Like stars in Hollywood, the rarest stars in the Universe have a short lifespan. The most massive stars consume their nuclear fuel within a period of a few million years, as short as the history of the human species. What happens in the last year of their life?
The death of massive stars is expected to be rather tumultuous, with substantial mass loss to their immediate environment in the last few years. Once their core collapses, the star’s fate is dictated by whether the collapse triggers an outward moving pressure wave within ten seconds from the formation of a central remnant. A strong shock wave could eject the envelope of the star and result in a bright supernova explosion, leaving behind a remnant neutron star with roughly the mass of the Sun and the size of a city like Boston. If no shock emerges, the stellar core implodes to a black hole. Such an implosion could be dark without any detectable fireworks. However, if the black hole gains enough spin from the rotation of the core, it could in turn launch a jet along its rotation axis that penetrates the stellar envelope and appears as a brilliant flash of gamma-rays to an aligned observer. The resulting “long-duration gamma-ray burst” is often followed by an afterglow and a hypernova as the jet dissipates its energy in the stellar envelope and the surrounding interstellar gas.
A gamma-ray burst within our own Milky-Way galaxy would pose an existential threat to life on Earth if its “gun barrel” is pointed at Earth from a short distance. The star Eta Carina has roughly a hundred solar masses and is located 7,500 light-years away. It’s “Great Eruption” made it the second-brightest star in the night sky between 11–14 of March in 1843. Given its advanced evolutionary phase and its distance, it is possible that the star had already collapsed and its explosion energy is making its way towards us at the speed of light. However, it is unlikely that this star is capable of producing a gamma-ray burst in our direction. More generally, in a paper with David Sloan and Rafael Batista at Oxford University, we showed that the event rate of gamma-ray bursts and supernovae in our galaxy is small enough for policy makers not to worry about this threat.
Another massive star, Betelgeuse, is merely 550 light-years away but just about 18 times the mass of the Sun. As a red supergiant, Betelgeuse is one of the largest stars visible to the naked eye. If it were at the center of our Solar System, its envelope would have engulfed the asteroid belt and the orbits of Mercury, Venus, Earth, and Mars. The outer layer of Betelgeuse is diffuse with a diameter wider than the orbit of Mars. Convection brings bubbles of hot rarefied matter to its surface, from where it cools and sinks again. Betelgeuse was observed to dim starting in October 2019, and by mid-February 2020 its brightness had dropped by a factor of a few. By 22 February 2020, Betelgeuse started to brighten again. Infrared observations found no significant change in brightness over the last 50 years, suggesting that the dimming was due to a change in extinction by large dust grains. Data from the Hubble Space Telescope in 2022 suggested that occulting dust was created by a surface mass ejection and caused the dimming. Its recent brightening represents a gradual return to its normal brightness, most likely because the cloud went away from our sightline.
But wherever there is risk, there is also opportunity. A common sight on the beaches of Hawaii is a crowd of surfers taking advantage of a powerful ocean wave to reach a high speed. Could extraterrestrial civilizations have similar aspirations for sailing on a powerful flash of light from an exploding star like the examples mentioned above?
A light sail weighing less than half a gram per square meter can reach the speed of light even if it is separated from the exploding star by a hundred times the distance of the Earth from the Sun. This results from the typical luminosity of a supernova, which is equivalent to a billion suns shining for a month. The Sun itself is barely capable of accelerating an optimally designed sail to just a thousandth of the speed of light, even if the sail starts its journey as close as ten times the Solar radius — the closest approach of the Parker Solar Probe. The terminal speed scales as the square root of the ratio between the star’s luminosity over the initial distance, and can reach a tenth of the speed of light for the most luminous stars.
Powerful lasers can also push light sails much better than the Sun. The Breakthrough Starshot project aims to reach several tenths of the speed of light by pushing a lightweight sail for a few minutes with a laser beam that is ten million times brighter than sunlight on Earth (with ten gigawatt per square meter). Achieving this goal requires a major investment in building the infrastructure needed to produce and collimate the light beam.
Alternatively, a civilization that happens to reside near a massive star, like Betelgeuse or Eta Carinae, could park numerous light sails around it, cleverly awaiting the powerful explosion that would launch these sails to the speed of light at a minimal cost.
Of course, there are challenges. First among them requires patience. Massive stars live for millions of years and it is difficult to forecast the exact timing of their explosion, just as it is challenging to predict in which year an old person might die after approaching the average life expectancy.
The sails can be transported to their destination well in advance of the explosion using cheap chemical rockets. The journey would take millions of years across the molecular cloud that gave birth to the massive star. Only civilizations in the vicinity of that cloud could use chemical propulsion to reach the star before it explodes. The same rocket engines would enable the sails to hover in the appropriate orientation relative to the star, given the desired direction for their journey after the explosion.
As in Starshot, the sails must be highly reflective so as not to absorb too much heat and burn up. Once the sails are placed in orbit around the massive star, they will be pushed away by the bright starlight or mass loss prior to the explosion. To avoid this danger, one could deploy the sails in a folded configuration and equip them with a switch that would open them up like umbrellas as soon as the explosion flash begins to rise. Even though the launch can start from a distance that is a hundred times larger than the size of the exploding star, care must be taken in selecting particularly empty acceleration paths — clear of any stellar debris. With a relative speed approaching the speed of light, dust particles would puncture the sail like miniature atomic explosions and gas particles would slow down the sail as soon as it sweeps ambient matter with a weight comparable to its own. Once the sail reaches its terminal speed, it could fold into a needle-like configuration with a small cross-sectional area along its direction of motion to minimize damage and friction.
Electric sails could also approach similar speeds while surfing on the relativistic winds produced by pulsars or black hole jets, as discussed in a paper I wrote with my former postdoc, Manasvi Lingam.
Is there any evidence for fast moving material in supernova remnants? Yes there is, but it likely originates from natural causes. Supernova ejecta typically move out at a tenth of the speed of light, and faster moving material was detected in remnants such as Vela and W44. In addition, the most powerful explosions, such as hypernova or gamma-ray bursts, are known to produce natural outflows approaching the speed of light, and isolating artificial components within them would be challenging.
Sailing on natural flashes to the speed of light saves on the expensive construction costs of artificial launch systems. If we are lucky to have many technological civilizations in our Galaxy, there might be swarms of light sails around massive stars, patiently awaiting their explosions. But before we let travel agencies market these fireworks as attractive tourist destinations, it would be good to know the answer to one question: are the environments of massive stars already too crowded with surfers as some of the beaches of Hawaii?
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 chairs the advisory board for the Breakthrough Starshot project, and 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.