In an extended study investigating how elements are formed and galaxies chemically evolve, Dr Christopher Sneden from the University of Texas, Galactic nucleosynthesis: along with his team, look at stars within our own galaxy for vital clues. There are many diverse populations of stars located throughout the galaxy, from the galactic centre and spiral arms, through to a diffuse, random spread of the onset of element stellar objects in an area that cocoons the Milky Way, called the galactic halo. Dr Sneden’s research specifically concentrates on ancient, metal-poor halo production in objects to discover how primordial elements, such as hydrogen and helium, grow to form heavier elements that populate our surroundings. our galaxy e are all aware of the unsustainable and inevitably leads to a core range of substances that collapse supernova. During the following makes up the periodic high energy explosion, as the star obliterates, table. From hydrogen, the heavier elements are formed via a process lightest, most abundant of neutron capture. For this reason, iron substance in the universe, through to and neutron capture are important players Wcommonly used elements such as copper, within nucleosynthesis, and are elements Dr iron and uranium. All are familiar, but how Sneden pays particular attention to within his are they produced? Apart from hydrogen, research. helium and lithium, which together make up the three lightest elements forged during the NEUTRON CAPTURE AS A PATHWAY big bang, nearly all elements in the periodic TO HEAVIER ELEMENTS table, known as metals, are forged in nuclear Due to core reactions becoming fusion processes throughout a star’s lifetime. endothermic once iron has formed, all This ongoing chemical evolution is what Dr heavier elements in the periodic table are Sneden and his team have dedicated their formed during a star’s post main-sequence research to, with particular attention being phase or violent supernovae events. Neutron paid to metal-poor halo objects. This is capture is split between two time-variant critical to the research programme, as these mechanisms, called the slow (s) and rapid (r) ancient objects act as an intermediate source processes. of information linking past stellar activity with current chemical abundancies. S-PROCESS The s-process occurs mainly in stars that During a star’s lengthy participation on the range in size from 1 to 8 solar masses. When galactic stage it will go through various hydrogen to helium reactions (hydrogen phases of chemical evolution as lighter burning) cease to be the dominant source of elements, such as hydrogen, fuse to nuclear fusion within a stellar core, the Helium eventually form iron. Once iron is Carbon star expands and moves into the formed within a stellar core the Oxygen red giant phase of evolution. end of its life is imminent. Neon This particular period sees gnesiu Once this stage has Ma m hydrogen burning continue, Silicon been reached reactions although in a shell Iron in the core become Core formation enclosing the endothermic, meaning core. Helium burning now C more energy is put in to alcium becomes the star’s primary fusion processes than is fusion source in the dense released. This situation is central region, with helium
During a star’s lengthy participation on the galactic stage it will go through
"Kepler's supernova" was the various phases of chemical evolution last exploding supernova seen in our Milky Way galaxy - Johannes as lighter elements, such as hydrogen, Kepler, among others, observed it with the naked eye in 1604 fuse to eventually form iron
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moving up in the atomic weight scale to form carbon – the next producible element Detail in nucleosynthesis. It is this particular area that Dr Sneden has targeted for exploration, RESEARCH OBJECTIVES as carbon is by far the main by-product of Dr Sneden’s research focuses on s-process reactions within the core and any More data is required to fine tune the entire set of heavy elements, our observational astronomy with an surrounding envelopes. During this stage our understanding. What current, or Sun qualifies as "metal-rich". Stars in the emphasis on ground-based stellar neutrons are captured by atomic nuclei forthcoming, sky surveys are planned outer halo are more pristine H and He spectroscopy. His work is helping to in the interior stellar fusion zones. Due to for this purpose? objects, with the heavy elements making inform our knowledge of how the their neutral charge this can occur relatively The Gaia/ESO large spectroscopic up 1/10% down to about 1/1,000,000%. chemical elements that make up our easily. Once a neutron has been captured survey will be a major step in the right It is often difficult to detect the presence world came into being. it can then go through a beta-minus decay, direction, gathering more than 100,000 of any heavy elements in the spectra where the neutron decays under the weak high resolution spectra of stars that will of halo stars. Just as interestingly, the FUNDING nuclear force into a proton to increase the have very good parallaxes from the Gaia relative abundances are often decidedly NSF and NASA atomic number and form heavier elements. astrometric parallax mission. There are non-solar. For example we have detected This is known as the 'slow' process because also several other current spectroscopic r-process-rich and s-process-rich halos COLLABORATORS the neutron capture takes a relatively long very large-sample studies that can take stars, ones with more lithium than our • Caty Pilachowski (Indiana University) time compared to the beta decay processes. advantage of the Gaia database: e.g., Sun has, and ones that are anomalously • Raffaele Gratton & colleagues APOGEE, a dedicated near-infrared (thus enhanced in carbon. (Padova Observatory) R-PROCESS able to penetrate Galactic dust) survey, • Jim Lawler & colleagues (University of In stark contrast, the r-process occurs in and GALAH, a wide wavelength-coverage Is chemical evolution witnessed when Wisconsin) very different circumstances and timescales. spectroscopic survey of the Galactic disk we observe entire galaxies back • John Cowan (University of Oklahoma) Although the nuclear processes are the and halo. through time, at different cosmological same in principle, neutron capture is epochs? BIO realised during violent supernovae events What type of laboratory experiments Yes (see previous answer). Since After a BA in Astronomy from of massive stars greater than about 8 are used in conjunction with the halo stars are so metal-weak, Haverford College, Dr Chris solar masses. Once iron is produced astronomical observations? we believe that sometimes Sneden completed his PhD in deep within the interior, core-collapse is Abundances of elements in stellar the elements in a halo-star's Astronomy at the University of triggered. The following explosion causes atmospheres can be no more accurate atmosphere might be the product Texas at Austin, where he is now the release of an extreme number of free than the input atomic transition data, of the lives and deaths of just a few Rex G. Baker, Jr. and McDonald neutrons, which are then rapidly captured particularly transition probabilities, for (maybe even one) stars. This gives us Observatory Centennial Research by existing atoms within the debris. Time absorption lines that are seen in stellar direct insight into the nucleosynthetic Professor of Astronomy. He has published per reaction is believed to be in seconds spectra. We need these transition data process. about 260 peer-reviewed articles with rather than thousands of years, therefore for work on all kinds of stars, from nearly 22,000 citations from these. heavier elements are produced very quickly. Light echo around V838 Monocerotis metal-poor halo stars to metal-rich disk We can look back in time but not Dr Sneden points out that many ancient, stars. Fortunately, the efforts of several forward. How do you expect the Milky CONTACT metal-poor stars within the halo show laboratory atomic physics groups (in Way to evolve into the future, and what Chris Sneden greatly enhanced r-process elements, which isotopes, such as thorium and uranium, age FUTURE ADVANCES Wisconsin, London, Mons, and Lund, for will its final chemical state look like? Professor he states were produced by high-mass, constraints on the galaxy, and subsequently To date, research has advanced well. In example) have been rewarded in recent The overall heavy element content will The University of Texas at Austin rapidly evolving stars that existed prior to the universe, have been obtained. When his recent paper investigating iron group years with heightened accuracy in the grow with time. It is a bit difficult to Department of Astronomy the formation of any halo stars we observe compared to alternative estimates already elements within the halo star HD84937, Dr basic lab data – which have instantly distinguish "super metal-rich stars" (those 2515 Speedway, RLM 13.116 today. calculated from a diverse number of sources Sneden points out that observed chemical improved the stellar abundance results. with larger heavy-element contents Austin, Texas 78712-1205 (from thorium/uranium ratios in meteorites abundance levels do not always agree with than our Sun) because the present solar USA HOW OLD IS THE MILKY WAY? to satellite and ground-based sky surveys) prior abundance results for these elements How do chemical abundance levels vary chemical composition is the product of As with any form of research, alternative the new method provides a galactic age of (scandium through zinc) and ‘when carefully between the outer halo region and the many prior stellar generations of element T: +1 512 471 1349 verification methods are always 12–14 billion years. As Dr Sneden states: analyzed can be excellent measures of prior galactic plane? donors – one additional contribution won't E: [email protected] sought to confirm a theory. Using a ‘Nucleocosmochronology offers promise nucleosynthesis events’. This confirms metal- The dominant effect is the overall greatly change things that much. Nevertheless, W: http://www.as.utexas.edu/~chris/ distance-independent process called as an independent dating technique. poor halo objects as perfect starting points diminished bulk levels of heavy elements there are several ongoing efforts to nucleocosmochronometry, Dr Sneden’s team The chronometric results are generally in for the development of galactic chemical in outer halo stars. Even though our Sun identify stars more metal-rich than our have also been able to provide, utilising their agreement with other age estimates such as evolution (GCE) theories. is composed of about 90% hydrogen and Sun, and to see if their relative abundance research, an alternative method of ageing globular cluster ages and cosmological age 9% helium, and the other 1% comprising ratios are different than the Sun. the Milky Way. Utilising observed radioactive estimates.’ For this reason, further detailed investigations into metal-poor stars are required. This should be realised as results from large- The efforts of several laboratory atomic scale observation surveys are obtained. As The vast majority of elements in the Dr Sneden concludes: ‘additional precise physics groups have been rewarded in abundance determinations in metal-poor halo periodic table, known as metals, are stars should be undertaken; such studies may recent years with heightened accuracy in help to identify the nature of the first stars as the basic lab data forged in nuclear fusion processes well as to discover additional r-process-rich, throughout a star’s lifetime ultra-metal-poor stars.’
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