Origin of Elements OUR CONNECTION TO THE STARS

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1 Origin of Elements OUR CONNECTION TO THE STARS

2 The smallest piece of an element is a single atom. What makes an atom of one element different from an atom of another element? Atoms are made of: Atoms and Elements Protons + Neutrons Electrons _

3 Atomic # 1 1 proton 5 5 protons 2 2 protons 6 6 protons 3 3 protons 7 7 protons 4 4 protons 8 8 protons

4 Atomic # 1 H 5 B 2 He 6 C 3 Li 7 N 4 Be 8 O

5 Abundance of Elements Relative abundance of elements in the Solar System

6 What is the origin of elements in the universe? Three elements formed in the first minutes after the big bang (hydrogen, helium and a small amount of lithium). Astronomers think that almost all other elements formed in stars.

7 About 100 million years after the Big Bang Universe is cold and dark, filled with vast clouds of hydrogen and helium gas. Clumps in these clouds attract more gas because of their gravity. As they pull in more and more gas, the pressure and temperature in their centers increases. When the pressure and temperature in the center becomes high enough (about million K), hydrogen begins to fuse into helium. The first stars are born and stellar nucleosythesis begins. Note: Gravity and pressure must be balanced or in equilbrium to form a star

8 Hydrogen Fusion proton proton proton helium proton Neutron Positron Neutrino The main process responsible for energy production and nucleosynthesis in stars like the Sun is the fusion of four hydrogen nuclei into one helium nucleus, releasing energy.

9 This process only produces helium. What about heavier elements? We shall soon find out that heavier elements are produced near the end of a stars life.

10 Stars Gravity is always trying to pull the material of the star together, into a smaller, denser, tighter ball. For most of their lives, nuclear fusion in the star s core generates heat and pressure. The outward push of pressure balances the inward force of gravity. Gravity Pressure Gravity

11 Stars: The Beginning of the End What happens when hydrogen in the core is used up? The helium-rich core can no longer support fusion and the balance is upset. Gravity wins and the core contracts. Gravity Pressure Gravity

12 Red Giant Stars Sun Red Giant Star As the core contracts, temperature and pressure in the core increase. As temperature and pressure increase, the outer layers of the star expand and cool. The star s cooler surface becomes redder in color. Larger, redder the star has become a red giant.

13 Red Giant Stars Hydrogen burning shell Red Giant Star Helium rich core Surrounding the core is a shell where hydrogen fusion still occurs. When the temperature in the core becomes high enough (about 100 million K), helium begins to fuse into carbon and oxygen.

14 Helium Fusion 4 He 8 Be 4 He 4 He 12 C Energy production and nucleosynthesis in the core of a red giant (post-main sequence) star. Three helium nuclei fuse to form a 12 C nucleus. The 12 C nucleus in turn may fuse with another 4 He nucleus to produce 16 O.

15 Asymptotic Giant Branch Star (low to intermediate mass stars end life) Image Credit: NOAO For stars with a mass less than about eight or nine times the mass of the Sun, the last major phase of life is as an Asymptotic Giant Branch (AGB) star. At this point the star has an inert carbon-oxygen core, surrounded by two separate layers where fusion still occurs - an inner layer of helium and an outer layer of hydrogen. These layers are in turn surrounded by a strongly convective outer envelope.

16 The End for a Sun-like Star Sun-like stars do not have enough mass to create the temperatures and pressures needed to fuse carbon and oxygen into heavier elements. When the core is filled with carbon and oxygen, fusion in the core will end. The final frantic fusion that occurs along with a gravitational collapse of the core produces a final burst of energy. This last burst of energy causes the outer layers of the star to be blown out into space.

17 Planetary Nebula Credit: Matt Bobrowsky (Orbital Sciences Corporation) and NASA Credit: The Hubble Heritage Team (AURA/STScI/NASA) Credit: Bruce Balick (University of Washington), Vincent Icke (Leiden University, The Netherlands), Garrelt Mellema (Stockholm University), and NASA This uncovers the very hot core. Ultraviolet light from the core cause the layers of gas to glow, creating what is called a planetary nebula. In these layers are elements heavier that hydrogen and helium. The colors of the nebula are produced as ultraviolet light is absorbed and reemitted by some of these elements such as oxygen and nitrogen.

18 Red Super Giant Stars Stars larger than 8 times the mass of the Sun begin their lives fusing hydrogen into helium like smaller stars. When the core runs out of hydrogen the star then becomes a red super giant. Red super giants have enough mass to create higher core temperatures than smaller stars. Hubble Space Telescope image of Betelgeuse, a red supergiant Credit: Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA

19 Red Super Giant Stars After these stars fuse helium into carbon and oxygen, fusion continues producing successively heavier elements, up to iron. Each successive process requires a higher temperature The structure of a red super giant becomes like an onion, with different elements being fused in layers around the core. Convection brings the elements near the star s surface, where the strong stellar winds disperse them into space. C Ne, Na, Mg, O H He Ne O, Mg O Si, S, P He C, O Si Fe, Co, Ni

20 Red Super Giant Stars Fusion continues in red super giants until iron is formed in the core. Because iron has the most stable nucleus of all the elements, fusion of iron does not release energy. When the core rapidly fills with iron, energy production stops. C Ne, Na, Mg, O H He Ne O, Mg O Si, S, P He C, O Si Fe, Co, Ni

21 Red Super Giant Stars With no energy (gas pressure) to counteract gravity the core collapses. Gravity pulls electrons onto protons to form neutrons. At this point the core stiffens and rebounds. As a result, an explosive shock wave travels out from the core heating the surrounding layers. Most of the star s mass is blown off into space, in what is called a supernova. Astronomers refer to this as a Type II supernova. Credit: NASA, ESA, J. Hester and A. Loll (Arizona State University)

22 Heavier-than-Iron Elements Heavier-than-iron elements form only when gas pressures and temperatures are extremely high, such as during a supernova. Supernovae create all of the heavy elements, such as iodine, xenon, gold, and platinum. The remains of a supernova first seen in 1604 Credit: NASA/ESA/JHU/R.Sankrit & W.Blair

23 Neutron Stars Can Fuse Heavy Elements If the mass of the collapsed core that is left behind is not big enough to become a black hole, the end result is a ball of neutrons a few miles in diameter called a neutron star. Heavier-than-iron elements can form if two neutron stars orbit each other, as they will eventually collide. This collision will again create the temperatures and pressures needed to fuse heavy elements. This artist's conception portrays two neutron stars at the moment of collision. Credit: Dana Berry, SkyWorks Digital, Inc.

24 Supernova Aftermath The shock wave from a supernova can compress nearby clouds of hydrogen and helium now seeded with heavier elements. This will cause the cloud to collapse and form another generation of stars. The heavier elements can also condense to form rocky objects like the inner planets and asteroids of our solar system. Credit: NASA, ESA, R. O'Connell (University of Virginia), F. Paresce (National Institute for Astrophysics, Bologna, Italy), E. Young (Universities Space Research Association/Ames Research Center), the WFC3 Science Oversight Committee, and the Hubble Heritage Team (STScI/AURA)

25 Stellar Nucleosynthesis Star life cycles Interstellar gas STAR BIRTH gravitational contraction Stars element mixing STAR DEATH Expansion or explosion thermonuclear reactions Credit: C.R. O'Dell (Rice University), and NASA Credit: SOHO (ESA & NASA)

26 What is the origin of elements in the universe? Three elements formed in the first minutes after the big bang. Other lightweight elements form in fusion reactions in stars. Massive stars near the end of their lives form elements up to iron. Still heavier elements are thought to form in supernova explosions.

27 Origin of Elements - Recap Hydrogen, Helium and small amounts of Lithium formed in the moments after the Big Bang. Elements like carbon and oxygen form in fusion reactions in stars. Elements such as calcium and iron formed in large stars. Heavier elements such as gold are thought to form during supernovae or when two neutron stars collide. The solar nebula, from which our solar system was formed, was seeded with these elements, and they were present at Earth s formation