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Stellar Evolution Marielle Deconinck 2 Outline I Birth of a Star -stellar nurseries II Life of a Star -protostars -main sequence stars III Death of a Star -white and black dwarfs -supernovae -neutron stars -black holes Marielle Deconinck 3 Brief History of Stellar Observation • Religion, celestial navigation, orientation, calendars • Oldest accurate star chart ancient Egypt, 1534 BC • Lascaux Paleolithic cave paintings, circa 17,300 years ago BBC News Marielle Deconinck 4 What is a Star? • Sphere of plasma Lifetime of Stars as a Function of their Masses (logarithmic scale) • Form a star cluster or galaxy • Vary with mass • Main source of energy nuclear fusion Lifetime (Million Years) 10000 1000 100 10 1 1 1.5 3 10 Mass (Solar Mass) 30 60 Marielle Deconinck 1.98855×1030 kg The Sun 3.828×1026 W 6.95700×108 m 5,777 K • Yellow dwarf • Magnetic field • 99.9% of total mass of Solar System • 70 % Hydrogen, 28 % Helium, 1.5 % Carbon, Nitrogen, Oxygen 0.5 % Other Elements NASA/SDO Marielle Deconinck 5 6 Birth of a Star – Stellar Nurseries • Dense regions in molecular clouds (>one million particles per cm3) • If the cloud is big enough, it will undergo gravitational collapse Protostars NASA/JPL-Caltech/W. Reach (SSC/Caltech) Marielle Deconinck 7 Protostars Continue to grow by accretion of gas and dust Mass < 0.08 M☉ Core temperature reaches 10 million K Proton-proton chain reaction initiated allowing H to fuse to 2H, then to He Temperature too low for nuclear fusion of H Brown dwarf Main sequence star Marielle Deconinck 8 Main Sequence Stars Hertzsprung-Russell Diagram (1910) Relationship between luminosity and effective surface temperature Small, cold stars stay for hundreds of billions of years Massive, hot stars will leave after a few million years Richard Powell Marielle Deconinck 9 Death of a Star No more nuclear fusion no force to counteract force of gravity collapse Depending on mass, 3 possible outcomes: 1) White and Black Dwarf 2) Neutron Star 1) Black Hole Marielle Deconinck 10 White and Black Dwarfs Sirius A Over 100,000 K at surface Small (< 0.5 M☉ ) mainly He Medium (around 1 M☉ ) C, O NASA, ESA, H. Bond (STScI) and M. Barstow (University of Leicester) Large (> 1 M☉ ) mainly O, Ne, Mg Radiates heat until all energy is used up Black Dwarf Marielle Deconinck 11 Neutron Stars Stellar core collapses, electrons and protons fuse Electron capture: p + e− → n + νe Neutrons collapse into a dense ball • Extremely small (10 km radius) • Extremely dense (1017 kg/m3) • Extremely short rotation period (1.5 ms) NASA/Andrew Fruchter (STScI) Marielle Deconinck 12 Supernovae http://www.novacelestia.com/images/stars_supernova_process_medium.jpg Marielle Deconinck 13 Black Holes Escape velocity from the surface of the star > speed of light (gravitational radius) Extremely high mass neutron degeneracy pressure insufficient to prevent collapse below gravitational radius (NASA/Dana Berry/SkyWorks Digital) Marielle Deconinck 14 References Books: Carlos A. Bertulani – http://www.worldscientific.com/worldscibooks/10.1142/8573 Longair, M. S. (2008). Galaxy Formation (2nd ed.). Springer. p. 478. Prialnik, Dina (2000). An Introduction to the Theory of Stellar Structure and Evolution. Cambridge University Press. Stahler, S. W. & Palla, F. (2004). The Formation of Stars. Weinheim: Wiley-VCH. Websites: http://science.nationalgeographic.com/science/space/solarsystem/neutron-stars/ NASA/SDO: http://sdo.gsfc.nasa.gov/assets/img/browse/2010/08/19/20100819_0 03221_4096_0304.jpg Marielle Deconinck 15 Summary Marielle Deconinck