August 21, 2022
Sunday, August 21, 2022
A couple of days before the high school students arrived on campus my wife was wondering what she was going to do with her science students since her students were not getting their textbooks and chrome books until the fifth day of the semester. Most of the conversation at the Strobel dinner table centers around teaching and physical science which is exciting stuff for two empty-nester STEM teachers. She decided to do a paper-pencil exercise about the Periodic Table of Elements. I suggested an astronomy connection with the “cosmic connection to the elements”.
The Periodic Table of Elements is a graphical representation of the different types of atoms in the universe, such as hydrogen, helium, carbon, oxygen, iron, uranium, etc. Each type of atom called an “element” has a unique chemical property. There are ninety-two naturally-occurring elements in the universe and over a couple dozen ones that we've synthesized in our high-energy laboratories. Each atom has two basic parts: a central massive nucleus made of protons and neutrons and a cloud of low-mass electrons swirling around the nucleus. A lot of chemistry depends on the arrangement of the outer electrons, so the Periodic Table arranges the elements in groupings of similar outer electron configurations and increasing total number of electrons from left to right and from top to bottom. Since the number of positive-charged protons matches the number of negative-charged electrons in a neutral atom, the increasing number of protons in the elements of the Periodic Table goes with the increasing number of electrons. The neutrons are along for the ride and help keep the nucleus together, so most of chemistry ignores the neutrons.
The cosmic connection comes when talking about where the atoms originally came from. Chemists, like most people, will often say different materials are created from different types of chemical reactions, such as when carbon dioxide and water react with the energy of sunlight to create sugars plus oxygen in the process we call photosynthesis or when the combustion of coal or oil creates carbon dioxide.
Snobbish astronomers and nuclear physicists pooh-pooh that “creation” and say that chemical reactions “just shuffle the atoms around”. The number of the different types of atoms—the elements—in a chemical reaction stays the same but how the various elements connect or bond to each other changes in a chemical reaction. However, where did the individual atoms themselves come from?
A whole lot of science has led us to discover that all of the atoms on Earth, planets, moons, etc. came from cosmic processes, most having to do with the stars. Most of the hydrogen, which has one proton in the nucleus, was created in the first micro-second after the universe began expanding (the Big Bang) when the universe was extremely hot and dense. Most of the helium in the universe was created in the first few minutes after the start of the expansion when some of the hydrogen was smashed together in a process called nuclear fusion, similar to what occurs at the cores of stars like the sun right now. With hydrogen fusion, four hydrogen nuclei—four protons—are smashed together to make a helium nucleus which has two protons and two neutrons. It turns out that the helium nucleus has less total mass than the combined mass of the four original protons. The mass that was lost in nuclear fusion was converted to energy—light! That's what makes stars shine. Stars are now slowly increasing the amount of helium in the universe.
Nuclear fusion requires extremely high temperatures and densities. When stars get old, they run out of hydrogen to fuse in the core and the core is filled with helium. The core compresses and heats up enough for helium to fuse to make heavier elements such as lithium, carbon, oxygen, silicon, and iron—the more massive a star is, the more types of elements it can make because to create nuclei with more protons requires ever higher temperatures and densities.
Explosive stellar events called supernovae can make many of the even heavier elements such as nickel, copper, zinc, etc. from a super-rapid nuclear fusion process that lasts just a few minutes. Supernovae happen when very massive stars die and their cores collapse suddenly or when already dead low-mass stars called white dwarfs in a binary system suck up too much gas from their close companion star. It turns out that high-mass dead star cores called neutron stars also briefly have nuclear fusion when they collide with other neutron stars. Most of the gold and uranium in the universe comes from that.
Our theory of the creation of all the different elements from nuclear fusion correctly predicts the observed abundances of all of the naturally occurring heavy elements seen everywhere in the universe. We understand now why some elements like carbon, oxygen, silicon, and iron are common and the heaviest elements like gold, mercury, and uranium are so rare. In order to create a planet like the Earth (and life on such a planet), enough of the heavy elements have to be created in previous generations of stars and then get concentrated in the interstellar clouds to collect into sizable chunks around forming stars. There is necessarily a "lag" between the beginning of the universe and the beginning of life.
Put the Dark Sky Festival at Sequoia and Kings National Parks on your calendar for September 24. The Kern Astronomical Society is going to have a lot of telescopes for people to use there under a truly dark sky!
—
Nick Strobel
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com
