Giant and subgiant stars
within 100 ly

Including Sol itself, around ? giant and ? suspected subgiant stars are located within 100 light-years or (30.7 parsecs) of Sol have evolved off the main sequence of core hydrogen fusion. Most of these relatively bright but rare stars can be seen with the unaided Human eye in Earth's night sky. While there are no giant or supergiant stars located with 10 parsecs (pc) of Sol, several stars have fused so much hydrogen and accumulated so much helium ash in their cores that they are beginning to swell and emit more yellow, orange, and red light -- changing their spectral type. As more massive stars exhaust their core hydrogen faster, the largest O and B stars in the 10-parsec Solar neighborhood appear to have already moved off the main sequence and relatively brief, giant and supergiant stages to become white dwarfs and possibly as yet undetected neutron stars and black holes).

Nearby Bright Stars by Type, Number, and Mass

A minimum mass of 0.6 Solar-masses was assumed for orange-red
stars without mass estimates.

NASA Observatorium




See a discussion of
the "main sequence"
as part of stellar
evolution and death.

NASA Observatorium


See a discussion of
"core helium burning"
as part of stellar
evolution and death.

As a star that has evolved out of the "main sequence," Arcturus has fully shifted from the fusion of hydrogen to helium in at its core to the fusion of helium to carbon and oxygen, with trace activity of other nuclear processes. However, Arcturus is somewhat brighter than expected for a stable helium-burning star. Among the heavier elements and molecules detected in its stellar atmosphere have been iron (Fe-0.5) and CN. This helium-burning, orange-red giant stage is relatively brief, lasting tens to hundreds of million years (e.g., lasting around 700 million years for a star of one Solar mass like the Sun).

Eventually, the star will lose much of its current mass, from an intensified stellar wind that eventually puffs out its outer gas envelopes of hydrogen and helium (and lesser amounts of higher elements such as carbon and oxygen) into interstellar space as a planetary nebula. The result will be a planet-sized, white dwarf core that gradually cools and fades in brightness from the shutdown of thermonuclear fusion. (Nearby white dwarfs include solitary Van Maanen's Star and the dim companions of Sirius, Procyon, and 40 (Omicron2) Eridani.)

H. Bond (STSci), R. Ciardullo (PSU), WFPC2, HST, NASA -- larger image
(White dwarfs are remnant stellar cores that have cast off their outer gas
layers, like planetary nebula NGC 2440.)

Stars like Sol evolve through three stages that could foster life. The first lasts for about 10 billion years, as the star burns hydrogen in its core during the main sequence. When the star exhausts its core hydrogen and begins to burn hydrogen in a shell around a growing helium core, it brightens and expands and becomes a "subgiant," during which its habitable zone moves outward with the increased heat. When our Sun, Sol, becomes a subgiant, its habitable zone may extending from two to 9 AUs. Thus, the inner edge of this zone remains habitable for several billion years while the outer extreme, where Saturn currently orbits, is habitable for a few hundred million years. The star then fluctuates in brightness for about 20 million years as it switches to core helium burning almost exclusively, before becoming a red giant and swelling to 10 times the diameter of the Sun. For about a billion years afterwards, the habitable zone around the red giant extends from 7 to 22 AU, the outer edge of which lies beyond the orbit of Uranus. Hence, planets that are currently very cold and icy can warm up and become potentially habitable. The time period over which these conditions change is very long -- long enough for life to form. Moreover, bacteria could be transported by meteorites from an inner planet where life is ending to an outer planet where conditions are warming up.
http://www.fp.sfasu.edu/astro105/Stellar_DeathLMnotes.htm
http://www.journals.uchicago.edu/PASP/journal/issues/v115n812/203116/203116.web.pdf