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Stellar Evolution - Cycles of Formation and Destruction
Mid-Sized Stars:
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Sun
Credit: SOHO - EIT Consortium, ESA, NASA |
Thermonuclear fusion in stars with masses between ~0.8 and 8 solar masses, similar to our Sun, produces the outward radiation pressure to counterbalance gravitational forces for approximately ten billion years. When all the hydrogen nuclei have been converted to helium nuclei and fusion stops, gravity takes over and the core begins to collapse. The layers outside the core collapse too - the layers closer to the center collapse more quickly than the ones near the stellar surface. As the layers collapse, the gas compresses and heats up. The temperature becomes high enough for helium nuclei to fuse into carbon and oxygen nuclei, with hydrogen fusing in a thin shell
surrounding the core. The outer layers expand to an enormous size
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Mira
Credit: M. Karovska (CfA) et al., FOC, ESA, NASA |
and the star is now called a red giant. The star brightens by a factor of ~1,000 to 10,000, and the surface temperature of the extended envelope drops to about 3,000K - 4,000K, giving the star its reddish appearance. A strong wind begins to blow from the star's surface, carrying away most of the hydrogen envelope surrounding the star's central core. During the
final shedding of its envelope, when the mass loss is greatest,
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| Mira Light Curve |
the star pulsates - the surface layers expand and then contract in repeating cycles - with periods from several months to more than a year. During this pulsating stage the star is called a Mira
variable star.
The pulsations of Mira variable stars result in a change in the magnitude, or brightness, of the star. A plot of the change in brightness over time is call a light curve. During this stage, as mid-sized stars evolve to the giant branch, they move through an area referred to as the Mira instability strip - on the H-R
diagram shown here this area is further divided into long-period and semiregular variables. Not all stars go through these stages of variability.
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Helix Nebula
Credit: NASA, WIYN, NOAO, ESA, Hubble Helix Nebula Team, M. Meixner (STScI), & T. A. Rector (NRAO) |
Eventually, the material ejected by the star forms an envelope of gas called a planetary nebula which expands into the surrounding interstellar medium at ~17-35 km/hr. The core of the star left in the center of the planetary nebula is called a white dwarf. The planetary nebula is very tenuous, and becomes so thin that after ~50,000 years it is no longer visible - therefore all planetary nebulae that we see are very young, less than ~50,000 years old.
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Animation: Planetary Nebula
Credit: NASA, ESA, and J. Gitlin (STScI) |
The white dwarf is extremely dense. It is held in equilibrium with gravity by electron degeneracy pressure. The repulsive forces of the electron clouds of the individual atoms are strong enough to stop any further gravitational contraction. The mass limit for a white dwarf to remain in equilibrium between gravity and electron degeneracy pressure is 1.4 solar masses - the Chandrasekhar limit. Eventually the white dwarf will radiate all of its remaining energy away and become a black dwarf - a cold, dark mass. The universe is not old enough for any white dwarf to have become a black dwarf, so black dwarfs are not considered as part of the evolutionary stage of a star.
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