SANTIAGO (Reuters) - Astronomers have discovered lithium in a type of stellar explosion known as a nova for the first time, a find that helps clear up a longstanding mystery in astrophysics about the quantity of the element that has been observed in stars.

Using two telescopes in Chile, astronomers detected tiny amounts of the chemical element lithium in Nova Centauri, which exploded in 2013 - the brightest nova so far this century, the European Southern Observatory said this week.

That could be the first step in solving the so-called lithium riddle. Models of the Big Bang at the birth of the universe 13.8 billion years ago allow astronomers to calculate quite accurately how much lithium should be present.

But older stars do not have as much lithium as the models suggest, while younger ones have more.

Astronomers have long speculated that the latter part of the problem could be explained by novae expelling the element, "seeding" space with lithium and enriching the interstellar medium from which new stars are born.

But no clear evidence to date has been found of lithium in novae.

The discovery of lithium being expelled at some 2 million kilometers (1.24 million miles) per hour in Nova Centauri could, when extrapolated to the billions of other novae that have exploded in the Milky Way's history, explain the unexpectedly large amount of lithium in our galaxy, the ESO said.

"If we imagine the history of the chemical evolution of the Milky Way as a big jigsaw, then lithium from novae was one of the most important and puzzling missing pieces," said Massimo Della Valle, a coauthor of the study.

The mismatch between the observed amount of lithium in older stars and the abundance estimated from the Big Bang, however, still remains an open problem, said Della Valle and team leader Luca Izzo.

Results of the study were published in the Astrophysical Journal Letters.

(Reporting by Rosalba O'Brien; Editing by Matthew Lewis)