Stellar Evolution. Sam Wilmarth PHYS 43 Younes Ataiiyan SRJC SPRING 2011.
SRJC SPRING 2011
Around 400,000 years after the big bang, matter primarily existed in the form of protons, electrons, and lightweight atoms such as 2H floating through space. The particles slowly began to coalesce into nebulous gas clouds.
Gravity pulls the particles together, increasing their kinetic energy and their temperature. As the matter collapses, the core becomes denser and hotter, radiating energy.
FUSION!If the star is massive enough, fusion occurs.Hydrogen is converted to helium through several fusion reactions.1H + 1H ⇒ 2H + e+2H + 1H ⇒ 3He + γ3He + 3He ⇒ 4He + 2 1H
Hydrogen fuses into helium in the core of the star. The star is stable as the pressure of the expanding gas counteracts gravity.
As the helium core grows, hydrogen fusion moves out into the star, and the size increases.
The star eventually runs out of hydrogen it can fuse, and collapses.
If the star is big enough, the collapse produces enough energy and heat to start helium fusion into carbon.
Depending on the mass of the star, it will continue the cycle of expansion and contraction, fusing heavier elements each time.
Each successive cycle is shorter than the last, and even the largest stars stop at iron fusion.
A medium size star that ends hydrogen fusion and begins helium fusion may either continue the cycle onto heavier elements or cease fusion there.
Either way, fusion will stop and the core will cool.
The hydrogen layer expands away from the core and cools, turning red in color. The star becomes a red giant.
As the hydrogen layer expands farther and cools, the star turns into a planetary nebula.
Eventually the gas cloud dissipates into space, leaving a white dwarf that cools over millions of years.
A small star never undergoes helium fusion, and turns directly into a white dwarf.
The composition of elements in a white dwarf depends on the size of the original star.