Stellar Evolution Life Cycle of stars Stars Mass

Stellar Evolution Life Cycle of stars

Stars ● Mass determines a star’s temperature, luminosity, and diameter. ● Hydrostatic equilibrium is when the inward pressure of gravity is equal to the outward pressure from fusion and radiation. ● If this equilibrium does not exist the star will expand or collapse.

We mentioned the idea of equilibrium within the star-the inward pressure of Gravity and the outward pressure of Fusion. Discuss how these forces would have to act in order to cause: 1) expansion 2) Collapse

Fusion ● Hydrogen molecules fuse together to form helium in the core of a young star. ● Some older, bigger stars can either fuse helium to form other elements or no fusion happens at all.

Star Formation-The beginning of all stars ● All stars form in the same manner. ● The star begins as a cloud of interstellar gas and dust called a nebula. ● The nebula collapses on itself as a result of its own gravity. ● The cloud begins to rotate around the center and when the center gets hot it is called a protostar.

Fusion Begins-A true star is born ● The heat of the protostar increases until it is hot enough to start fusion in the center. ● Once fusion begins it is now stable and a true star. ● This is called the “Main Sequence” stage! Star spends most of it’s life here!

Average size star-Red Giant ● The rest of the life cycle depends on the MASS of the star. ● Only the core of a star is hot enough to fuse hydrogen into helium, when the hydrogen is gone the star begins to expand. ● This expansion turns the star into a red giant.

● When the star is a red giant it begins to lose gas from its outer layers. ● The star gets so large its gravity isn’t strong enough to hold some of the gases together. ● The core however heats up, so hot that helium now can fuse to carbon. ● When the core has used up all of the helium it is now entirely carbon.

Back to nebula ● The stars mass will never get high enough to fuse carbon, so no more energy is produced. ● The outer layers of gas expand are driven off. ● This gas is called a planetary nebula. ● Only the core is left which is a white hot ball of carbon called a white dwarf.

White Dwarfs ● White Dwarfs are about the same size as Earth and are considered dead stars.

Massive Stars ● For stars bigger than the sun a slightly different path is taken. ● They form about the same way, only hydrogen is used up faster, because they are so bright. ● These massive stars become red giants many times, each time it uses up a new layer of gases by fusing different elements together.

Massive Stars ● The star expands to a larger size and becomes a supergiant. ● As the star expands, each time it loses some gases in the outer shell and gets smaller in mass. ● Eventually the star becomes a white dwarf. Instead of being made of only carbon it can be made of many different elements.

Supernovae ● Some stars do not lose enough mass to become a white dwarf. ● These stars are too massive to be stable and meet a violent end. ● Once reactions in the core have made IRON, no more energy is produced and the core collapses in on itself. ● Protons and electrons merge to form neutrons and become a neutron star!

Supernovae ● Neutron stars are incredibly dense, 3 times the mass of our sun but only 10 km in radius! ● The neutron star forms so fast that the gases around the star begin to collapse ● When the gases reach the dense core they explode outward into a supernova. ● Supernovae spread the heavier elements around the universe.

Supermassive Stars (Black Holes) ● Some stars skip the neutron star stage because they are supermassive. ● The gravity is so strong that not even light can escape.

H-R Diagrams ● The properties of mass, luminosity, temperature and diameter are closely related. ● A Hertzsprung –Russell diagram puts all the stars on a graph based on luminosity and surface temperature. ● 90% of all stars fall along a broad strip called the “main sequence” which runs diagonally from top left (hot, bright) to bottom right (cool, dim)