When Julius Caesar's final remark of "Et tu, Brute" commented on the underlying motivations for Brutus's fatal action, no one could have known that the metal of the blade used was created in a star and flung into interstellar space by a seeding distribution of metals and exotic elements forged in the cataclysmic self-destruction of a stellar explosion - the supernova, such as that of 1987A shown above which showered the Earth with neutrino radiation in the year of its discovery: 1987.
Stars are born, go through an early embryonic phase of condensation and contraction, further phases that may be conveniently described as young and middle age during which their nuclear fires start and stabilise, and an advanced old age in which many go through grotesque contortions of size and appearance such as the hugely bloated Red Giants before either settling down as minuscule anonymous and selfish dwarfs or ending their careers in a final act of altruistic suicide. Altruistic, because it is during supernova explosions that the simpler constituents of stars which have matured into more advanced atomic species over millions of years during their late careers, abruptly evolve and explode into the vast reaches of their interstellar medium (ISM) to seed and enrich the birth of following star generations.
| This image taken by the Hubble Space Telescope, HST, shows gaseous pillars towering in the Eagle Nebula, M16. At the apex of each pillar are "stellar eggs" where new star formation is taking place as the cloud is eroded by ultraviolet radiation and differentiation of the material takes place. The image is hyperlinked to a video (763kB MPEG) zooming in on the egg location of the top left pillar - it's well worth the wait of loading it: a truly amazing sight! |  |
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The initial gravitational condensation of a star's Hydrogen material from the ISM in the case of our Sun lasted only about twenty years and caused a contraction of its initially discernible envelope from twenty-thousand to about a hundred Solar radii. A further contraction down to one solar radius took some fifty million years, at which point the internal core pressure and temperature rose to a level sufficient to start the nuclear burning process. Bodies with a mass exceeding at least 0,6 solar masses can undergo nuclear fusion and become true stars. |
Bodies with mass between 0,01 and 0,1 solar masses will be luminous due to the release of gravitational energy as they contract but after some 10e+9 years will fade completely to a state known as black dwarves. A large planet like Jupiter is in fact a failed companion star to our Sun and radiates intensely in the infrared band of wavelengths.
Three major time scales may be distinguished in stellar evolution: the nuclear, thermal and dynamic periods. The nuclear period is the time taken for a stably evolved star (said to be on the Main Sequence) to convert all its Hydrogen into Helium by nuclear burning. This period is directly proportional to the mass "M" of the star but inversely proportional to its Luminosity "L". The luminosity is proportional to the fourth power of the star's mass but the nuclear period varies as the cube root of the star's mass - it follows that the more massive a star, the more luminous it is but the faster it converts its nuclear fuel. As the two exponential powers are not reciprocal but an order of magnitude apart, this also means that more massive stars actually have a shorter lifetime. Stars of the order of twenty solar masses have average lifetimes of a mere million years compared to the 10e+10 (ten thousand million, American: ten Billion) years for a star of normal solar mass.
The thermal time scale is the time taken for newly released energy to diffuse from its centre of creation to the surface of a star. For the Sun, this period is ten million years - a relatively lengthy period. The dynamic time scale is appreciably shorter and is the period of propagation of shockwaves or pressure waves through a star. For the Sun this period is about thirty minutes. As an example, if the nuclear burning at a star's centre suddenly stopped and the radiation pressure caued by this burning could no longer support the tremendous gravitational attraction that the core of the star exerts on its gaseous envelope, the surface would become aware of this catastrophic event in the time that it takes for that negative pressure wave to propagate from the core to the surface. More accurately, from the limb of the nuclear burning core-shell in the star, to the surface itself.
During the long, stable phase of its life, a star owes its existence to, and has its characteristic size and emission characteristics due to a dynamic equilibrium between the nuclear radiation pressure from inside, and the gravitational attraction caused by its mass tending to collapse the star in on itself.
All stars finally suffer a depletion of their main fuel, Hydrogen, when most of this has been converted to Helium and this is accompanied by a reduction in radiation pressure, the core contracts causing a rise in temperature, a compensating increase in radiation pressure occurs and the gravitational contraction of the outer envelope is halted due to the hotter and faster nuclear burning of a new inner shell of a higher atomic species: a Carbon shell inside the older Helium shell, itself contained in the oldest, thin and inadequate, Hydrogen shell. In small to medium sized stars of a few solar masses the converted Helium core grows until it is too large to support itself against the gravitational pressure of the outer layers and finally collapses on a thermal time scale of a few million years. During this phase the outer envelope distends hugely and changes its radiative colour from yellow to red, so forming what we know as a Red Giant The star has now entered a last short and unstable period and has moved has off the lengthy and stable main sequence. These giants can rather gently blow off whole shells of hot gases which may deplete the core of enough matter and energy to stop the nuclear burning process - such stars become White Dwarfs.
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A dwarf surrounded by such a radiating shell is often called a Planetary Nebula, of which the photographic example here, Abell 39, is one. The dying star in a planetary nebula can waft off these shells in repeated successive envelopes of luminous gases. |
Some cores do not stop contracting as they contain degenerate matter where the electron cloud which normally surrounds atomic nuclei is violently pushed into the nucleus, partially converting such matter into pure neutrons. When such a core reaches temperatures high enough for remaining fuel burning, no pressure increase occurs and the core keeps on collapsing until the temperature is raised sufficiently for the degenerate gas to revert to a normal gas causing a Helium flash. Heavier cores completely convert to neutron material, forming the densest cosmic bodies known, the neutron stars. Rotating neutron stars, emitting highly directional and narrow radio beams which can "sweep" an observer like a lighthouse are known as pulsars and have extremely regular periods measurable in the range of milliseconds to seconds.
| This is the Crab Nebula, the remnant of a supernova explosion that occurred in AD 1054 and was observed and recorded by Chinese astronomers. The actual explosion occurred about 6000 years earlier, corresponding to the Nebula's distance in light years. The central pulsar has a rotational period of 33ms, imagine a ball 25km across with the mass of our Sun, rotating 30 times per second! The pulsar's position is indicated by the small arrow. |  |
In larger stars exceeding several solar masses, the almost completely converted Helium core can undergo increasingly more energetic burning through several nuclear species, form Helium to Carbon, to Oxygen, Magnesium, Silicon and then Calcium, each of these leaving the ashes of its nuclear fire enveloped in successively enclosed core-shells. All these nuclear burning processes are exothermic except when during a final attempt to keep going, the star converts its core to Iron in an endothermic process. This phase therefore refrigerates the core and the cooled core cannot maintain radiation pressure, implodes on itself and develops an extremely high neutrino flux which blasts off the star's outer layers in a supernova. It is during this final act that the heaviest elements in the universe are formed, many to exist only transiently. A supernova can outshine all the emitted light of its own parent galaxy for a period of weeks.
| Very heavy stars of several solar masses that have reached the end of their nuclear fuel burning phase may have core collapses that under the influence of their increasing gravitational concentration do not stop collapsing at all. The volume of degenerate matter is not proportional to its mass, more massive white dwarfs are actually smaller than lighter ones - beyond the so-called Chandrasekar Limit of 1,4 solar masses, the core can collapse completely to disappear beyond an event horizon, forming a Black Hole. Here is NGC4216 with such a black hole at its centre consuming infalling gas and dust. |  |
One such black hole candidate is the strong X-ray source in the Swan constellation, Cygnus X-1, a massive body of about six solar masses that has irregular flarings ranging from tenths of a second to events on a monthly scale. Such irregularities would be typical of a black hole's limitless consumption of all matter that strays within its Schwarzschild radius or "event horizon", that radius from the centre of a black hole where the escape velocity of any particle or body equals the speed of light. Although these exotic objects were named after the fact that nothing can escape from them once it has passed their event horizons, black holes do in fact slowly radiate away energy by means of Hawking radiation, discovered by the British cosmologist Stephen Hawking.
| This occurs when, subject to the Heisenberg Uncertainty Principle, a packet of energy springs into being near the event horizon as a particle-antiparticle pair (this not violating the law of energy conservation) - one of the pair may fall into the event horizon and cannot therefore annihilate its antiparticle as would normally occur. The remaining free particle, subject to its initial velocity vectors, would radiate away into space. |  |
The black hole as seen from a distance is therefore continually radiating free electrons and positrons which are effectively carrying away energy from the hole, causing it to slowly shrink and evaporate away into space over a lengthy time period. Within the event horizon, time and space do not follow the classical rules of science and therefore cannot be treated with standard calculation - they behave as mathematical singularities.
