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Stellar Evolution Page 7

Massive Stars

Massive stars are brighter, fuse faster, and perish more dramatically. There is no sharp definition of a “massive” star, but it generally means a star with a mass of more than
8 Msun. There is a practical upper limit to the mass of a star. A star that is more massive than about 120 Msun will expel its outer layers by the extreme radiation until it becomes less massive. They will continue to shine as very bright blue giant stars.

 

Beta Centauri (the blue one of the two "pointers" in the Southern sky, left in the image) is a very luminous star of spectral class B1.
It is 8 times the size and 15,000 times the luminosity of our Sun.
(see HR-diagram on page 4)
Image source: http://catherinesherman.wordpress.com

 

 

Initially the end stage in the life of a massive star is very similar to that of a less massive star as described above.
In the Red Giant stage, the star becomes much larger to form a Red Super Giant.

When the Helium in the core is exhausted the temperature will increase to something like a billion degrees. Under these conditions, the Carbon and Oxygen will fuse to form nuclei of heavier elements such as Neon, Magnesium, Silicon, Sulphur and finally Iron (Fe). Beyond Iron a fusion reaction will not generate but consume energy and this is the natural end stage of fusion in the immensely hot core. But the scene is likely set for disaster.

Nuclear fusion generates energy
only up to the element Iron (Fe)

 

Periodic Table

The Periodic Table lists the chemical elements according to increasing number of protons in the nucleus (atomic number). The table should be read like a book, from top to bottom and left to right. Hence the order of the elements is H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca, Sc, Ti, V, Cr, Mn, Fe, etc.

At first further collapse of the core is halted by the immense radiation from the ever faster nuclear fusion reactions. Depending on the mass of the remaining core, fusion will stop at a particular type of fusion reaction. If the core’s mass is below the Chandrasekhar limit of 1.4 Msun the star will still end up as a white dwarf, but with different nuclear composition than for Sun-like stars, depending on what type of fusion reactions have happened.

If the mass is above this limit and the Iron stage ends and fusion stops, the electron degeneracy pressure of the
atoms within the core is no longer able to stop further collapse of the star against gravity.

In less than a second, the core collapses from a diameter of about 8000 kilometres to less than 20 kilometres - the collapse happens so fast that the outer layers have no time to react or to collapse along with the core. The electrons in the atomic structure are forced into the atomic nuclei where they combine with protons and become neutrons and neutrino’s. The neutrinos radiate off at speeds near the speed of light, generating further nuclear reactions in the star’s outer region.