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Large amounts of beryllium-7, an element that decays into lithium, have been found inside nova Sagittarii 2015 N.2
Lithium, the lightest solid element in existence, plays an important role in our lives, both at the biological and the technological level. Like the majority of chemical elements, its origins stem back to astrophysical phenomena, but its point of genesis was so far unclear. Recently, a group of researchers detected enormous quantities of beryllium-7 –an unstable element which decays into lithium in 53.2 days– inside nova Sagittarii 2015 N.2, which suggests that novae are the main source of lithium in the galaxy.
Practically every chemical element has an astronomical origin. A first
genesis took place in what is known as primordial nucleosynthesis,
shortly after the Big Bang (between ten seconds and twenty minutes
after). Light elements were then formed: hydrogen (75%), helium (25%)
and a very small amount of lithium and beryllium.
The remaining chemical elements were formed in stars, either through
fusion of other elements inside the nucleus –which begins with the
fusion of hydrogen into helium and produces increasingly heavy elements
until iron is reached- or through other processes such as supernovae
explosions or reactions in the atmosphere of giant stars where, among
others, gold, lead and copper are produced. Those elements in turn were
then recycled into new stars and planets until the present day.
"But lithium posed a problem: we knew that 25% of existing lithium
comes from primordial nucleosynthesis, but we were not able to trace the
origins of the remaining 75%", says Luca Izzo, researcher at the
Institute of Astrophysics of Andalusia (IAA-CSIC) involved in the study.
Solution to the lithium enigma
The solution to the enigma of the origin of lithium lies, according to
this study, in the novae, explosive phenomena occurring in binary star
systems in which one of the stars is a white dwarf. The white dwarf can
nab material from its twin star and form a superficial layer of hydrogen
which, when it reaches a certain density, will trigger an explosion –a
nova– which can increase the brightness of a star up to one hundred
thousand times. After a few weeks the system stabilizes and the process
Artist concept of a binary system similar to the one that originated the
nova Sagittarii 2015 N.2
Credit: David A. Hardy y PPARC.
The researchers studied nova Sagittarii 1015 N.2 (also known as V5668 Sgr), which was detected on March 15th,
2015, and remained visible for more than eighty days. The observation,
made with the UVES instrument of the Very Large Telescope (ESO) in the
course of twenty four days, made it possible for the first time to
follow the evolution of the beryllium-7 signal inside a nova and even to
calculate the amount of it present. "Beryllium-7 is an unstable element
which decays into lithium in 53.2 days, so its presence is an
unequivocal sign of the existence of lithium", says Christina Thöne,
researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC).
The existence of beryllium-7 had been previously documented in another
nova, but the measure of the amount of lithium which would be ultimately
produced from it on nova Sagittarii 1015 N.2 came as a surprise. "We’re
talking about an amount of lithium ten times greater than that in the
Sun," says Luca Izzo (IAA-CSIC). "With these amounts in mind, two
similar novae a year would suffice to account for all the lithium in our
galaxy, the Milky Way. Novae seem to be the predominant source of
lithium in the universe," he concludes.
P. Molaro, L. Izzo et al. "Highly Enriched 7Be in the ejecta of Nova Sagittarii 2015 No. 2 (V5668 Sgr) and the Galactic 7Li origin". Monthly Notices of the Royal Astronomical Society, Vol. 463