Editor’s note: A version of this has been previously published on the author’s website.
As Joni Mitchell sang 50 years ago, “We are stardust.”
Members and guests of the Utah Astronomy Club learned that anew, and heard about the close connections we all have with distant titanic events in the universe, on Feb. 14 during the group’s monthly meeting at the University of Utah. Seth Jarvis, the recently retired director of Clark Planetarium, used examples concerning the color red to make the point.
Jarvis began working in what was then Hansen Planetarium, 15 S. State, Salt Lake City, in the fall of 1978. He started out as an usher making $2.25 an hour — a step down financially from his previous job at $2.50, he said, but a step up in terms of an exciting and educational career. He moved up from usher to cashier to science demonstrator.
By the time he retired from the top position, the planetarium itself had moved, becoming the Clark Planetarium, operated by Salt Lake County at 110 S. 400 West, Salt Lake City, in The Gateway mall. Under his leadership, the planetarium developed innovative displays and produced numerous star shows. According to the planetarium’s latest annual report, “We currently have star shows playing in over 52 countries across the globe. We are proud to engage audiences of all ages and nationalities in discovering and learning about space and science.”
Jarvis spoke about ways to understand the scale of the solar system and nearby stars, folklore and stories concerning star patterns, the value of parks, the importance of fighting air and light pollution, and the need to protect natural and historical treasures. He related the creation of elements beyond hydrogen and helium, for which stars are responsible. Illustrated with a table of elements, he showed which ones are formed by fusion in the interior of stars of different sizes, which are slammed into existence during supernovas and, surprisingly, which ones come about only through the collision of neutron stars.
Gold is among the neutron collision elements, Jarvis said. “That’s the only place you get energetic enough neutrons to create gold nuclei.”
Elements in existence soon after the Big Bang, just short of 14 billion years ago, were hydrogen, helium and small amounts of lithium and boron, he said. Hydrogen atoms drew together, compressed by gravity, and stars formed. They ignited when they began fusing hydrogen into helium.
NASA says: “As the main sequence star glows, hydrogen in its core is converted into helium by nuclear fusion. When the hydrogen supply in the core begins to run out, and the star is no longer generating heat by nuclear fusion, the core becomes unstable and contracts. The outer shell of the star, which is still mostly hydrogen, starts to expand. As it expands, it cools and glows red. The star has now reached the red giant phase. It is red because it is cooler than it was in the main sequence star stage and it is a giant because the outer shell has expanded outward. In the core of the red giant, helium fuses into carbon. All stars evolve the same way up to the red giant phase. The amount of mass a star has determines which of the following life cycle paths it will take from there.”
Actually, while that is a generally good explanation, NASA is wrong in saying “all stars evolve the same way up to the red giant phase.” Red dwarf stars, which are among the most common type, never reach the red giant stage. At the end of a red dwarf’s exceptionally long life, it collapses into a white dwarf star.
Red giants fuse helium into carbon. Eventually, low-mass stars like the sun will burn up their helium fuel and undergo a core collapse. ”As the core collapses, the outer layers of the star are expelled. A planetary nebula is formed by the outer layers. The core remains as a white dwarf and eventually cools to become (a cosmic cinder called) a black dwarf,” NASA adds.
Jarvis said a red supergiant star like Antares “is so big that if you put it in our solar system it would fill the solar system up out to about the orbit of Mars or beyond. It’s a really big star. It shines red because it’s in the process of dying.” Temperature determines star color. A red supergiant is larger and cooler than a normal star. Although in photos it may look orange, such a star is termed a red supergiant.
As a red supergiant, a star 10 or more times as massive as the sun will fuse carbon iron, which centers in the core. Iron, Jarvis said, is the end of the line for the process.
Prior to supernova, aging stars are layered like onions, he said, with each layer representing a different type of fusion. From the outer envelope of hydrogen going toward the core, each layer produces a heavier element. Gravity causes the denser, heavier elements to reside closer to the center. “By the time you get to the core of these giant dying stars, the innermost core is just iron, and iron is the death element for a star because it takes more energy to attempt to fuse iron than you release.”
Insufficient energy is released to maintain the star’s extent. It loses its lifelong battle with its own gravity and collapses in a supernova explosion. “The supernova can be as bright as the galaxy itself.” A bigger star in this scenario continues to collapse and forms a black hole, from which nothing escapes. A smaller one will halt its fall and continue to release energy as a neutron star. Elements created by fusion inside the star, and others that are made in the explosion, scatter through the vicinity.
Think of the debris from star deaths “as interstellar fertilizer drifting through space,” Jarvis said. Iron in our blood, calcium in our bones, oxygen in our lungs were created as stars died, he added.
”Want to hear a really cool story about the color red?” he asked.
Imagine yourself in southern Utah’s Capitol Reef National Park at sunset. Surrounding you is the beautiful red-rock country. Above, at the heart of the constellation Scorpius, Antares glows red, not because of elements but because of its temperature. Sometimes Mars is near Antares. The star’s name derives from its similar color to Mars the planet, which itself was named for the war god of ancient Rome (war = blood). Antares means Anti-Ares — Ares was the earlier Greek name for the war god. Antares’ color marks it as a challenger to the planet.
When you look at Mars, it is reddish, the color of the American southwest, he said. Both have that color because of the abundant element iron, minted in the centers of dying stars, which combines with another common product of star fusion, oxygen. Together they make rust, with its familiar ruddy hue.
Iron in human blood colors it red. “Your blood is red because stars have died.”
Jarvis asked why barns are usually red. The answer, he said, is that they are enormous buildings of rough-hewn wood. Paint soaks into the wood. The most frugal way to paint a barn is with red paint, because it’s “the least expensive paint.” Its red oxide is abundant, present even in dirt.
”You are more intimately connected to the universe than any story that any astrologer is ever going to tell you,” Jarvis said. “... You are quite literally a way for stars to be aware of themselves.”
Joe Bauman, a former Deseret News science reporter, writes an astronomy blog at the-nightly-news.com and is an avid amateur astronomer. His email is joe@the-nightly-news.com.