The discovery of gravitational waves in 2015 sparked a new era of astronomy. Heavy stellar black holes have been seen interacting and merging, and some of them must have been created by stars much bigger than our Sun. This led researchers to wonder: how many of these stars are there in the universe?
As reported in The Astrophysical Journal Letters, the researchers estimate that less than 14 percent of all massive stars in the universe will become black holes around 30 times the mass of the Sun that will eventually merge. Meanwhile, black hole pairs between two and four times as heavy as the Sun are the result of less than 0.8 percent of all massive stars.
"Researchers up until now have theorized the formation and existence for pairs of black holes in the universe, but the origins of their predecessors, stars, still remains a mystery," Vanderbilt astrophysicist and lead author of the study Karan Jani said in a statement. "With this study, we did a forensic study of colliding black holes using the astrophysical observations that are currently available. In the process, we developed a fundamental constraint, or budget, which tells us about the fraction of stars since the beginning of the universe that are destined to collide as black holes."
The astronomers' starting point was the type and location of gravitational wave events detected by the LIGO and Virgo observatories. On top of that, they placed constraints on the number of available stars in the universe, the number of stars in the right mass range to form these large black holes, their end-of-life processes, the formation of close-by similarly sized black holes, and, eventually, their collision. Many of these collisions are now being detected by LIGO. These constraints produced a framework that allowed the team to estimate the budget.
"From the current observations, we find that 14 percent of all the massive stars in the universe are destined to collide as black holes. That's remarkable efficiency on nature's part," added Jani. "These added constraints in our framework should help researchers trace the histories of black holes, answering old questions and undoubtedly opening up more exotic scenarios."
As always with science, future observations and new theories might change this value, but it is interesting to have a starting point for the discussion.