, or in other words, the singularity has no event horizon. However, this corresponds to a case where {\displaystyle Q/{\sqrt {4\pi \epsilon _{0}}}} exceeds {\displaystyle M{\sqrt {G}}} (or in Planck units, {\displaystyle Q>M}), i.e. the charge exceeds what is normally viewed as the upper limit of its physically possible values. Also, actual astrophysical black holes are not expected to possess any appreciable charge.

A black hole possessing the lowest {\displaystyle M} value consistent with its {\displaystyle J} and {\displaystyle Q} values and the limits noted above, i.e., one just at the point of losing its event horizon, is termed extremal.

Before Stephen Hawking came up with the concept of Hawking radiation, the question of black holes having entropy had been avoided. However, this concept demonstrates that black holes radiate energy, which conserves entropy and solves the incompatibility problems with the second law of thermodynamics. Entropy, however, implies heat and therefore temperature. The loss of energy also implies that black holes do not last forever, but rather evaporate or decay slowly. Black hole temperature is inversely related to mass.^[16] All known black hole candidates are so large that their temperature is far below that of the cosmic background radiation, which means they will gain energy on net by absorbing this radiation. They cannot begin to lose energy on net until the background temperature falls below their own temperature. This will occur at a cosmological redshift of more than one million, rather than the thousand or so since the background radiation formed.