|MadSci Network: Astronomy|
Excellent question Mukund. Blue supergiants are a stage in the life of very massive stars. Ordinary stars, like our sun, will leasurely "burn" (thermonuclear fusion) Hydrogen, making Helium. Since Helium is heavier than Hydrogen, it sinks to the core. But the temperatures and pressures are not adequate to ignite this Helium ash. A shell of burning Hydrogen surrounds the Helium core, but struggles against gravity to maintain equilibrium of the stellar atmosphere. The star shrinks, and converts gravitational energy into heat - causing the outer atmosphere of the star to expand and cool -- a Red Giant. Eventually, however, enough ash builds up, the Hydrogen burning can no long sustain the thermal pressure needed to counteract gravity and the star collapses. The collapse increases the temperature and pressure to the point where Helium burning occurs briefly. For a star like the sun, a Red Giant is the mark of old age and death is not far behind.
But what about Blue Supergiants? Stars more massive than a few of our suns can sustain their fuel cycle further up the chain. Hydrogen burning is much more rapid for a larger star, and it marches quickly from hydrogen burning, to helium burning and beyond. For very large stars (greater than about 8 suns), the end is Silicon burning. The end product, iron cannot fuse and release energy - and the star dies (catastrophically) when enough iron ash builds up in the core or these massive stars. These stars live short lives and die spectacularly - as supernovae (or if massive enough, as hypernovae). For these massive stars, the buildup of Helium ash in the core leads to the formation of a Red Supergiant, similar to our sun's red giant phase - just bigger. But these stars are massive enough to ignite and sustain the next element in the fuel chain - Helium burning. When the star shrinks enough to have the immense temperature and density necessary to fuse helium atoms, it ignites the core. This renewed power source in the core increases the atmosphere temperature - moving its spectrum toward the blue (blue is hotter - red is cooler). It becomes a Blue Supergiant. As the Helium is depleted, it again moves toward the red, as a Red Supergiant again, until it ignites the next element in the fuel cycle - carbon. Carbon burning (actually carbon - oxygen burning)pushes the supergiant back toward the blue spectrum again as the atmosphere heats dramatically. As I said, for massive enough stars - this progresses through the remaining light elements until finally arriving at Silicon burning. The products of silicon burning build an iron core. But nuclear physics spells doom for the star, as one cannot fuse iron and release energy. When enough iron ash builds up (very, very hot and very very dense ash - from hundreds of millions degree to about 4 billion degrees or more and a density so incredible, its roughly equivalent to trying to cram all of humanity into a single volkswagen. But that is just the start. These stars have no nuclear fuel to counteract gravity once enough iron builds up. So the core collapses, rapidly and catastrophically. When the density reaches that of an atomic nucleus, the core collapse stalls, and hits the infalling matter. A massive shock wave is generated, destroying the star in one of the most spectacular events in our universe - a supernova. For a brief time, the supernova will outshine the collective brightness of all the other stars in the galaxy. Thus ends the life of a Blue Supernova. The progenitor star (the star that gave birth to) supernova 1987A was a Blue Supergiant. Antares and Betelguese are examples of Red Supergiants, Rigel an example of a Blue Supergiant. Its just a matter of time (hundreds of years, thousands of years, or 100,000 years) before one of them explodes in a supernova.
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