Most people probably know that the universe is full of elementary particles – but not everyone knows that it is also full of extremely fast stars that move freely through space like barracudas through the ocean. These stars are ejected by gravitational slingshots, which are at the focal point of galaxy mergers – where two supermassive black holes grow together as they throw stars out of the host galaxy like a batsman hitting a series of house runs out of the park. When the pair of black holes contracts, their orbital speed increases and creates an even stronger momentum. Ultimately, this process brings some stars up to the speed of light, according to Albert Einstein’s special theory of relativity, and turns them into what astrophysicists call “relativistic”.
In 2014, I and my former postdoc James Guillochon calculated the frequency of relativistic stars in free fall in the vast space between galaxies and the difficulties of discovering them at great distances. It must be exciting to live on a planet orbiting one of these ejected stars and witness its journey through space. The journey begins in the center of the mother galaxy, leads through many sights to the edge of the galaxy’s halo within a million years and culminates in intergalactic space that passes cosmological destinations for billions of years – all of which we can barely see through telescopes.
These relativistic stars make up the most attractive travel packages intergalactic travel agents can offer, and they also offer health benefits. Traveling close to the speed of light gives you the benefit of time dilation which slows the natural aging process of all travelers compared to those who left them behind.
Even without a galaxy merger, stars dragged by strong gravity near a black hole in the center of a galaxy could reach the speed of light. Half of the 2020 Nobel Prize in Physics was jointly awarded to Reinhard Genzel and Andrea Ghez for their program that monitors stars moving at a few percent the speed of light near the supermassive black hole of the Milky Way, Sagittarius A *. It is expected that relativistic stars in the centers of many other galaxies are gravitationally bound to black holes.
When relativistic stars collide in a galactic core, the resulting frontal collision can create an explosion that is much more energetic than a typical supernova – an explosion caused by the collapse of a massive star after its nuclear fuel runs out. For the two-star collision to occur almost at the speed of light, the central black hole must weigh more than 100 million suns. At lower masses, as is the case with black holes like Sagittarius A *, which weighs “only” four million suns, the strong tidal force of the black hole spaghetti stars as they approach it. The destroyed stars then spread out in a gas stream long before they can approach the horizon of the black hole to reach the speed of light, as shown in the doctoral thesis. Thesis of my former student Nick Stone.
At higher masses and at their event horizon, the gravitational tide – which scales inversely to the square of the black hole’s mass – is sufficiently weak not to disturb a passing star. Stars orbiting at great distances from either type of black hole move at slower speeds, and their collisions result in weak explosions, as shown in a preprint paper with my former PhD student Doug Rubin and in a reprint paper with Shmuel Balberg and Re’em Sari, both at the Hebrew University of Jerusalem.
What happens near the most massive black holes, where stars can orbit near the speed of light without disrupting the tides? In a new work, my current PhD student Betty Hu and I show that collisions of stars near these large black holes trigger the most energetic explosions in the universe and release up to a thousand times more energy than normal supernova explosions. These super-luminous explosions in galactic cores could be detected at the edge of the universe with the LSST (Legacy Survey of Space and Time) camera at the Vera C. Rubin Observatory, which is expected to start operating in a few years.
There is another way to launch stars from galactic centers at high speed. A pair of bound stars passing near a supermassive black hole can be separated by its gravitational tide. One of the stars is kicked closer to the black hole while the other is ejected at high speed, as theoretically predicted by Jack Hills in 1988. The kick a star receives towards the black hole could be responsible for the stars closest to Sagittarius A *, discovered by Genzel and Ghez. The ejection of their companions is the likely origin of the hypervelocity stars that Warren Brown and his coworkers discovered in the halo of the Milky Way in 2005. These hypervelocity stars move at up to 2 percent the speed of light and may carry planets with them. Planets liberated by the ejection process represent a population of hypervelocity planets as theorized in a 2012 article I wrote with my former student Idan Ginsburg.
Overall, galactic cores offer launch sites for the fastest habitable platforms that nature offers for free. It would not be surprising if advanced technological civilizations migrated toward galactic centers for the same reason astronauts and spectators flock out during the rocket launch to Cape Canaveral, Florida. With this perspective in mind, the search for extraterrestrial intelligence should look for radio signals originating from drivers of hypervelocity stars. We might also notice celebratory fireworks from their relatives in the galactic center when a high speed star is shot out from there.
This is an opinion and analysis article.