STELLAR EVOLUTION

By: Brian Smith 2ndp.

INTERSTELLAR CLOUD At this stage cloud of dust and gas start

to clump together. As these clumps gather more mass their gravitational attraction increases, pulling more atoms into the clump.

CLOUD FRAGMENT Next, a fragment will eventually form a

star like the Sun, and estimated to span a few hundredths of a parsec across. It will look like a fuzzy, gaseous blob and will still be about 100 times the size of our solar system.

PROTOSTAR Now it has become a protostar and it

will start to evolve, and will start to change. It will start to shrink, density will increase, and temperature rises, both in the core and at the photo-sphere.

MAIN SEQUENCE STAR Now the star is at this time in its life that

it is in hydrostatic equilibrium and is turning hydrogen into helium. The star is very stable in this stage. A low-mass star will increase in luminosity as well as temperature as it fuses its hydrogen.

SUB GIANT BRANCH After most of the hydrogen in a star's

core has been fused to helium, fusion stops and the new helium core begins to contract because of its own gravity. Now the layers above the core fuse hydrogen a lot faster, because of increased temperature. 

HELIUM FLASH This will only happen for a star of a

mass less than 2 solar masses. The star will have a degenerate helium core at the time of helium fusion and will begin this new fusion process very violently in which is why its called helium flash.

HORIZONTAL BRANCH Right after the helium flash, the star has

a pretty low luminosity but then moves back towards the main sequence in a horizontal type way. In this state helium fusion has become stable.

ASYMPTOTIC GIANT BRANCH After all of the helium has been fused in

the core of a star, the star starts to change, similar to the changes it went through after the main sequence phase. The now carbon/oxygen core begins to contract under its own gravity as the outer layers of the star again expand and cool.

CARBON CORE As the carbon/oxygen core contracts,

the outer layers become more unstable, and eventually start to expand off of the star completely and creates a planetary nebula.

WHITE DWARF For low mass stars, after a long time the

star's outer layers will dissipate into a planetary nebula because of the low gravitational pull, revealing a mainly helium core that is known as a helium white dwarf.

SOURCES http://rainman.astro.illinois.edu/ddr/stellar/evolution.html

http://sunshine.chpc.utah.edu/labs/star_life/starlife_main.html

http://www.telescope.org/pparc/res8.html