The life cycle of stars
I originally posted the info below on another message board ('Keep It Real - Exploring the Sciences' forum) in answer to a question re whether all stars (including our sun) had the same life cycles.
Any star’s particular life cycle is determined by its mass. And its mass is determined by the amount of matter there was in the original interstellar cloud it was born from. The stellar process begins when a cloud of interstellar gas - mostly hydrogen - forms a dense clump in its midst (a core) and with the increasing density of the core, gravity increases, dragging in more material from the cloud to form a spinning disc known as a ‘protostar.’ The protostar gains mass and continues to contract under gravity and spin faster, which heats it up, until (at approx 10-15 million (!) degrees C) nuclear fusion reactions begin in its core and we have a ‘star.’ Pressures now exerted by the superheated gas in the core stop the contraction and the star enters what I referred to in the post to Ice Smile above as the ‘main sequence,’ which is where our sun currently is, and will be for a good while yet.
Generally, the larger a star’s mass, the shorter its life span. The more mass it has, the more luminous it is, the hotter it burns and the faster it goes through its fuel. This is why supergiant stars such as Antares and Betelgeux (mentioned in my first post) have a dramatically shorter life span than a main sequence star like our sun. In the main sequence stage, nuclear fusion converts hydrogen in the star’s core into helium. The pressure in the core is around 200 billion times (yikes!) that of the air pressure on the earth’s surface and with that serious heat already mentioned the hydrogen nuclei bash into each other so violently that some of them fuse together to form helium nuclei. Each complete ‘bash into’/reaction welds four hydrogen nuclei into one nuclei of helium. The key point here is that the mass of the helium nucleus is about 0.7% less than the mass of the particles that went into it…so…the excess energy has to go somewhere and it does. It is liberated in the form of energy we see as starlight. (This is where Einstein’s whole E=mc2 thing comes into play. ) A star normally stays on the main sequence for billions or tens of billions of years.
Eventually, all good things must come to an end, and so the star’s life on the main sequence. The supply of hydrogen fuel in the star’s core begins to run out and the core loses its stability and begins to shrink. The star’s outer shell, still made up mainly of hydrogen, begins to expand, but because the inner core is shrinking and the outer layer expanding it takes more energy to maintain the main sequence temperature over the larger surface so the surface in fact begins to cool, from white or yellow down to red. (With the principle that something ‘red hot’ ain’t as hot as something ‘white hot.’) So the star now becomes a huge puffy red ball – a ‘red giant’ in astronomyspeak. Pretty well all stars evolve in the same way up to the red giant phase, then the amount of mass contained in the original star determines what happens from there.
For medium size stars like our sun; though the surface temperature is cooling off to a balmy 3000 deg c or so, the core temp hits maybe a couple of hundred million degrees at which point the helium atoms fuse into carbon. At this point (in about another 5-7 billion years for our sun) the outer shell flies apart and bits of it form a circle of gas surrounding the core. This is known as a ‘planetary nebula.’ Here is a cool picture of a planetary nebula.
Eventually the last of the helium nuclei in the core are gone and the star begins to die. The last of the core compacts into a fuzzy white ball known as a white dwarf, where it stays until all the energy burns out, then it becomes a black dwarf, forever. Note: this is a black dwarf, not to be confused with a ‘black hole’ which is a very very different beast.
For massive stars; (ie larger than 1.5 times the mass of the sun), the route from the red giant phase is somewhat more dramatic. The carbon atoms that form in the core continue to be pulled together with the increasing gravity (by the star’s higher basic mass) to eventually form iron, and the fusion process stops dead as the iron atoms absorb energy and keep on absorbing it until…ka boom!!! A supernova. The temperature in a supernova explosion can reach 1 billion degrees. All but the iron core of the star is blown away into space in one almighty explosion. Here is a cool pic of the remnant of a supernova explosion.
This is supernova 1987A, the remnant of the final blaze of glory of a star astronomers had named Sanduleak SN 69 202, which exploded 160,000 years ago. The event itself was witnessed only in 1987.
The core of the former star is left as a tiny superdense neutron star, only 5-10 miles in diameter but more massive than our sun.
If the original star was of a mass roughly 8 times or more that of our sun, then the imploded core remains massive after the supernova. Since there’s no nuclear fusion left at this point, the core is devoured by its own gravity (so to speak) and becomes an ever smaller, ever more massive magnet for anything that comes within its event horizon. Nothing escapes it, not even light. So it is now known as a ‘black hole.’
The matter ejected from the supernova follows a rather different history. With the elements present, mainly hydrogen, also oxygen, nitrogen, carbon, sulphur and iron and vast quantities of dust, ripe and ready for forming into clumps that will form into a core so the whole process begins again. There is a field of astronomical/cosmological study now looking at what happens when an object like a comet passes through these nursery clouds and picks up their seeds to transport them around the universe and maybe crash into the odd planet now and then. Here's an image of some 'starbirth' clouds from the M16 Eagle Nebula, taken by the Hubble telescope.
When the pillars of dust themselves erode away there remains pockets of dense gas in which exist embryonic stars. The pockets are known as 'Evaporating Gas Globules' as clever name astronomers have assigned them to give us the entirely appropriate acronym - EGGS.
(As an indication of size, check out the nifty little fingertip thingees on the cloud's upper crown. Each of those tiny nodues would dwarf our entire solar system.
Something spooky. In the last few years astronomers and cosmologists have settled on an object ‘observable’ by radio wave and X Ray emissions from the region of the constellation of Sagittarius that has all the hallmarks of a supermassive black hole. The object, which is known as Sagittarius A* has been estimated to have the mass of 3,000,000,000 suns. It is likely to be the hub of our entire Milky Way spiral galaxy.
Edited by: Sanduleak at: 1/6/02 10:48:36 pm
White dwarves are a great joke. The diamond industry on Earth casually forgets all the giant diamonds floating around the dark corners of the universe.
It's all an interesting phenomena that black holes and neutron stars, while having such intense gravitational pulls, still eject an immense amount of matter and energy. Also, it's predicted that black holes, although presumably 'immortal' can actually dissipate over time. I think it was Steven Hawkings that came up with that, but I'm not sure.
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