Yorkshire Philosophical Society

Event Information

Stellar Alchemy and the Origin of Cosmic Elements
Jordi José, Dept. Physics, UPC Barcelona

Where do the nitrogen (N) in our DNA, the calcium (Ca) in our bones, the iron (Fe) in our blood, or the oxygen (O) we breathe come from? As the late planetary scientist Carl Sagan used to say, we are made of starstuff. Most of the ordinary (visible) matter in the Universe, from a terrestrial pebble to a human, a giant star, or a galaxy, is made of protons and neutrons arranged in different configurations called elements. There are 82 elements that have stable isotopes, all the way from hydrogen (H) to lead (Pb), except for technetium (Tc) and promethium (Pm). Several dozen elements have only unstable isotopes, naturally abundant or artificially synthesized in nuclear physics labs.

During eons, the Cosmos was a chemically hostile environment, almost devoid of elements, except for the very light ones [(hydrogen (H), helium (He), and some marginal amounts of lithium (Li)], created during the first hundreds seconds after the Big Bang. For about 200 million years, during the so-called Dark Ages, the Universe just expanded and cooled down. But the emergence of the first stars, formed by effect of gravity, changed the picture completely: such nuclear furnaces have been responsible for turning the handful of light elements synthesized right after the Big Bang into a suite of heavier elements. Indeed, most elements up to Fe are synthesized by nuclear fusion in “normal” stars of different masses, while elements heavier than Fe are produced in a number of astrophysical sites, including the late stages of the evolution of low- and intermediate-mass stars, core-collapse supernovae, and neutron star mergers.

This talk summarizes a breathtaking 13.8 billion-year cosmic journey, from the Big Bang to the role played by stellar explosions in shaping the chemical abundance pattern that we see today in the Universe.

Crab Nebula image: courtesy NASA and STSci

Dr Alison Laird’s lecture will follow at a later date and she will introduce ProfessorJosé

Astrophysics mini theme

Member’s report

When he was sick as a child, a Flash Gordon comic book caught Professor José’s imagination because it showed how the hero escaped by recognising a depiction of the periodic table of elements. The elements we recognise here on the Earth are ubiquitous across the universe. Alchemy, the creation of new elements from other lighter elements, happens in stars. Initially the big bang only created hydrogen and helium in abundance with tiny amounts of deuterium and lithium. As stars started to form from two hundred million years after the big bang, new heavier elements were formed in the fusion reactions that powered the stars. A star like our sun will have a lifetime of ten thousand million years. In its reaction it will form elements up to the weight of iron. Stars much bigger than the sun are much hotter in their cores, have shorter lifetimes, but produce the heavier elements. As stars age, they can become red giants and then collapse under their own gravity into white dwarfs finally exploding as super novae. The elements formed in the stars are thereby scattered throughout the galaxy and eventually collapse into new generations of stars in regions known as stellar nurseries. The lecture was illustrated with spectacular stellar images and peppered with Jordi’s infectious humour and mental arithmetic.

Andy Marvin