Life Cycle of Stars

1. Life Cycle of Stars

2. 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

3. There are Nebulas…..

4. And then there are FAMOUS Nebulas….!

5. 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 *106 K • Repulsive forces between nuclei overcome • Higher temperatures = higher velocities • Hydrogen nuclei combine in fusion • 4H →1He + energy • E=mc2

6. 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

7. 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

8. 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

9. 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*106 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

10. Red Giant

11. 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

12. A Low Mass Star’s Demise • Mass < .5 Sun’s mass • 2*1030 kg or 330,000 * Earth’s mass (5.98*1024 kg) • These stars are like the Prius • Burn through hydrogen fuel very slowly • Main sequence star for 100 billion years! • Are low mass, 1st generation stars still alive? • Core temperature never reaches 100*106 K for helium fusion to occur • Does not enter Red Giant phase • Goes straight to the White Dwarf phase

13. 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!

14. Planetary Nebula

15. 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

16. 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”

17. Supernova

18. 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?

19. 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)

20. 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!

21. H-R Diagram