Life after Death
The stars with masses between 0,8 e 8 times the solar mass, on the other hand, end up their lives as white dwarfs, where the free electrons (not bound to an atomic nucleus) exert a pressure towards the exterior that is not anymore depending on the temperature, rather just on the very high density beyond which it can't be further compressed anymore. They are called the degenerated electrons.
The star contracts until incredibly high densities are reached (10 000 times the lead's density) in a sphere with a size that is comparable with the Earth's.
From the moment when it can't be contracted anymore, it starts to irradiate only the fossil energy resulting from the previous phases of nuclear fusion and contraction. As the red dwarfs, in the end it shall become a completely inactive black dwarf.
The stars with more than 8 solar masses end up their days in a huge explosion when the centre is completely filled with iron, which is an element unable to be used as fuel for subsequent nuclear reactions.
The core contracts until the electrons are combined with the protons to be transformed into neutrons, impelling the star to assume incredibly high densities, of about 100 000 tons / mm3, comparable to the density of the atomic nuclei.
The exterior layers contract with the core and fall on it when the latter refrains its their contraction. This way, these layers are compressed and very strongly heated, originating the ignition of violent nuclear reactions, producing so high amounts of energy that the explosion becomes unavoidable.
The explosion releases huge amounts of energy, under the shape of radiation (visible and invisible) and the ejection of a vast quantity of matter into space, at about 10 000 km / second. The process that determines the explosion of a supernova also provokes an explosive emission of neutrinos.
Meanwhile, the bubble of hot matter resulting from the explosion and composed by ionized gas expands until several hundred thousand years later, when it interacts with the interstellar space and vanishes in the end. The supernovae are a precious source of material that will be used as a basis to the formation of new stars.
The supernova 1987 A (to the left), and the star that would originate its explosion (to the right)
Internal structure of a neutron star (Dany P. Page)
In the centre of the explosion a neutron star is likely to be formed. Neutron stars have diameters of just a few kilometres (inversely proportional to the mass) and are entirely made by neutrons, which sum a whole mass of 1,44 or more times the mass of the Sun (the Chandrasekhar limit).
Neutron stars are covered by solid and 1 km thick crusts, which are composed by elements like the iron. Due to the violent pressures exerted by the gravity and the centrifugal forces, these crusts sometimes crack and originate "starquakes". The matter inside these stars behaves like a superfluid, or in other words, it displays neither frictions nor any other kind of resistance against the crossing of the electric current.
It's estimated that there is a limit of about 2 to 3 solar masses for the neutron stars. Beyond that limit, the material shall be even more compressed until a black hole is formed. Some models support the idea that in the neutron stars' cores the neutrons decay to quarks, the ultimate theoretical condition of the matter before its squeezing compression into black holes. Some scientists defend that this mass of quarks takes the shape of strange matter, composed by a blend of "up", "down" and "strange" quarks, which in a few minutes after the supernova convert all the ordinary matter into strange matter. This strange matter may eventually become the overwhelming component of the star's volume.
Since a discovery about a star called RXJ185635-375, previously thought to be a neutron star, there is some speculation about the existence of quark stars, where matter would decay to even more extreme conditions. These stars would be composed by quarks not combined in neutrons or protons, so they could be more compressed than matter in a neutron star. The characteristics of this star were discovered in April 2002 and it was found to be cooler (less than 1 million ºK) and smaller (with a diameter perhaps as small as 8 km) than a neutron star would be expected to be.
Possible sighting of a quark star (NASA – Chandra X-Ray observatory)
The magnetic field axis - blue arrow - rotates as the pulsar swings around its rotation axis - in red (The Imagine! Team)
Fast rotating and strongly magnetized neutron stars produce beams of radio waves that act as cosmic lighthouses, in such a way that the observers only receive the radio signal when the beam crosses their eyesight. The speed of the pulses is connected to the star's rotation and it's uniform, although it tends to decrease as the star gets older. The already mentioned "starquakes" in neutron stars may, nevertheless, occasionally change the pulses' periodicity. To these stars is given the name of pulsars.
In super-massive stars, with more than 30 solar masses, not even the pressure exerted by the neutrons (or quarks), compressed to the mentioned high densities, is enough to counterbalance to gravity force. These stars are destined to be transformed in black holes.
In the black holes, the matter is compressed into a tiny dot, called singularity, and the gravity force becomes so strong that the escape velocity (necessary for a close object to escape from its gravity attraction) ends up exceeding the light speed, the maximum velocity that is allowed by the physical laws.
Everything placed inside a determined distance from the singularity (known as the event horizon) is relentlessly condemned to be swallowed by it and to never be able to escape. Because not even the light, under such circumstances, is able to escape from the gravity fields of these objects, they are given the name of black holes.
Black Holes Types
A black hole that is not provided with any rotation movement is known as a Schwarzchild black hole. However, more probably, the black holes preserve the rotation of the stars that gave birth to them.
To the rotating black holes is given the name of Kerr black holes. The rotation forces the event horizon to become elliptical, provoking also the reduction of its surface area. In the limit, when the rotation speed is high, the area of the event horizon becomes zero, allowing the singularity to be seen from the exterior (a naked singularity). Outside the event horizon an ergosphere is formed. Inside it the objects are forced to rotate with the black hole, but they are not necessarily destined to be swallowed by it. The limit of the ergosphere is called the static limit. Finally, a second event horizon, deeper (more central) than the first one is formed inside the black hole. If someone crosses that second horizon, he will emerge in another universe or in other place of the same universe, through objects with characteristics that are opposite to the black holes' - the white holes.
Theoretically, the black holes may, besides the rotation, being also endowed with electric charge, being those ones called the Kerr-Newman black holes. It's nevertheless improbable that a black hole is endowed with any expressive electric charge.
The Black Holes Radiation
The physician Stephen Hawking predicted that, although not even the light is able to escape from them, the black holes may emit a light amount of radiation.
Virtual pairs of particles/anti-particles form constantly in space and they spend a very short time between their mutual creation and annihilation. The law of the quantum uncertainty allows the formation of these entities in an empty space, called the false vacuum. Because it's impossible to create energy from nothing, one of the members of the pair must have negative energy, while the other must have positive energy. On the other hand, we may see the anti-particle of the pair (traveling forwards in time) as a particle that travels backward in time or vice-versa. A virtual pair won't be then more than a particle cyclically traveling forward and backward in time.
It's possible that in the outskirts of the black hole one member of the pair is, in a short time lapse, attracted to its interior, while the other manages to escape. It will look like the black hole emitted a particle or radiation, which endows it with a temperature. The falling member, because it's captured by the intense gravitational field, will assume a negative energy, while the other member assumes a positive energy, like the ordinary matter.
When the negative energy member falls inside the black hole, it will reduce this one's energy and, therefore, it deprives it from a small portion of its mass. The smaller the black hole is, the higher its temperature will be.
This way, the mini-black holes formed during the Big Bang evaporate much faster than the stellar black holes, and these ones faster than the giant black holes of the galactic centres. It's estimated that the average life expectancy of a mini-black hole is about 10 billion years - close to the Universe's age.
The slow evaporation of a black hole (MoonRunner Design, UK)