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Thursday, November 7, 2024
Dr. Brown: The physics and chemistry of stars and life — part 1
"Stars forge heavier, more complex elements from simpler elements under the intense gravitational forces and temperatures in the cores of stars," writes Dr. Brown.

Much of the information we have about the universe comes from ground and space-based telescopes designed to pick up energy signals in various bandwidths associated with the Big Bang, and the formation and life cycles of what’s visible in the universe — such as stars, galaxies and some massive black holes (because of the energy generated by their coronas). 

In other cases, such as the massive black hole that lies at the center of our galaxy, the evidence is more indirect and based on the gravitational effects of the black hole on the trajectories of nearby stars.

Or, in the case of the development of young galaxies, there may not be enough ordinary matter to gravitationally account for their development, hence there must be some other source of mass to make up for the apparent surfeit of mass.

That enigmatic matter called dark matter, because it doesn’t interact with the electromagnetic spectrum, turns out to be six times more common than ordinary matter.

Beyond these discoveries in the last century, there were other triumphs in cosmology: The discovery of gravitational waves in space-time generated by massive events such as the collisions of black holes or neutron stars and discoveries about how all the naturally occurring elements were generated — save hydrogen, helium and a tiny bit of lithium, which were created by the Big Bang. 

But beyond the latter three, how were most of the rest of the elements created? 

Stars forge heavier, more complex elements from simpler elements under the intense gravitational forces and temperatures in the cores of stars.

The simplest example of this is the fusion of pairs of hydrogen nuclei to create single helium nuclei, in the process losing a tiny bit of mass and releasing an enormous amount of energy. 

The reason for such a huge amount of energy stems from the relationship between energy and mass, which was so clearly expressed in 1905 by Albert Einstein’s most famous equation, where energy equals mass multiplied by the square of the speed of light (E = mc2). 

Heavier elements are created in a similar fashion by nuclear fusion of successively heavier elements (nuclei) in the cores of stars.

However, as heavier nuclei are created in the core, nucleosynthesis becomes less and less efficient as a source of expansile energy to offset the growing inward gravitational force created by the increasingly massive core in the center of the star. 

Finally, gravity wins out over expansion and the outer shell of the star gravitationally collapses and crashes into the now very dense core, which force is then reflected backward, scattering the elements created by the star in its natural lifetime and yet more massive nuclei created by the even more extreme heat and pressures generated by the implosion-explosion (supernova) — outward into the neighbourhood of the former star.

Finally, the collapse of the now very dense core typically implodes and creates either a neutron star, jampacked with as you might guess, mostly neutrons, or a black hole.

Supernovas mark the end of most medium to large-sized stars and light up the sky for several days or even a few months, providing standard candles for determining the distance to the supernova and spectral analyses, which tell us which elements were created in the stars’ lifetime and death.

Creation of the heaviest elements, such as gold and tungsten, requires the much higher pressures and temperatures generated by the collision of a pair of neutron stars — what’s been called kilonovas. 

In a nutshell, that’s how all the natural elements were and are created — except for hydrogen, helium and a tiny bit of lithium nuclei, created shortly after the Big Bang. 

It all sounds very complex, and it is because the formation of stars, galaxies and the natural elements combines the dominant themes of physics in the first half of the 20th century: General relativity, which relates mass to space-time, and quantum physics, which governs the universe of the atomic universe.

That period between 1900 and 1940 I’ve likened to the Camelot period in physics — what followed in physics in the latter half of the century into the 21st century, was stunning but more consolidation. 

The second Camelot period in science was not in physics but in biology, beginning with the discovery of how DNA codes genetic information and was soon followed by a series of equally fascinating discoveries in molecular biology, recently including a means for precisely editing the genome and a whole other universe of not the atom, but the cell.  

Next week, we turn to the more complex matter of the nature of life.

Dr. William Brown is a professor of neurology at McMaster University and co-founder of the InfoHealth series at the Niagara-on-the-Lake Public Library. 

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