Re: X-force Fusion Lifecycle 2010 Download
Kiera Mcintyde
It is very poetic to saythat we are made from the dust of the stars. Amazingly, it's alsotrue! Much of our bodies, and our planet, are made of elements that werecreated in the explosions of massive stars. Let's examine exactly howthis can be.Life Cycles of StarsA star's life cycle is determined by its mass. The larger its mass, theshorter its life cycle. A star's mass is determined by the amount ofmatter that is available in its nebula, the giant cloud of gas anddust from which it was born. Over time, the hydrogen gasin the nebula is pulled together by gravity and it begins to spin.As the gas spins faster,it heats up and becomes as a protostar. Eventually the temperaturereaches 15,000,000 degrees and nuclear fusion occurs in the cloud'score. The cloud begins to glow brightly, contracts a little,and becomes stable. It is now a mainsequence star and will remain in this stage, shining for millions tobillions of years to come. This is the stage our Sun is at right now.As the main sequence star glows, hydrogen in its core is convertedinto helium by nuclear fusion. When the hydrogen supply in the corebegins to run out, and the star is no longer generating heat by nuclearfusion, the core becomes unstable and contracts. The outershell of the star, which is still mostly hydrogen, starts toexpand. As it expands, it cools and glows red. The star has nowreached the red giant phase. It is red because it is cooler than itwas in the main sequence star stage and it is a giant because theouter shell has expanded outward. In the core of the red giant, heliumfuses into carbon. All stars evolve the same way up tothe red giant phase. The amount of mass a star has determines which ofthe following life cycle paths it will take from there.
The life cycle of a low mass star (left oval)and a high mass star (right oval).The illustration above compares the different evolutionary pathslow-mass stars (like our Sun) and high-mass stars take after the redgiant phase. For low-mass stars (left hand side), after the heliumhas fused into carbon, the core collapses again. As the corecollapses, the outer layers of the star are expelled. A planetarynebula is formed by the outer layers. The core remains as awhite dwarf and eventuallycools to become a blackdwarf.On the right of the illustration is the life cycle of a massive star (10 times or more thesize of our Sun). Like low-mass stars, high-mass stars are born innebulae and evolve and live in the Main Sequence. However, their lifecycles start to differ after the red giant phase. A massive star willundergo a supernova explosion. If the remnant of the explosion is 1.4 to about 3 times asmassive as our Sun, it will become a neutron star. The core of amassive star that has more than roughly 3 times themass of our Sun after the explosion will do something quite different.The force of gravity overcomes the nuclear forces which keep protonsand neutrons from combining. The core is thus swallowed by its owngravity. It has now become a black hole whichreadily attracts any matter and energy that comes near it. Whathappens between the red giant phase and the supernova explosion isdescribed below.From Red Giant to Supernova: The Evolutionary Path of High Mass StarsOnce stars that are 5 times or more massive than our Sun reach thered giant phase, their core temperature increases as carbon atoms areformed from the fusion of helium atoms. Gravity continues to pullcarbon atoms together as the temperature increases and additional fusionprocesses proceed, forming oxygen,nitrogen, and eventually iron.
The two supernovae, one reddish yellow and one
blue, form a close pair just below the image center
(to theright of the galaxy nucleus)
Image Credit: C. Hergenrother, Whipple Observatory,
P. Garnavich, P.Berlind, R.Kirshner (CFA).When the core contains essentially justiron, fusion in the core ceases.This is because iron is the mostcompact and stable of all the elements. It takes more energy to breakup the iron nucleus than that of any other element. Creating heavier elementsthrough fusing of iron thus requires an input of energy rather than therelease of energy. Since energy is no longer being radiated from thecore, in less than a second, the starbegins the final phase of gravitational collapse. The core temperaturerises to over 100 billion degrees as the iron atoms are crushedtogether. The repulsive force between the nuclei overcomes the forceof gravity, and the core recoils out from the heart of the star in ashock wave, which we see as a supernova explosion.As the shock encounters material in the star'souter layers, the material is heated, fusing to form new elements andradioactive isotopes. While many of the more common elements are madethrough nuclear fusion in the cores of stars, it takes the unstableconditions of the supernova explosion to form many of the heavier elements.The shock wave propels this material out into space.The material that is exploded away from the star is now knownas a supernova remnant.The hot material, the radioactive isotopes, as wellas the leftover core of the exploded star, produce X-rays and gamma-rays. For the StudentUsing the above background information, (and additional sources of informationfrom the library or the web), make your own diagram ofthe life cycle of a high-mass star. For the StudentUsing the text, and any external printed references, define the following terms: protostar, life cycle, main sequence star, red giant, white dwarf, blackdwarf, supernova, neutron star, pulsar, black hole, fusion, element, isotope,X-ray, gamma-ray.Reference URLs:Supernovae
Life Cycles of Stars
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#railAlign width:700px;margin-right:-22px; A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Andy Ptak (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC