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Discover Red Giants

Page history last edited by stephen labarge 12 years, 7 months ago

By: Stephen Labarge and Kevin Kearney 



Red Giants 

A red giant is a luminous giant star of low or intermediate mass that is in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower. The appearance of the red giant is from yellow orange to red, including the spectral types K and M, but also class S stars and most carbon stars. Astronomers hypothesize that our Sun will become a red giant in about five billion years. It is red because of its relatively low temperature, and it is one of the largest of all star types, 1,000 times as luminous as our Sun. Betelgeuse, Antares, Aldebaran, and Arcturus are some well-known red giant stars visible from Earth with the naked eye. As a matter of fact, about 1/10 of the stars that can be seen with a naked eye from earth are pulsating red giants.





Younger stars create energy through hydrogen fusion, which creates helium in the process, gradually causing the helium to hydrogen ratio inside the star to increase. Hydrogen is in the core of younger stars, but as a star ages and uses up its hydrogen store, the hydrogen becomes confined to an outer shell, while the core is only helium.When the star has run out of hydrogen fuel to fuse into helium within its core the core will begin to collapse and heat some more. To counter the core's collapse the outer envelope expands causing the temperature to drop at the surface but also increasing surface area and thereby the luminosity of the star. Within the core temperatures will rise to begin fusion of helium into carbon. Since Iron nuclei cannot be used to create energy for the red giant and therefore the iron nuclei gather at the center of the star. A shell around the core will rise to such a temperature as to ignite further hydrogen fusion in that region of the star. The helium produced falls onto the core where it can be used as fuel. This time in the life of a Red Giant is very short compared to the main sequence lifetime, only a few million years. Eventually, the pulsations help drive off the outer parts of the star, producing a circumstellar shell of gas and dust and leaving behind a remnant white dwarf core. For a few thousand years before it cools, the hot white dwarf may illuminate the gases around it, giving rise to the phenomenon of planetary nebula. About one tenth of stars visible to the unaided eye are pulsating red giants, spectral type K5 or cooler, mostly with very small amplitude.




The "habitable zone" of a red giant extends from about 630 million miles to 2 billion miles. Astronomers argue that about 150 sub-giant and red giant stars are situated within 100 light years of Earth. NASA's Terrestrial Planet Finder space mission will focus only on main sequence stars. These stars will have habitable planets that are further from their suns, and will therefore be easier to find in the glare of the parent stars.

   In the late 1970's Olin Eggen spent years studying and observing red giants. Thanks to him we know that there is a complete behavior to the pulsating of red giants. Sometimes the pulsations go from being periodically to irregular. The Hipparcos mission in 1989-1993 has enabled us to determine the distances to many red giants with good accuracy. Before this mission little was known on how to measure the distance to red giants.




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