Stars Part Two:

1. Stars Part Two: Stellar Evolution

2. Overview of the life of a star: • Formation of protostar from a cloud of mostly Hydrogen gas. • Main sequence star • Red giant • White dwarf or… • Supernova - • Neutron star or • Black hole

3. Formation of protostar: • Gaseous clouds contract under their own gravity. • Regional areas of initial high density accrete more and more gas. • Gravitational potential turns to heat. • Heat and pressure start fusion.

4. Birth of a star IP Demo: Star_Birth.ip

5. Birth of a star • As the cloud of gas and dust collapses, a small rotation becomes big (Conservation of angular momentum) • The rapidly spinning protostar often needs to get rid of angular momentum before it can start fusion. • The magnetic field channels rapidly spinning material out of polar jets

7. Birth of a star • Eventually, the spin slows enough to allow fusion • The newly born star often blows away the nebula it came from with its radiation. • The remaining material (still spinning) stays around the newly formed star in an accretion disk.

9. Icy and Gassy Stuff Rocky Stuff The New Star Birth of a solar system: Accretion Disk

10. The Solar System National Geographic Magazine

11. The Inner Planets:: Mercury Venus Earth Mars Asteroids • Close together (Relatively) • Terrestrial (made of rock like Earth)

12. The Outer Planets:: Jupiter Saturn Uranus Neptune Pluto • Spread out (Relatively) • Gas giants

22. Life on the Main Sequence: • Energy comes primarily from the Proton-Proton cycle: • 1H + 1H = 2H + e+ + ν • 1H + 2H = 3He + γ • 3He + 3He = 4He + 1H + 1H • (requires heat and pressure)

23. Heat - Thermal Agitation Gravity - Crushing Pressure Thermal Agitation balances the tendency of gravity to crush a star:

24. The rate of burn depends roughly on the cube of the mass • Even though larger stars have more fuel, they burn the fuel they have at a much faster rate. • Big stars are Brief, Bright, and Blue • Diminutive stars are Durable, Dim and reD

25. .01 Billion Years 1 Billion Years 100 Billion Years .1 Billion Years 10 Billion Years 500 Billion Years From Robert Garfinkle’s “Star Hopping”

26. From Jay Pasachoff’s “Contemporary Astronomy”

27. A Star trying to be too big From Jay Pasachoff’s “Contemporary Astronomy”

28. 4He accumulates in the core of the star:

29. The death of a star: • When most of the Hydrogen in the core has been used up, leaving a Helium core, the star cools down. (The Helium displaces the fusing Hydrogen) • Heat energy no longer balances gravity. • Gravity collapses the He core. • The heat generated by the implosion of the core spurs more fusion of the remaining Hydrogen. • The outer envelope of the star expands, and cools. It is now a Red Giant

30. Expands Cools Down Collapse of the He Core:

31. Turning into a Red Giant : • A star the size of the sun would expand to the orbit of Venus, or maybe the earth. • As a red giant, the star blows off a great deal of its mass into space. • A star 8 time as massive as the sun will have a residual mass of 1 or 2 times the mass of the sun after its red giant stage. • Stunning image from the Hubble:

33. Helium Fusion: • When the core gets hot and dense enough, He begins to fuse: • 4He + 4He = 8Be + γ • 4He + 8Be = 12C + γ • The star contracts slightly and heats up, moving along the horizontal branch • Before the He is used up these reactions also occur: • 4He + 12C = 16O + γ (mainly) • 4He + 16O = 20Ne + γ • 4He + 20Ne = 24Mg + γ

34. Heats up and contracts Helium Fusion:

35. Carbon Fusion: • When most of the Helium in the core has been used up, leaving a Carbon core, the star cools down. • Heat energy no longer balances gravity. • Gravity collapses the Carbon core. • The heat generated by the implosion of the core spurs more fusion of the remaining Helium. • The outer layer of the star expands, and cools briefly.

36. Cools and Expands again Collapse of the Carbon Core:

37. Carbon Fusion: • If the remaining part of the star is more than .7 times the mass of the sun, the core gets hot and dense enough to start Carbon fusion: • 12C + 12C = 24Mg + γ • 16O + 16O = 28Si + 4He • Nuclei as heavy as 56Fe and 56Ni can be created if the star core is hot enough. • Nucleosynthesis and fusion stop with 56Fe and 56Ni as larger nuclei would require the input of energy, because of binding energy

38. Most tightly bound nuclei (If you go from less to more bound you release energy) 56Fe and 56Ni From Douglas Giancoli’s “Physics”

39. So far: Collapse of C core Helium Fusion Carbon Fusion (if > .7 Msun) Collapse of He Core Hydrogen Fusion stops Degenerate Matter

40. How do we know all this? By observing Globular clusters…

42. How do we know all this? • Globular clusters are thousands of stars that all formed at more or less the same time. • Globular clusters are much smaller than galaxies. • Galaxies create stars in an on-going process. • The stars in a globular cluster accrete suddenly and nearly simultaneously. By observing Globular clusters…

45. Planetary Nebulas: • Some stars with mass 1-7 times the sun’s mass. • While the star is fusing carbon, it shrinks and gets hotter. • The material blown off by the red giant phase is overtaken by the material blown off by the carbon core collapse. • The rapidly spinning core creates a strong magnetic field that channels the expulsion of the outer envelope. • Some planetary cores might have a companion.