| Ramblin' ramblin' ramblin' ramblin'......... |
[Jul. 12th, 2008|07:12 pm] |
hrm.
OK, so most of the heavier elements were forged in megastars that existed briefly during the dawn of the Universe. We're talking monsters, 1000 times the mass of Sol, that came into existence about 500 million years after the Big Bang as the first molecular clouds condensed out of the interstellar gas to form the first mini-galaxies - they lived fast and died young - intensely bright stellar objects, their high UV-output re-ionized the Universe, which had been cold and neutral since the time, 300,000 years after the Big Bang, that interstellar gas cooled enough to become transparent to light. After only a few million years the hydrogen fuel ran out, then the helium ran out, and so on, until they reached the iron stage of fusion - the tipping point, energy-wise - fusing together anything lighter than iron generates energy, anything heavier saps energy - so the cores of the megastars cooled, catastrophically, causing an implosion of stellar material, and the sudden intense heating of the core caused by the in-falling matter lead to a titanic explosion, a supernova - it was in these last seconds of the star's life, at it's brightest moment, shining brighter than it's own host galaxy, that all the elements heavier than iron were created. The core, being crushed by this implosion, becomes a neutron star if the megastar was on the smallish side - a neutron star 20km in diameter with about 5-20 solar masses - an object so dense a spoonful weighs about as much as Everest and with a gravitational pull so strong it has forced all of the electrons and protons in the elements that make up the core to fuse into neutrons. If the megastar was on the biggish side, the implosion crushes the core into an object so unimaginably dense that no force in nature can prevent it collapsing further, at light speed, for all eternity - a black hole of 25+ solar masses is formed.
Sol is what we call a main-sequence star - that is it fuses hydrogen into helium (via 4H-1 -> 2He-2 > 2H-2 -> He-4), and to a lesser extent fuses helium into oxygen and carbon. Main-sequence stars live a long, sedate life, and die relatively quietly. Sol formed about 5 billion years ago and was originally about 25% dimmer than it is today - as Sol's ages its temperature slowly increases, and a few billion years from now it will run out of hydrogen. At this stage it starts to fuse elements together outside the core, the core starts shrinking, and the outer layers start expanding - eventually Sol will be 1000 times bigger than it is today, and will have swallowed up Mercury, Venus, and Earth - Earth being last, would take about 1000 years to get dragged into a death spiral by the tenuous outer layers of Sol, and would slowly boil away as it spiralled towards Sol's shrinking core. Eventually, Sol will give up - the core, now essentially a dense ball of superheated carbon and oxygen, will shrink to the smallest size it's going to be, about 10,000km in diameter, and start to give off intense ultraviolet light - this UV light blows away the outer layers, forming a planetary nebula, a cosmic flower lit up from within by the hot core, that will be visible to neighbouring star systems for about 10,000 years - and slowly, the core cools, dims into a white dwarf, cools... cools... eventually, after hundreds of billions of years it fades away, the remaining planets orbit in darkness.
But... there's another type of star... even more short-lived than a megastar... they would have existed for a very brief time as the first mini-galaxies collided - I can't even think of a name for them - but, let's call them black hole stars, due to, well, you'll shortly see. Truly utter behemoths, as massive galaxy-sized gas clouds collided and condensed, spheres of gas with as much as a million solar masses appeared - as fusion kicked off in their rapidly compressing and heating cores, they started to shine as brightly as a small supernova - except unlike a supernova, this was no brief flash - these things shone that intensely non-stop for up to a million years. However, it wasn't fuel running out that killed them - as the core temperature passed the 620 million degree mark, the energy density became so huge, so unfathomably hot, that single photons were packing enough energy to spontaneously convert into neutrinos - matter condensing out of pure light. This was catastrophic - the core density of these beasts was already teetering on the edge of runaway collapse even with hydrogen for fusion still plentiful - now, suddenly, with all these photons converting into neutrinos and zipping away from the core, the core effectively bled out all its heat into empty space - causing near-instant collapse, and being so massively dense already it becomes a super-massive black hole with thousands of solar masses - but it's still surrounded by the mantle of the star that formed it! As this falls in towards the event horizon, it gets so highly compressed and so heated up that, you guessed it - a supernova of biblical dimensions - creating a burst of energy of such a short (hence high-energy) wavelength and of such intensity that we can detect them here on Earth as gamma-ray bursts from 10 billion light-years away (and hence these are faint images from 10 billion years ago). It's thought that the supermassive black holes at the centers of galaxies formed this way...
...hrm. I gotta stop - starting to feel like an amoeba in a vast ocean... |
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