Life Cycle of Stars

Birth of a Star Born from interstellar matter (dust & gases) – Denser portions of the nebula Nebula begins to contract – Due to gravity – Shockwave from nearby explosion Temperature at the core rises – Gravitational energy is converted into heat energy

There are Nebulas…..

And then there are FAMOUS Nebulas….!

Childhood A star is born once fusion begins. If fusion does not occur, it is not a star – Brown Dwarfs (ex. Jupiter) Core temperature must reach 10 *10 6 K – Repulsive forces between nuclei overcome – Higher temperatures = higher velocities – Hydrogen nuclei combine in fusion 4H →1He + energy E=mc 2

Childhood continued Once fusion begins, the star now enters the protostar stage Gravitational contraction continues (nothing to counteract it) As fusion continues, pressure from the gases builds at the core, creating an OUTWARD force

Adulthood Eventually, the outward force will balance the inward force, and the star will become stable; its size will remain fixed (not contracting or expanding) The star is now a main sequence star – Spends approximately 90% of its life in this stage Hydrogen fusion continues

A Senior Citizen Eventually the hydrogen core is turned into helium through fusion H fusion can continue in outer shells Core begins to contract (why?) – Gravity is no longer balanced by the gas pressure from fusion Temperature rises in core, heat is transferred to outer shells, which expands – Creates a giant star

Senior Citizen continued Red Giant Stage – Temperature at core becomes hot enough for helium fusion to form carbon, and possibly oxygen Temperature needs to be 100*10 6 K Temperature at surface cools – Why are these stars called RED GIANTS? Red Giant stage is shorter for more massive stars – Similar to cars: SUV vs. Sedan

Red Giant

Death of a star A star begins to die once it runs out of all useable fusion fuel Next phase is determined based on the star’s mass

A Low Mass Star’s Demise Mass <.5 Sun’s mass – 2*10 30 kg or 330,000 * Earth’s mass (5.98*10 24 kg) These stars are like the Prius – Burn through hydrogen fuel very slowly – Main sequence star for 100 billion years! Are low mass, 1 st generation stars still alive? Core temperature never reaches 100*10 6 K for helium fusion to occur – Does not enter Red Giant phase – Goes straight to the White Dwarf phase

Medium-Mass Star’s Demise Masses similar to the sun Red Giant phase goes rather quickly Red Giants blow off their outer layers – Like a mini-explosion – Planetary nebula Forms a cloud of gas and interstellar matter Then goes into the white dwarf phase Fun Fact: When the sun expands into the Red Giant phase, it will go past Earth!

Planetary Nebula

A Massive Star’s Destruction Mass > 4 suns Short life spans (like a Humvee!) Becomes a SUPER Red Giant Temperature becomes so hot that fusion of heavier elements can occur (nucleosynthesis) – Occurs rapidly over a short amount of time – Formation of elements in periodic table – The mass determines what heavy elements can form

Going out with a BANG! When it runs out of fuel, gravitational contraction occurs rapidly because it is SO MASSIVE! Star collapses and IMPLODES as a supernova Creates a shockwave that destroys the star, and throws matter into space (IMPORTANT!) – Distribution of elements in universe – Responsible for forming everything, stellar bodies – “We are all made out of star dust”

Supernova

White Dwarfs Only low and medium mass stars Once fusion ends, the star contracts – Extremely high density – Temperature rises, including at the surface Why are these stars called WHITE DWARFS? Similar to the wick of a candle – No heat source – White Dwarfs will cool and dim over time – Once all light / heat is gone, it becomes a black dwarf Why is it named a BLACK DWARF?

Neutron Stars Formed after a supernova Stronger gravitational pull – More contraction Massive, denser, and smaller than white dwarfs Atoms take up less space (just like with a white dwarf) – Instead of electrons closer to the nucleus, they combine with protons to form NEUTRONS (hence the name)

Black Holes Most massive stars collapse to become black holes – Mass > 20 Suns – Denser and more massive than neutron star Gravitational field is so strong not even light can escape!

H-R Diagram