Low mass stars. Sequence Star Giant. Red. Planetary Nebula. White Dwarf. Interstellar Cloud. White Dwarf. Interstellar Cloud. Planetary Nebula.

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1 Low mass stars Interstellar Cloud Main Sequence Star Red Giant Planetary Nebula White Dwarf Interstellar Cloud Main Sequence Star Red Giant Planetary Nebula White Dwarf

2 Low mass stars Interstellar Cloud Main Sequence Star Red Giant Planetary Nebula White Dwarf Interstellar Cloud Main Sequence Star Red Giant Planetary Nebula White Dwarf

3 Fusion H fuses into He What could He fuse into?

4 Red Giants The He core collapses until it heats to 108 K He fusion begins ( He C) called the triple- process The star, called a Red Giant, is once again stable. gravity vs. pressure from He fusion reactions red giants create and release most of the Carbon from which organic molecules (and life) are made

5 Red Giants

6 Helium fusion begins at the center of a giant While the exterior layers expand, the helium core continues to contract and eventually becomes hot enough (100 million kelvins) for helium to begin to fuse into carbon and oxygen core helium fusion 3 He C + energy and C + He O + energy occurs rapidly - called the Helium Flash

7

8 Low mass stars Interstellar Cloud Main Sequence Star Red Giant Planetary Nebula White Dwarf Interstellar Cloud Main Sequence Star Red Giant Planetary Nebula White Dwarf

9 Planetary Nebulae When the Red Giant exhausts its He fuel the C core collapses Low & intermediate-mass stars don t have enough gravitational energy to heat to 6 x 108 K (temperature where Carbon fuses) The He & H burning shells overcome gravity the outer envelope of the star is gently blown away this forms a planetary nebula

10 Planetary Nebulae

11 Planetary Nebulae Cat s Eye Nebula Twin Jet Nebula

12 Helix Nebula--125 pc

13

14

15 Planetary Nebulae Ring Nebula Hourglass Nebula The collapsing Carbon core becomes a White Dwarf

16 Low mass stars Interstellar Cloud Main Sequence Star Red Giant Planetary Nebula White Dwarf Interstellar Cloud Main Sequence Star Red Giant Planetary Nebula White Dwarf

17

18 What provides the pressure in White Dwarf stars? Pauli exclusion principal two identical particles cannot exist in the same place at the same time this effect in stars is called electron degeneracy pressure and is not dependent on temperature the star is supported by the fact that the electrons cannot get any closer together

19 Degeneracy Pressure Two particles cannot occupy the same space with the same quantum state (i.e., mass, energy, momentum etc.). For very dense solids, the electrons cannot be in their ground states, they become very energetic---approaching the speed of light. the electrons play a game of musical chairs The pressure holding up the star no longer depends on temperature.

20 Low-Mass Stellar Evolution Summary

21 Low-Mass Star Summary 1. Main Sequence: H fuses to He in core 2. Red Giant: H fuses to He in shell around He core 3. Helium Core Burning: He fuses to C in core while H fuses to He in shell 7. Double Shell Burning: H and He both fuse in shells Not to scale! 5. Planetary Nebula leaves white dwarf behind

22 Reasons for Life Stages Ø Core shrinks and heats until it s hot enough for fusion Ø Nuclei with larger charge require higher temperature for fusion Ø Core thermostat is broken while core is not hot enough for fusion (shell burning) Ø Core fusion can t happen if degeneracy pressure keeps core from shrinking Not to scale!

23 Nuclear Fusion: star is mostly hydrogen pressure at core is enormous hydrogen combines to form helium and releases energy process continues to produce heavier elements--stops at iron

24 Iron is a dead end for fusion because nuclear reactions involving iron do not release energy (Fe has lowest mass per nuclear particle) So the Fe core continues to collapse until it is stopped by something new and different

25 But wait it gets wilder

26 But wait it gets wilder The Story of the Massive Stars!!!!!!!!

27 High Mass Main Sequence Stars The CNO cycle is another nuclear fusion reaction which converts Hydrogen into Helium by using Carbon as a catalyst. Effectively 4 H nuclei go IN and 1 He nucleus comes OUT.

28 High Mass Main Sequence Stars The C nucleus has a (+6) charge, so the incoming proton must be moving even faster to overcome the electromagnetic repulsion!! CNO cycle begins at 15 million degrees and becomes more dominant at higher temperatures. The Sun (G2) -- CNO generates 10% of its energy F0 dwarf -- CNO generates 50% of its energy O & B dwarfs -- CNO generates most of the energy

29 They have enough gravitational energy to heat up to 6 x 108 K. C fuses into O C is exhausted, core collapses until O fuses. The cycle continues. O Ne Mg Si Fe Supergiants What happens to the high mass stars when they exhaust their He fuel?

30 A series of different types of fusion reactions in high-mass stars lead to luminous supergiants

31 A series of different types of fusion reactions in high-mass stars lead to luminous supergiants When helium fusion ceases in the core, gravitational compression increases the core s temperature above 600 million K at which carbon can fuse into neon and magnesium. When the core reaches 1.5 billion K, oxygen begins fusing into silicon, phosphorous, sulfur, and others At 2.7 billion K, silicon begins fusing into iron This process essentially stops with the creation of iron and a catastrophic implosion of the entire star.

32 Supergiants on the H-R Diagram As the shells of fusion around the core increase in number: thermal pressure overbalances the lower gravity in the outer layers the surface of the star expands the surface of the star cools The star moves toward the upper right of H-R Diagram it becomes a red supergiant example: Betelgeuse For the most massive stars: the core evolves too quickly for the outer layers to respond they explode before even becoming a red supergiant

33 Degenerate Core Leftover The central star collapses, heats up, and ejects a Planetary Nebula. The star has insufficient mass to get hot enough to fuse Carbon. Gravity is finally stopped by the force of electron degeneracy pressure. The star is now stable... White Dwarf

34 High-Mass Stellar Evolution Summary

35 High-Mass Stars interstellar cloud main sequence (high luminosity) run low on fuel (hydrogen) core collapses, outer layers expand red giants (cool) pressure in core increases burns and fuses to form iron PROCESS CEASES!! CRUNCH (collapse takes a few seconds!) boom (type II supernova) neutron star or black hole

36 High mass star Neutron Star Black Hole Interstellar Cloud Big Main Sequence Star Red Giant Type II Supernova Interstellar Cloud Big Main Sequence Star Red Giant Type II Supernova Neutron Star or Black Hole

37 Life Stages of High-Mass Star 1. Main Sequence: H fuses to He in core 2. Red Supergiant: H fuses to He in shell around He core 3. Helium Core Burning: He fuses to C in core while H fuses to He in shell 7. Multiple Shell Burning: Many elements fuse in shells Not to scale! 5. Supernova leaves neutron star behind

38 Life of a 20 M star Life of a 1 M star

39 Death of a high mass star results in a supernova type II: neutron star (the really big ones) black hole (the really really big ones) the whole story depends on mass

40 What have we learned? How do we classify stars? We classify stars according to their spectral type and luminosity class. The spectral type tells us the star s surface temperature The luminosity class Why is a star s mass its most important property? A star s mass at birth determines virtually everything that happens to it throughout its life.

41 What have we learned? What is a Hertzsprung- Russell diagram? An H R diagram plots stars according to their surface temperatures and luminosities.

42 What have we learned? How is the Luminosity of a star related to its temperature and size? L = 4πR2σT4, so for the same luminosity, is temperature goes up then size, R, must go down. Remember Luminosity is power What is the main sequence? What happens to a low mass star?

43 What have we learned? What happens to a high mass star?

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