According to the Chabot Space and Science Center’s ground-floor Destination Universe exhibit, we are all made of stardust! Dr. Mary Barsony, Adjunct Professor of Physics and Astronomy at San Francisco State University and Research Scientist at the Carl Sagan Center for the Study of Life in the Universe at the SETI Institute, brought this point home during this month’s volunteer enrichment lecture.
Dr. Barsony’s talk chronicled the early history of the universe, starting with the Big Bang and continuing through the formation of lighter and then heavier elements. To begin, she outlined the four basic forces holding our world together: gravity, electromagnetism, and the strong and weak nuclear forces (holding positively charged nuclei together, and allowing the particles to decay during certain reactions). Gravity has proved mysterious due to its relative weakness at the subatomic level, leaving physicists searching for a new unifying theory to explain this.
Also, she oriented us to the scale of the world around us: if an atom’s nucleus were the size of an 0.5mm pencil point at the pinnacle of St. Peter’s Cathedral, the rest of its electron cloud would likely extend throughout the entire dome. And, this ordinary matter, composed of electrons, protons, neutrons and subatomic particles known as quarks, only represents a little over four percent of the universe. The rest, according to current theories, consists of invisible ‘dark matter’ (22.7 percent) and ‘dark energy’ (72.8 percent ). Dark energy is thought to be responsible for the expansion of the universe. We observe this expansion when we see galaxies moving farther away from us. We can detect changes in the wavelengths of light reaching us as galaxies move, a phenomenon known as a ‘redshift’ because the light moves towards the redder, longer-wavelength sections of the light spectrum.
Next, Dr. Barsony explained why scientists are fairly sure that the known universe began with a Big Bang, a point when hot, condensed matter began expanding and cooling. Today’s relative abundances of light elements, such as hydrogen and helium, make sense given the Big Bang theory, as does the background microwave radiation we have detected in the far reaches of the universe.
During the Big Bang, hydrogen and helium formed first, as these elements have the smallest number of protons in their nuclei. The reactions creating these elements were exothermic, giving off heat rather than requiring an input of energy.
Every other, heavier element in the periodic table formed later, within stars at various points of their life cycles. Stars carry out nuclear fusion to power themselves, and a star’s mass determines how much fuel it has and thus how long it will last. When a star runs out of fuel, it can decay to become a red giant, expanding to make room to radiate out the last of its luminous energy. Or, if it is larger, it may become a supernova or even a black hole, an object that has collapsed in on itself, where not even the light reaching it can escape the pull of its gravity.
The universe contains stars of all sizes, although many more are lower-mass than higher-mass. A star near the mass of our sun can fuse elements all the way up to iron in its core, and higher-mass stars can create other elements through a process of slow neutron capture (s-capture). Highly massive stars can produce very heavy elements through rapid neutron capture (r-capture), where more than one neutron gets captured at once. Some heavy elements form under endothermic conditions, where a certain amount of heat is required to start the reaction. The extreme conditions required for heavy metal formation explain the rarity, and thus the value, of precious metals, such as gold, silver and platinum.
Finally, Dr. Barsony illustrated her point by tracing the origin of the elements making up an average person. The iron in our blood originates from fusion in large stars. The oxygen and carbon in the food we eat and the water we drink come from dying solar-mass stars, and the hydrogen in our water from the Big Bang itself. And our gold jewelry definitely arose from deep within a supernova.