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Outline Introduction The Life Cycles of Stars The Creation of Elements A History of the Milky Way Nucleosynthesis since the Beginning of Time INTRODUCTION Abundances of the elements Composition of stars How do we learn what stars are made of? The Composition of Stars 90% hydrogen atoms 10% helium atoms Less than 1% everything else M Finding the Composition of a Star Rainbows from starlight The Solar Spectrum – In Detail! Stellar Spectra Abundances of the Elements Log e (H=12) 14 11 H He CNO 8 Fe 5 Sr, Y, Zr Sc 2 Li, Be, B Ba Eu -1 10 20 30 40 50 Atomic Number 60 70 80 Solar Abundances EXPANDED CNO Log e (H=12) 8 Fe 5 Sr, Y, Zr Sc 2 Ba Li, Be, B Eu -1 10 20 30 40 50 Atomic Number 60 70 80 The Life Cycles of Stars • Fundamental Properties of Stars – – – – Mass Temperature Luminosity or radius Age • Star Birth • Middle Age • Stellar Senior Citizens But only certain sizes and colors are allowed! Stars come in many sizes and colors The Main Sequence The sun is an ordinary, yellow main sequence star Giants are cool and very large Most stars occur in these main groups in the luminosity-temperature diagram Main Sequence Giants Supergiants White Dwarfs Quiz: Which star is the biggest? A B C D Quiz: Which star is the smallest? A B C D The Evolution of Stars The Ages of Stars How old is the Jewelbox? The Jewelbox Cluster is about 12 million years old Star Birth • New stars are born continuously • Stars form in dense regions of gas and dust Stellar Middle Age Stars produce energy by converting hydrogen into helium through nuclear reactions in their interiors. The Proton-Proton Cycle Stars produce energy by fusing hydrogen nuclei into helium, and helium into heavier nuclei Stellar Senior Citizens When stars finally deplete their nuclear fuel, they become white dwarfs, neutron stars, or black holes. In the process, much of the stellar material is returned to interstellar space The Creation of Elements… “Just between you and me, where does it get enriched?” Primordial Nucleosynthesis Hydrogen and helium were created during the Big Bang while the Universe was cooling from its initial hot, dense state. About 10% of the lithium in the Universe today was also created in the Big Bang. We’re still not sure where the rest comes from. Hydrogen Burning Stars burn hydrogen in their interiors to produce helium. Hydrogen burning also rearranges carbon, nitrogen, and oxygen. Helium Burning Three helium atoms combine to form carbon Alpha-Process Elements CNO Log e (H=12) Carbon and oxygen atomsFecombine 8 to form elements with even 5 numbers of protons. Sr, Y, Zr Sc 2 Ba Li, Be, B These elements come from supernovae. -1 10 20 30 Eu 40 50 Atomic Number 60 70 80 The Iron Peak Metals In the cores of massive stars during supernova explosions, atomic nuclei exchange protons and neutrons to form the iron peak metals. CNO Log e (H=12) 8 Fe 5 Sr, Y, Zr Sc 2 Ba Li, Be, B Eu -1 10 20 30 40 50 Atomic Number 60 70 80 Heavy Metals All heavier elements are formed when iron peak elements capture neutrons Heavy Metals from Ancient Superstars Some heavy metals are made in supernovae when massive stars explode The explosion produces many free neutrons, which combine with iron to make heavier metals Making Heavy Metals in Low Mass Stars • In low mass stars like the sun, neutrons are created when protons mix into the hot, helium-burning region • With only a few neutrons available, a different set of heavy metals are created Neutron Capture Elements in the Solar System Eu 1 Br Fraction of r-process As Se Rh Te Sb Ag Ru Rb Pd Ge Kr Zn Sm Xe Pr Cd Mo Y Nb Tm Lu Yb Ta Dy Gd Er Os Au Re Pt Th Bi Ir U Hf Hg Sn I Sr La Nd Ce W Tl Pb Ba By studying Purple elements the abundance were formed patterns of in the supernovae heavy metals, (the r-process) we can learn and whether pink elements the metals were were formed madeinin supernovae red giant stars or in low (themass s-process) stars. Ga Zn As Kr Cs In Zr 0 Tb Ho Y Mo Pd In Te Cs Ce Sm Tb Er Lu W Ir Hg Bi A History of the Milky Way What is a galaxy? Structure of the Milky Way How did our galaxy form? You are here. The Milky Way…. Halo Flattened Inner Halo Disk Bulge Thick Disk Dwarf Spheroidal Companions Dark Matter Corona Formation of the Milky Way Galaxy Collisions and Mergers Chemical Evolution The creation of elements in stars leads to chemical enrichment of the Galaxy The composition of the Galaxy depends on How fast stars form What kinds of stars form How long it’s been since stars started to form The Chemistry of Stars The chemical compositions of stars reflect the star formation histories of stellar populations The complexity of the Milky Way’s history is reflected in the compositions of its stars Heavy Metals in the Early Galaxy In the early galaxy, elements were forming very quickly from new star formation. The chemical mixture we see is different from the Solar System We find a much smaller amount of “metals” mixed in with the hydrogen and helium Heavy metals come only from supernovae – not from low mass stars Nucleosynthesis Since the Beginning of Time • By studying stars of different ages, formed at different times in the Galaxy’s history, we can trace the history of the Milky Way Low Mass Stars Begin to Contribute Heavy Metals Low mass stars begin to contribute heavy metals when the Galaxy reaches an age of a few hundred million years. The Mystery Metals The abundances of the “light” heavy metals in ancient stars are too high to explain with our current theory of supernova r-process production Where to the first metals come from? Ancient Superstars! Formation of stars as “pre-galactic” objects from small density fluctuations Masses from a few tens to a few hundred solar masses Low mass star formation is suppressed because the first, massive stars reheat the gas These stars form the first metals Metals in Very Distant Galaxies Studies of the most metal-poor stars in the Galaxy give us access to the state of the Universe at very early times The most metal-poor stars in our Galaxy contain only 1/10,000 of the amount of metal that the sun has. The most distant galaxies we can study contain 1/100 (1%) of the amount of metal that the sun has The Epochs of Galactic Chemical Evolution Primordial Epoch -The Big Bang (hydrogen, helium, lithium) Epoch of Massive Stars – the first few million years Ca, O, and the “mystery metals” Supernova Epoch - r-process elements from 8-10 MSun SNII The first few 10’s of millions of years The Red Giant Epoch yields s-process elements The first few hundreds of millions of years The Iron Epoch – most of the iron comes from low mass stars The first billion years The Lithium Epoch – Where does lithium come from?? Websites of Interest Jewels of the night http://www.noao.edu/education/jewels/home.html National Optical Astronomy Observatory Image Gallery http://www.noao.edu/image_gallery Space Telescope Science Institute http://www.stsci.edu Amazing Space http://amazing-space.stsci.edu NASA’s Astronomy Picture of the Day http://antwrp.gsfc.nasa.gov/apod/astropix.html Astronomical Society of the Pacific http://www.astrosociety.org The Stonebelt Stargazers http://www.mainbyte.com/stargazers/ n-capture Synthesis Paths La Ba 138 139 p s,r 130 132 134 135 136 137 138 p p s s,r s s,r s,r 133 Cs Xe s,r 128 129 130 131 132 134 136 s s,r s s,r s,r r r s-process path r-process path