Star Life Cycle Fill in the chart when you see a yellow star. Take notes on the stars and events as well.
STARS • All stars begin the same way, but the last stages of life depend on it’s mass. • The birth place of stars are Nebulas, often referred to as “stellar nurseries”. • Nebulas are clouds of dust and gas.
3 TYPES OF NEBULAE • Emission: emit radiation, usually appear red • Reflection: reflect light of nearby star, usually appear blue. • Dark: block light, appear black.
Nebula: Accretion disc • Gravitational attraction causes gas and dust to condense, spin and heat up which forms a proto-star.
Proto-stars • There are no nuclear reactions inside the proto-star, it is not a star yet!
Brown Dwarfs • If there is not enough mass to create a protostar, a Brown Dwarf forms. • Some astronomers consider Jupiter a Brown Dwarf…
A star is born… • Eventually the gas shrinks enough that its temperature and density become high enough, that a nuclear fusion reaction starts in its core! -- It becomes a giant Hydrogen Bomb!
A star is born… • At 10 Million K, Hydrogen begins nuclear fusion to form helium and the star begins to shine. • It will now be visible on an H-R Diagram.
Main Sequence Star • The star shines as nuclear reactions inside produce light and heat.
How it works: • How long and how hot the star burns is determined by the star’s mass but… • Eventually, stars begin to run out of their fuel hydrogen. • The problem is that pressure begins to decrease but gravity stays the same causing contraction, which raises pressure which increases temperature.
How it works… • Hydrogen shell begins to burn rapidly (red layer in the diagram) and this causes the non-burning helium ‘ash’ (yellow layer) to expand. • The core shrinks and heats up, the outer layers expand and cool. This is a red giant.
Red Giant • Star of less mass expands and glows red as it cools.
Planetary Nebula • In the core, helium begins fusing to make carbon. Temperatures are not high enough to make heavier elements. • Helium flash: burning of helium becomes explosive and the outer layers of red giant are ejected in an envelope called a planetary nebula.
Planetary Nebula • Outer layers of gas puff off. • Hot core will be exposed as white dwarf.
White dwarf • As the envelope recedes, the core becomes visible.
White Dwarf • Small, dim and hot. • No nuclear reactions • Dying star that is slowly cooling off
Nova • A nova is a white dwarf star that suddenly increases enormously in brightness, then slowly fades back to its original luminosity. • Novea are the result of explosions on the surface of the star caused by matter falling onto their surfaces from the atmosphere of a larger binary companion.
White dwarf cooling • Star cools and reddens.
Black Dwarf • Eventually the white dwarf cools off completely and becomes a cold dense ball called a black dwarf because it does not radiate any energy.
Black Dwarf • Star stops glowing.
Now lets talk about massive stars! • Remember, low mass and massive stars for the same way up until the red giant phase. • To be considered massive, a star must be about 8 times larger than our sun.
Massive stars • Massive stars are hot enough to continue to fuse elements until the core becomes iron. • Nuclear reactions in stars can’t make heavier elements than iron.
Supergiant • Star of greater mass expands, cools, and turns red.
Type II supernova • Core releases an explosive shock wave expelling the outer layers of the star in a tremendous explosion called a type II supernova.
Supernova • Supergiant explodes, blasting away outer layers.
After a supernova: option 1 = Neutron Star • Intense pressure in the core causes electrons to fuse with protons creating neutrons.