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Synthesis of the Elements in Stars Takes Place

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Synthesis of the Elements in Stars Takes Place ~l "'r $7 ~ L. Vot.UME 29, NUMnaa 4 OcToaaR, 1957 ' '~ Synt iesis oI. t.~e . .ements in Stars K. MARGARET BURBIDGE) G. R. BURMDGE) %ILIIAM: A. Fowl, ER) AND F. HQYLK Eellogg EadiatzonlLaboratory, California Instztute of Technology, and Mount Wilson and Palonzar Observatories, Carnegie Institution of Washington, Calzfornia Instztute of Technology, Pasadena, California "It is the stars, The stars above us, govern our conditions"; (Eing Lear, Act IV, Scene 3) "The fault, dear Brutus, is not in our stars, But in ourselves, " (Julius Caesar, Act I, Scene 2) TABLE OF CONTENTS IQge I. Introduction. 548 A. Element Abundances and Nuclear Structure. , . 548 B. Four Theories of the Origin of the Elements. 550 C. General Features of Stellar Synthesis. 550 II. Physical Processes Involved in Stellar Synthesis, Their Place of Occurrence, and the Time-Scales Associated with Them 551 A. Modes of Element Synthesis. , . 55j. B. Method of AsslgnIIlent of IBotopes atnong PI'ocesses (i) to (vill) . 553 C. Abundances and Synthesis Assignments Given in the Appendix . 555 D. Time-Scales for Different Modes of Synthesis. 556 III. Hydrogen Burning, Helium Burning, the e Process, and Neutron Production . ~ ~ ~ ~ t ~ ~ ~ o ~ o ~ ~ ~ 0 ~ ~ & r 559 A. Cross-Section Factor and Reaction Rates. , . 559 B. PuI'e HydI'ogen Burning. ,. ; . 562 C. Pure Helium Burning. ~. 565 D ~ 6 Process t ~ o + ~ ~ ~ ~ ~ + ~ o o s e ~ + ~ ~ ~ e s ~ 4 ~ ~ ~ ~ ~ ~ ~ l ~ ~ ~ ~ ~ I ~ ~ ~ ~ i ~ 567 E. Succession of NU. clear Fuels in an Evolving Star. ~ \ J ~ ~ 1 ~ ~ ~ ~ ~ ~ , . 568 F. Burning of Hydrogen and Helium with Mixtures of Other Elements; Stellar Neutron Sources . 569 IV. e Process . 577 V. s and r Processes: General Considerations. , . , . 580 A. "Shielded" and "Shielding" Isohars and the s, r, p Processes. 580 B. Neutron-Capture Cross Sections. 58j. C. General Dynamics of the s and r Processes. 583 VI. Details of the s Process. , . , . 583 Supported in part by the joint program of the ofBce of Naval Research and the U. S. Atomic Energy Commission. 547 Copyright O 1957 by the American Physical Society 8URB I DGE, B URB I DGE, FOOLER, AND HOYLE Details of the r Process . 587 A. Path of the r Process. , , . , . 588 B. Calculation of the r-Process Abundances. 593 Flow ~ ~ \ ~ . 596 C. Time for the r Process: Steady and Cycling . \ D. Freezing of the r-Process Abundances. .. 597 VIII. Extension and. Termination of the r Process and s Process. , . , . 598 A. Synthesis of the Naturally Radioactive Elements. , . , 598 B. Extension and Termination of the r Process. 598 C. Age of the Elements and of the Galaxy. 605 D. Termination of the s Process; the Abundances of Lead, Bismuth, ThorluIQ~ and Uranium. 608 IX. p Process . 615 X. g PIocess o ~ ~ ~ ~ ~ ~ ~ ~ s e t ~ s ~ e ~ e ~ o s ~ ~ e s ~ A. Observational Evidence for Presence of Deuterium, Lithium, Beryllium, and Boron in our Galaxy 618 B, Nuclear Reactions which Destroy Deuterium, Lithium, Beryllium, and Boron. , . 618 C. Synthesis of Deuterium, Lithium, Beryllium, and Boron . , . 618 D. Preservation of Lithium in Stars. ~ . 620 XI. Variations in Chemical Composition among Stars, and Their Bearing on the Various Synthesizing Pmcesses. , . \ ~ ~ t ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ . ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 620 A. Hydrogen Burning and Helium Burning . , . 621 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 1 ~ ~ ~ ~ t ~ ~ ~ ~ B~ Q Process i ~ ~ ~ ~ ~ ~ ~ i ~ ~ ~ ~ ~ ~ ~ ~ 1 ~ ~ 1 . 626 C. Synthesis of Elements in the Iron Peak of the Abundance Curve, and the Aging ERect as It Is Related to This and Other Types of Element Synthesis. 626 D. s Process. 627 E. r Process. 629 F. p Process. 629 G. x Process. 629 H. Nuclear Reactions and Element Synthesis in Surfaces of Stars. 629 General Astrophysics. ~. ~. ~ . , . 630 A, Ejection of Material from Stars and the Enrichment of the Galaxy in Heavy Elements. , 630 B. Supernova Outbursts. ~ ~ 4 ~ ~ ~ ~ 1 i ~ ~ ~ t ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ . 633 C. SupelIlova Light Curves. , . 635 D. Origin of the r-Process Isotopes in the Solar System. 639 XIII. Conclusion , . , ~ . , 639 Acknowledgments. 641 Appendix. , . , . 641 . 647 BlbllogI'aphy. - . ~ . ~ ~ I. INTRODUCTION of known nuclear species is almost 1200, with some 327 of this number known to occur in nature. In spite of A. Element Abundances and Nuclear Structure this, the situation is not as complex as it might seem. 'AN inhabits a universe composed of a great Research in "classical" nuclear physics since 1932 has variety of elements and their isotopes. In Table shown that all nuclei consist of two fundamental build- I,i a count of the stable and radioactive elements and ing blocks. These are the proton and the neutron which isotopes is listed. Ninety elements are found terrestrially are called nucleons in this context. As long as energies and one more, technetium, ls found 1n stars) only pI'o- below the Ineson pmduction threshoM are not exceeded, methium has not been found in nature. Some 272 stable all "prompt" nuclear processes can be described as the and 55 naturally radioactive isotopes occur on the earth. shufHing and reshuRhng of protons and neutrons into In addition, man has been able to produce artificially the variety of nucleonic packs called nuclei. Only in the be- the neutron, technetium, promethium, and ten trans- slow beta-decay pmcesses is there any interchange uranic elements. The number of radioactive isotopes he tween protons and neutrons at low energies, and even nu- has produced now numbers 871 and this number is there, as in the prompt. reactions, the number of gradually increasing. cleons remains constant. Only at very high energies Each isotopic form of an element contains a nucleus can nucleons be produced or annihilated. Prompt with its own characteristic nuclear properties which are nuclear processes plus the slow beta reactions make di6erent from those of all other nuclei. Thus the total it possible in principle to transmute any one type of SYNTHESIS OF ELEMENTS IN STARS TAsr z I,1.Table of elements and isotopes /compiled from Chart of the Xgcjides (Knolls Atomic Power Laboratory, April, 1956)). Elements Isotopes tO Stable 81 Stable Radioactive: Radioactive: 1' iid II Natural (Z& 83) Natural (A &206) 8 (Zi83) 9b (A &206) 44 5 Natural: Natural: Stable and Radioactive 91 Stable and Radioactive 327 Radioactive: Radioactive: Kl Arti6cial (Z& 83} ic Artlflc ial (A &206) 702 UJ (Zg83) 10 (A &206) 169 I- Total 102 Total 1198 K Neutron 1 ¹utron 1 x~ l- 2 103 1199 C3 C) R Tc, observed in 8-type stars. b Including At and Fr produced in weak side links of natural radioactivity ~r-DECAY e Pm, not observed in nature. d Including Hg, C~4, and Tc». s- -2- YCLE nuclear material into any other even at low energies of interaction. t l With this relatively simple picture of the structure 50 l00 l50 200 and interactions of the nuclei of the elements in mind, ATOMIC WEIGHT it is natural to attempt to explain their origin by a FIG. I,i. Schematic curve of atomic abundances as a function synthesis or buildup starting with one or the other or of atomic weight based on the data of Suess and Urey (Su56). Suess and Urey have employed relative isotopic abundances to both of the fundamental building blocks. The following determine the slope and general trend of the curve. There is still question can be asked: What has been the history of considerable spread of the individual abundances about the curve the matter, on which we can make observations, which illustrated, but the general features shove are now fairly well established. These features are outlined in TaMe I,2. Note the produced the elements and isotopes of that matter in overabundances relative to their neighbors of the alpha-particle the abundance distribution which observation yields? nuclei A = 16, 20, ~ ~ 40, the peak at the iron group nuclei, and the = This history is hidden in the abundance distribution of twin peaks at A 80 and 90, at 130 and 138, and at 194 and 208. the elements. To attempt to understand the sequence of events leading to the formation of the elements it is the sun were first derived by Russell (Ru29) and the necessary to study the so-called universal or cosmic most recent work is due to Goldberg, Aller, and Muller abundance curve. (6057). Acc111'a'te relative lsotoplc Rbu11daIlces al'e Whether or not this abundance curve is universal is available from mass spectroscopic data, and powerful not the point here under discussion, It is the distribu- use was made of these by Suess and Urey in compiling tion for the matter on which we have been able to make their abundance table. This table, together with some observations. We can ask for the history of that par- solar values given by Goldberg et ul. , forms the basic ticular matter. We can also seek the history of the data for this paper. peculiar and abnormal abundances, observed in some It seems probable that the elements all evolved from stars. We can finally approach the problem of the uni- hydrogen, since the proton is stable while the neutron versal or cosmic abundances. To avoid any implication is not. Moreover, hydrogen is the most abundant that the abundance curve is universal, when such an element, and helium, which is the immediate product of implication is irrelevant, we commonly refer to the hydrogen burning by the pp chain and the CN cycle, number distribution of the atomic species as a function is the next most abundant element. The packing-frac- of atomic weight simply as the atomic abundance dis- tion curve shows that the greatest stability is reached tribution. In graphical form, we call it the atomic at iron and nickel. However, it seems probable that iron abundance curve. and nickel comprise less than 1% of the total mass of The 6rst attempt to construct such an abundance the galaxy.
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