ASTR 1020 1st Edition Lecture 13Summary of Stellar Lives- Stars “cook” H into He and heavier elements via fusion.- Everything depends on stellar mass: more mass = hotter = more fusion = shorter lifetime- Supernovae release heavy elements into space where they can form more stars, planets,and life!- Small stars (<Msun) live for 10-20 billion years / fuse HHe,red giants,white dwarf- Massive stars (>10 Msun) burn briefly (<1 billion years) / fuse H..Fe, explode as SN, creating heaviest elements  neutron star or black holeMultiple Layers of Nuclear Fusion = how high-mass stars dieAs gravity presses inwards, the star burns heavier elements faster and hotterOnce depleted, the star’s core collapses slightly until hot enough to burn the nextIron is the most tightly bound nucleus• No energy generation → gravity collapses the star’s innermost core very quickly• gravity is only stopped when the neutrons in the atoms slam together → neutronstar (or black hole) protons + electrons → neutrons + neutrinos regular matter becomes neutronium (nothing but neutrons)SUPERNOVA- Exploding remnant of massive star disperses heavy elements through the galaxy- Inside may be a neutron star a remnant core of pure neutrons!Observing Supernovae- About 1 per century per galaxyo Last one “observed” in the Milky Way: 1604 (Known ~1868)- Bright explosion visible for weeks/months These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.Supernova RemnantsVisible for 10,000+ years as huge bubbles and “veils”Eventually merge with other interstellar gasHow do we know all this?The Scientific Process:- Observation- Conceptual model (hypothesis) does the model have predictive power?- Test the model with more observationsPredictions:If elements above helium really are the results of nuclear fusion via helium capture, then there should be more elements with even numbers of atomic elementsIf elements heavier than iron (Fe) are made primarily by brief fusion reactions during a supernova, then these elements should be rareIf a supernova really results from core collapse of iron into neutrons and neutrinos, many neutrinos should be generatedIf all heavy elements are created inside high-mass stars (and released at their death), early stars (stars born near the beginning of the Universe) should have less heavy elements in them“metal” = anything heavier than Helium / Carbon (Z=6) and beyond anything made through fusion in stars METALLICITY –number of atoms an element X to hydrogen (compared with the same ratio as the sun) often [Fe/H] or [O/H]- Abundances of different elements track each other / increased O=increased C, Fe..- More star formation, particularly massive stars  more metalsQ: The Andromeda Galaxy is about the same size as the Milky Way, but has had more active star formation history/more generations of massive stars. Compared to the Sun, we would expect a star the same age in Andromeda to have…A. Higher Metallicity Metallicity/Age relationship- Estimator of relative stellar ages- Star with high metallicity formed more recently than a star with low metallicity “Fusion in stars” skips over Li, Be, B / no stable 5- and 8-nucleon isotopes Li, Be, B only created through fission of heavy elements by cosmic rays(energetic protons) in interstellar space, not in starsAlgol Binary System- Binary stars can have different masses but usually are formed at the same time- More massive star should have had a shorter main sequence lifetime BINARY MASS EXCHANGE- The 0.8 MSun star once was more massive (3.0 MSun), with a 1.5 MSun companion• As it became a red giant, it swelled and poured material onto its companion• The red giant is now less massive than its companion • Future: when the other star becomes a red giant, it may pour gas back…Stellar Graveyard- Lower mass stars (<8 Msun)  white dwarfso Gravity vs. electron degeneracy pressure- High mass stars (8-20 Msun)  neutron starso Gravity vs. neutron degeneracy pressure- Most massive stars (>20Msun)  black holeso Gravity winsWhite Dwarfs- M < 8 Msun = a hot core of carbono Can also be oxygen for intermediate mass starso Density -1 cm^3 is about 5 tons- Held up by electron degeneracy pressure- Cool from white-blue through red to black dwarfs- End of non-binary starsWhite Dwarfs in Binary Systems- Mass transfer from a companion red giant spirals into an accretion disk and eventually makes its way onto the WD- Red giant “feeds” the WD, increasing its massNOVA- Accretion of small amounts of hydrogen gas onto the white dwarf (binary mass exchange!)o Heats surface layers and fuses hydrogen- Star becomes much brighternova (new star)10^5 x Lsun (dimmer than supernova but still impressive)Nova vs. SupernovaDetonation of hydrogen fusion on surface Total explosion of a starWD in a close binary Occurs once for a given star, total destructionCan repeat as more material accumulates Luminosity of ~10^10 SunsLuminosity of ~10^5 SunsQ: If a lot of material from the accretion disk makes it into the white dwarf (before it can all fuse away). What will happen?A: The white dwarf will have enough mass to