Like ordinary stars, planets, and other celestial bodies, black holes, which astronomers have been able to detect in recent years, have certain physical properties that distinguish them from the others. Yet because of a black hole's extraordinary nature, especially the fact that it is black and invisible, very few of its properties can be directly measured from the outside. "From the outside," John Gribbin explains,



you can [calculate] the mass of the hole from its gravitational attraction and the speed with which it rotates. If it has an electric charge, you could measure that as well. But those  properties areallyou can ever measure. There is no way to tell what the matter that went into the hole was before it was swallowed up  whether it was a star, a great glob of water, or a pile of frozen TV dinners. There is no way to distinguish a black hole made of stellar material from one made of anything else, a property summed up by [scientists] in the expression "black holes have no hair [distinct, visible physical characteristics]," coined by [John] Wheeler and his colleague Kip Thorne in the early 1970s.





Physical properties



The simplest static black holes have mass but neither electric charge nor angular momentum. These black holes are often referred to asSchwarzschild black holesafter Karl Schwarzschild who discovered thissolutionin 1916.according toBirkhoff's theorem, it is the onlyvacuum solutionthat isspherically symmetric.this means that there is no observable difference between the gravitational field of such a black hole and that of any other spherical object of the same mass. The popular notion of a black hole "sucking in everything" in its surroundings is therefore only correct near a black hole's horizon; far away, the external gravitational field is identical to that of any other body of the same mass.



Solutions describing more general black holes also exist. Non-rotatingcharged black holesare described by theReissnerNordstrm metric, while theKerr metricdescribes a non-chargedrotating black hole. The most generalstationaryblack hole solution known is theKerrNewman metric, which describes a black hole with both charge and angular momentum.



While the mass of a black hole can take any positive value, the charge and angular momentum are constrained by the mass. InPlanck units, the total electric chargeQand the total angular momentumJare expected to satisfy



{displaystyle Q^{2}+left({tfrac {J}{M}}right)^{2}leq M^{2},}



for a black hole of massM. Black holes with the minimum possible mass satisfying this inequality are calledextremal. Solutions of Einstein's equations that violate this inequality exist, but they do not possess an event horizon. These solutions have so-callednaked singularitiesthat can be observed from the outside, and hence are deemedunphysical. Thecosmic censorship hypothesisrules out the formation of such singularities, when they are created through the gravitational collapse ofrealistic matter.[]This is supported by numerical simulations.



Due to the relatively large strength of theelectromagnetic force, black holes forming from the collapse of stars are expected to retain the nearly neutral charge of the star. Rotation, however, is expected to be a universal feature of compact astrophysical objects. The black-hole candidate binary X-ray sourceGRS 1915+105 appears to have an angular momentum near the maximum allowed value. That uncharged limit is



{displaystyle Jleq {frac {GM^{2}}{c}},}



allowing definition of adimensionlessspin parameter such that



{displaystyle {frac {cJ}{GM^{2}}}leq 1.}