starnut wrote:neufer wrote:... It is a "blue supergiant," ... With an original mass around 17 times that of the Sun, *RIGEL* is in the process of dying, and is most likely fusing internal helium into carbon and oxygen. The star seems fated to explode, though it might just make it under the wire as a rare heavy oxygen-neon white dwarf.
I am confused. From what I learned about stellar evolution, all stars become either a red giant or red supergiant, depending on their respective masses, when they leave main sequence and start fusing helium into carbon and oxygen. A high mass star, one having more than 8 solar masses, becomes a blue supergiant only after going through the red supergiant phase and just before it explodes as a supernova. Think of Sanduleak -69 202, the progenitor of Supernova 1987A, which was a blue supergiant.
So, is Rigel about to go supernova?
Sanduleak -69 202 was possibly an anomaly. It is more probable, based upon current theory, that Rigel will first transition into the faster burning red supergiant phase of a Betelgeuse, at least once, before it finally goes supernova. In any event, all supergiants burn their nuclear fuel too fast to live for very long.
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http://en.wikipedia.org/wiki/Blue_supergiant
<<Blue supergiants (BSGs) are supergiant stars (luminosity class I) of spectral type O or B.
They are extremely hot and bright, with surface temperatures of 20,000-50,000°C. They typically have 10 to 50 solar masses on the Hertzsprung-Russell diagram, and can have radii up to about 25 solar radii. These rare and enigmatic stars are amongst the hottest and brightest in the known Universe.
Because of their extreme masses they have relatively short lifespans and are mainly observed in young cosmic structures such as open clusters, the arms of spiral galaxies, and in irregular galaxies. They are rarely observed in spiral galaxy cores, elliptical galaxies, or globular clusters, most of which are believed to be composed of older stars.
The best known example is Rigel, the brightest star in the constellation of Orion. Its mass is about 20 times that of the Sun, and its luminosity is more than 60,000 times greater. Despite their rarity and their short lives they are heavily represented among the stars visible to the naked eye; their inherent brightness trumps their scarcity.
Blue supergiants represent a slower burning phase in the death of a massive star. Due to core nuclear reactions being slightly slower, the star contracts and since very similar energy is coming from a much smaller area (photosphere) then the star's surface becomes much hotter. Red supergiants can become blue supergiants if their nuclear reactions slow for whatever reason and the reverse can also occur imploding into Pulsars.
While the stellar wind from a red supergiant is dense and slow, the wind from a blue supergiant is fast but sparse. When a red supergiant becomes a blue supergiant, by contracting, the faster wind it produces impacts the already emitted slow wind and causes the outflowing material to condense into a thin shell. Almost all blue supergiants observable have this shell of material surrounding them, suggesting that they all once were red supergiants.
As the star evolves, it may swing back and forth between red supergiant (slow, dense wind) and blue supergiant (fast, sparse wind) several times and give concentric faint shells around itself. In between the transition, the star can be yellow or white in color, such as the star Polaris, the North Star. Eventually the star is likely fated to go supernova although a very small number of stars in the 8-12 solar mass range will form supergiants but will proceed to become a very rare oxygen-neon white dwarf. It is not well understood how or why these special white dwarf stars form from a star which should, by right, end up a small supernova. It is theorised, though not quantitavely, that significant mass-loss occurs during the star's supergiant phase and places it below the threshold for supernova. Either a blue supergiant or a red supergiant can go supernova as the process of a supernova is not related to the state of the star's envelope.
Since stars spend more time being red supergiants, we observe more red supergiants and most supernovae progenitors are red supergiants. It was assumed all supernovae were from red supergiants until Supernova 1987A forced revision as the progenitor was a B3 blue supergiant>>
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http://en.wikipedia.org/wiki/Red_supergiant
<<Red supergiants (RSGs) are supergiant stars (luminosity class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive. Betelgeuse and Antares are the best known examples of a red supergiant.
Stars with more than about 10 solar masses after burning their hydrogen become red supergiants during their helium-burning phase. These stars have very cool surface temperatures (3500–4500 K), and enormous radii. The five largest known red supergiants in the Galaxy are VY Canis Majoris, Mu Cephei, KW Sagitarii, V354 Cephei, and KY Cygni, which all have radii about 1500 times that of the sun (about 7 astronomical units, or 7 times as far as the Earth is from the sun). The radius of most red giants is between 200 and 800 times that of the sun, which is still enough to reach from the sun to Earth and beyond.
These massively large stars are little more than "hot vacuums", having no distinct photosphere and simply "tailing off" into interstellar space. They have a slow, dense, stellar wind and if their core's nuclear reactions slow for any reason (such as transitioning between shell fuels) they may shrink into a blue supergiant. A blue supergiant has a fast but sparse stellar wind and causes the material already expelled from the red supergiant phase to compress into an expanding shell.
The mass of many red supergiants allow them to eventually fuse elements up to iron. Near the end of their lifetimes, they will develop layers of heavier and heavier elements with the heaviest at the core.
The red supergiant phase is relatively short, lasting only a few hundred thousand to a million or so years. The most massive of the red supergiants are thought to evolve to Wolf-Rayet stars, while lower mass red supergiants will likely end their lives as a type II supernova.>>
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