Ch. 27.2 Stellar Evolution

• Nebula—a cloud of dust and gas. 70% Hydrogen, 28% Helium, 2% heavier elements.

• Gravity pulls the nebula together; it spins and flattens into a disk of matter with a central concentration called a protostar.

• Pressure and temperature increases in the protostar, until the core reaches about 10,000,000 degrees C, and then nuclear fusion starts, releasing energy, and a star is born.

Main-Sequence Stars

• Main-sequence stage is the longest stage in a star’s life.

• Hydrogen is converted to helium by nuclear fusion, and energy is released.

• The star is in balance. The inward pull of gravity is balanced by the energy from fusion.

Giants and Supergiants

• 3rd stage, when hydrogen in the star’s core is exhausted. Hydrogen fusion continues in the star’s outer layers.

• Core contracts due to gravity, and its temperature increases.

• Helium in the core fuses into carbon.

• Combined helium and hydrogen fusion causes outer shell of star to greatly expand.

• Star is now a red giant (10 times bigger than sun) or a red supergiant (100 times bigger than sun)

White Dwarf Stars

• For medium-sized stars, after helium fusion, the giant stage is over.

• Outer gasses are lost, and a core is revealed, which heats and illuminates the expanding gasses, forming a planetary nebula.

• The last inner matter collapses inward due to gravity, forming a hot, dense white dwarf star.

• It shines for billions of years, but eventually becomes cooler and fainter, until energy emmission stops, and it becomes a black dwarf.

Novas

• Explosions on the surface of some white dwarfs, causing brightness to increase by thousands of times for a few days.

• Believed to be caused by gas (from a companion star) buildup on the white dwarf’s surface.

Supernovas

• Occur when high-mass stars (10 to 100 times mass of the sun) finish the supergiant stage.

• Gravitational collapse causes fusion to restart…carbon is converted into iron…then gravity causes the iron core to collapse and fuse into all heavier elements, in a gigantic explosion.

Neutron Stars

• What may be left after supernovae explosions…super dense rotating balls of pure neutrons, with a diameter of about 30 km.

• Pulsars are neutron stars that emit beams of radiation.

Black Holes

• In the most massive stars, gravity crushes the matter down into zero volume.

• Detected by x-rays given off as surrounding matter falls into a black hole.