Giant and subgiant stars
within 100 ly

While there are no giant stars located with 10 parsecs (or 33.3 light-years) of Sol, six stars -- Altair, Procyon A, Delta Pavonis, Beta Hydri, Mu Herculis Aa, and Rana -- appear to have already fused so much hydrogen and accumulated so much helium ash in their cores that they are beginning to swell and emit more yellow, orange, red, and infra-red light as subgiant stars -- shifting their spectral type towards the red end of the spectrum. As more massive stars exhaust their core hydrogen much faster, the largest O and B stars that may have once been born in the immediate 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.

NASA Observatorium




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

Arcturus is a nearby star located just outside the immediate 10-parsec Solar neighborhood (about 37 light-years away) that has already evolved out of the main sequence into giant status. Astronomers suspect that the star may have already shifted from the fusion of hydrogen to helium in its core, to the fusion of helium to carbon and oxygen, with trace activity of other nuclear processes, although Arcturus is somewhat brighter than expected for a stable helium-burning star (see Professor Jim Kaler's discussion). 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 our Sun, Sol).

NASA Observatorium


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

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.)

In March 2005, some astronomers announced that stars like Sol evolve through three stages that could foster life (see NASA news release with images). 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 radiated.

NASA



Larger image



As a Sol-type star
evolves beyond core
hydrogen fusion, it
brightens and expands
which moves its
habitable zone
outwards (more).

When our Sun, Sol, becomes a subgiant, its habitable zone may extending from two to nine 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 around 10 times the diameter of the Sun.

NASA



Larger image



When Sol becomes
a giant star, its
habitable zone
will extend from
7 to 22 AUs -- an
outer edge beyond
the orbit of Uranus.

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 may be long enough for life to develop. Moreover, bacteria could be transported by meteorites from an inner planet where life is ending to an outer planet where conditions are warming up.