astrophysics: a lithium-rich stellar explosion

an attempt to decode the personality traits of a single person on the basis of their full set of Facebook friends. This would be a tough job, given the wide variety of individuals in the group, which might include friends, family and acquaintances. Similarly, if we assume t hat all connections are equivalent in a neuronal network, predicting the response selectivity of a cortical neuron may be difficult, because the inputs are so diverse. Although connectome projects will certainly generate valuable statistics about connectivity in the cerebral cortex, Cossell and colleagues demonstrate that identifying the weights of synaptic connections is essential to account for neuronal responses. When connectivity is considered in conjunc- tion with synaptic weighting, it should be possible to predict response selectivity. Despite the difficulties assoc iated with determining connection weight from anatomical measurements alone, there are hints that not all synaptic inputs are physically equal. Properties of synaptic contacts that may be linked to synaptic weight 10 include the size of postsynaptic structures such as dendritic spines, the number of neurotrans- mitter molecules available for release by the presynaptic terminal, and the extent to which subcellular compartments that are responsible for protein synthesis can form new postsynaptic structures. Understanding these links will be crucial for bridging the gap between functional and anatomical connectivity, so that neuroscientists can get closer to obtaining a functional connectome. ■ Benjamin Scholl and Nicholas J. Priebe are in the Department of Neuroscience, The University of Texas, Austin, T exas 78712, USA. e-mail: [email protected] 1. Binzegger, T., Douglas, R. J. & Martin, K. A. C. J. Neurosci . 24, 8441–8453 (2004). 2. Cossell, L. et al. Nature 518, 399–403 (2015). 3. Isaac, J. T . R., Nicoll, R. A. & Malenka, R. C. Neuron 15, 427–434 (1995). 4. Liao, D., Hessler, N. A. & Malinow, R. Nature 375, 400–404 (1995). 5. Allard, T ., Clark, S. A., Jenkins, W. M. & Merzenich, M. M. J. Neurophys iol. 66, 1048–1058 (1991). 6. Sato, M. & Stryker , M. P. J. Neurosci. 28, 10278–10286 (2008). 7. Kleinfeld, D. et al. J. Neu rosci. 31, 16125–16138 (2011). 8. Lichtman, J. W., Livet, J. & Sanes, J. R. Nature Rev. Neurosci. 9, 417–422 (2008). 9. Oh, S. W. et al. Nature 508, 207–214 (2014). 10.Bourne, J. N. & Harris, K. M Annu. Rev. Neurosci. 31, 47–67 (2008). This article was published online on 4 February 2015. MARGARITA HERNANZ T he origin of lithium observe d in today’s Universe is a long-standing problem. It is known that a fraction of this light chemical element was created during the Big Bang, along with hydrogen and helium, and that another fraction has formed since then through nuclear reactions induced by energetic cosmic rays. But comparison of chemical-evolution models and observed stellar lithium abundances in the Milky Way indicates that part of the lithium should also have been synthesized in old low-mass stars, such as red giants, and in stellar explosions known as novae. However, although lithium has been observed in giants, its detection in novae has remained elusive. On page 381 of this issue, Tajitsu et al. 1 provide the first observational evidence of lithium synthesis in novae. The authors detected radioactive beryllium-7 ( 7 Be), the paren t nucleus of lithium-7 ( 7 Li), during a nova explosion called V339 Del (Nova Delphini 2013). It has long been known that almost all of the chemical elements are produced in st ars by the nuclear fusion of light elements into heavier ones, starting with hydrogen fusion 2 . The synthesized elements can then be expelled to the interstellar medium — from which new stars will form — either by stellar winds or during supernova explosions and their dimmer relatives, novae. However , the main origin of the light elements lithium, beryllium and boron is not linked to nuclear reactions in stars. Instead, it is related to nucleosynthesis processes that are less efficient than stellar ones. This is why these elements are much less abundant in the Milky Wa y and the S olar System than heavier elements. Lithium has a complex origin. It is produced in three ways: by nucleosynthesis during the Big Bang; by nuclear reactions in the interstellar medium that are induced by energetic cosmic rays and are also responsible for the origin of beryllium and boron; and by nuclear reactions in stellar sources, such as red giants 3 . The stellar sources are required to reproduce the rise of lithium abundance in the Milky Way after the formation of the Solar System about 4.5 billion years ago. Inside stars, 7 Be, the subject of Ta jitsu and colleagues’ study, is formed by the fus ion of helium-3 and helium-4. This radioactive element then captures an electron and transforms into its daughter nucleus, 7 Li, within a short timescale ( 7 Be has a half-life of 53.22 days), releasing a 478-kiloelectronvolt-energy photon (Fig. 1). But efficient production of 7 Li requires this nuclear reaction to occur in hot, external stellar layers, and requires freshly produced 7 Be to be transported into cooler subsurface layers before it transforms into 7 Li. In this way, 7 Li is immune to destruction once it is created. This process, known as the Cameron–Fowler 7 Be transport mechanism, is responsible for 7 Li production in stars 4,5 . Novae are thermonuclear explosions, and take place on top of white dwarfs that pull hydrogen-rich material from a companion star. As more hydrogen accumulates on the white dwarf, it builds up a shell that reaches pressures and temperatures sufficient to trigger explosive runaway fusion of the hydrogen. This leads to the fast expansion and subsequent ejection of the white dwarf ’ s outer layers, and is accom- panied by a sudden large inc rease in the st ar’ s brightness. During this process, 7 Li is thought to be produced through the Cameron–Fowler 7 Be transport mechanism. The first studies of lithium production in novae were made in the 1970s 6,7 , but it was not until 1996 that the details of the process were 3 He 3 He 4 He 4 He 7 Be 8 B p p e – 478 keV 478 keV 7 Li 2p ASTROPHYSICS A l ithium-rich stellar explosio n The contribution of explosions known as novae to the lithium content of the Milky Way is uncertain. Radioactive beryllium, which transforms into lithium, has been detected for the first time in one such explosion. S L .381 Figure 1 | The main nuclear reactions involved in the synthesis of 7 Be and 7 Li in novae. Tajitsu et al. 1 have detected radioactive 7 Be in a nova explosion. 7 Be transforms into 7 Li, a neutrino and a photon of 478 kiloelectronvolts when it captures an electron. p, Proton; e – , electron; γ, γ-ray; 3,4 He, helium-3 and -4; 7 Be, beryllium-7; 7 Li, lithium-7; 8 B, boron-8; ν, neutrino. 19 FEBRUARY 2015 | VOL 518 | NATURE | 307 NEWS & VIEWS RESEARCH © 2015 Macmillan Publishers Limited. All rights reserved

astrophysics: a lithium-rich stellar explosion

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