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37 38

58

La Lettre

The sun inside the Milky Way (artist view)

© Caltech et NASA

Nuclear physics and the study of radioactivity (Becquerel, Rutherford, the Curie couple) would, between 1850

and 1950, provide the bases for an explanation of the origins of the elements. The sun then became a star like

the others, its energy deriving from reactions of nuclear fusion (Perrin, Eddington). Nucleosynthetis reactions

became known to occur within the stars which, at the end of their evolution, were acknowledged to enrich the

interstellar medium with heavier elements (Burbidge). The age of Earth was estimated, by means of natural

radioactivity counting techniques, at 1 and then 4 billion years during the first half of the 20

th

Century.

At the end of such progress, it has been established that the sun and its train of planets are in motion in

the Milky Way, our Galaxy, which comprises hundreds of billion stars more or less comparable to ours. The

Milky Way has become but a galaxy among hundreds of thousand others within the expanding Universe

(Shapley, Ross, Hubble).