Inspiration from the Stars
Some Hollywood stars, like James Dean, were brilliant but had a short life. The same holds true for the most luminous stars in the Universe, which shine for a short while in cosmological terms — merely a few million years.
Stars are nuclear fusion furnaces, held together by gravity. They burn the abundant fuel of hydrogen and helium, left over from the Big Bang. Once their fuel is consumed, they die. When I once mentioned to a receptionist at a front desk of an office that everything is transient including the Sun, she responded: “Wow, the idea that the Sun will die is in conflict with my religion.” I apologized for being the bearer of bad news but explained that this is not an idea but rather a sober fact based on the ten billion corpses of dead suns that are observed in the graveyard of the Milky-Way galaxy. These corpses are called “white dwarfs”. They kept about 60% of the mass of the Sun but cooled and contracted to the size of Earth. Their surface gravity is of order 100,000 times larger than that of Earth. A person who weighs 150-pound on Earth would weigh about 15 million pounds on the surface of a typical white dwarf.
Stars that are more massive than 8 solar masses often end their life in a supernova explosion that gives birth to a neutron star. The typical mass of the remnant, 1.4 solar masses, fills a sphere of radius 12 kilometers — comparable to the size of a city. A neutron star is made mostly of neutrons, packed at the typical mass density of an atomic nucleus.
The cores of more massive progenitor stars end their life as black-holes. I recently co-authored a paper led by Kishalay De, which reported the disappearance of a massive star in the Andromeda galaxy to a black hole (as described here). The phenomenon resembles the death of a rock star who suddenly has a heart attack and collapses on stage.
Nearly all of the very massive stars that formed during the 10-billion-year history of the Milky-Way collapsed by now to a population of about 100 million black holes. Some of these black holes were discovered recently as dark partners in orbit with a normal luminous star in the new Gaia catalog of stars (as reviewed here). If the progenitor star is sufficiently compact and rapidly spinning, the collapse of its core to a black hole can lead to the production of jets that penetrate through the progenitor’s envelope and appear as a short gamma-ray burst along the jets axis (as reviewed here).
Once the Sun will die, its envelope may engulf the Earth (as discussed here) and trigger the crash of the Moon on the Earth’s surface as a result of orbital drag on the solar envelope.
The expansion is more extreme for massive stars. At the end of their life, they expand to a scale comparable to the Solar system of planets. Last week, a new paper reported here about a real time observation of the expansion of a red supergiant star, named WHO G64, to a yellow hypergiant. A red supergiant is a cool, evolved massive star in its final evolutionary stage before exploding as a supernova. Since the 1980s, WOH G64 has been considered the most extreme red supergiant in the Large Magellanic Cloud with a size of order 7 times the Earth-Sun separation, a temperature of about 3,300 degrees Kelvin and a luminosity approaching a million solar luminosities. Its evolution over the past four decades reveals an extreme transition to a yellow hypergiant star with about half that radius and a hotter surface temperature of 4,800 (+/-300) degrees Kelvin, similar to the surface temperature of the Sun.
The most abundant and longest-lived stars are dwarf stars with a mass that can be as low as 7% of the mass of the Sun and a lifespan of up to 10 trillion years (as calculated here). This raises the fundamental question of why do we find ourselves living near a star like the Sun today rather than near a more common dwarf star in the future, as I discussed in a related paper here. The answer might be that dwarf stars are fainter and so their habitable zone is closer in, placing rocky planets at a higher risk of losing their atmospheres and liquid water reservoirs from flares and winds which are commonplace near dwarf stars.
Rather than imagining the neighbor’s grass to be greener, we should be grateful to our life on blissful Earth while the Sun still shines in our sky. But as in any long-term relationship, one must have contingency plans in case the partner dies. In anticipation of the death of the Sun within 7.6 billion years, we must develop a plan to embark on a journey out of the solar system towards interstellar space. The space platform that will carry humanity was labeled “Noah’s Spaceship” in my recent book “Interstellar” (available here), in analogy to Noah’s Ark — the vessel built to save life in the biblical story. Coincidentally, the dimensions of Noah’s Ark as specified in the bible: 135 meters long, 22 meters wide and 13 meters wide, are similar to those inferred for the interstellar object 1I/`Oumuamua.
Other civilizations may have built their interstellar Ark by now, because most stars formed billions of years before the Sun (as reported here). If we ever find their artifacts among the population of interstellar comets, we can learn useful technological tips on how to build our own vessel.
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In this essay, I featured four amazing watercolors from a series created by the celebrated artist, Greg Wyatt. These watercolors incorporate inspiring statements by Aristotle, Boethius and Cicero. This is the fifth in a sequence of essays, where Greg and I collaborate on the interface between art and science. The first essay in this series, titled “Music of the Cosmic Spheres,” appeared here; the second essay, titled: “Cosmic Waterfalls in Spacetime Cliffs,” appeared here; the third titled “Missing Elements in the Cosmic Jigsaw Puzzle,” appeared here; and the fourth essay, titled: “Why Do We Exist?”, appeared here.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
Professional website:
https://lweb.cfa.harvard.edu/~loeb/
Social media:
https://avi-loeb.medium.com/
https://www.youtube.com/@ProfessorAviLoeb
https://open.spotify.com/show/1zhndXkvSY2b8FdjspFpCd
https://x.com/ProfAviLoeb
