Enlarge image
Spiral galaxy NGC 1385 as seen by the Hubble Space Telescope
Photo: ESA / Hubble / NASA / J. Lee / PHANGS-HST
The universe, endless expanses.
And infinite amounts of stars, planets and moons.
But the lifetime of stars is limited.
Eventually, the fuel supply that powers these stellar power plants will run out.
Then they become, for example, red giants, white dwarfs, neutron stars or black holes - depending on how big they were.
In a few billion years, for example, a so-called red giant first develops from our sun.
Our star will expand and incorporate its neighbors, the planets Mercury and Venus.
In the end, a cooling white dwarf is left behind in a pretty planetary nebula.
Most stars end up as such, roughly earth-sized crystal balls.
But some also become so-called stellar black holes.
Astronomers are fascinated by these strange objects with their great gravitational pull, their mass concentrated in a tiny point.
It was known that there are many such gravitational miracles that attract light and make it disappear irretrievably.
But how many exactly?
Researchers led by Alex Sicilia from the Scuola Internazionale Superiore di Studi Avanzati in Trieste have now come up with an answer using computer simulations.
Their answer, which they published in The Astrophysical Journal: 40 x 1018
Expressed as a numerical word, it is 40 trillion or 40 billion billion - this corresponds to a 4 followed by 19 zeros.
"This is one of the first and most robust calculations of the masses of stellar black holes in cosmic history," Sicilia said in a statement.
The researchers included the entire duration of the observable universe in the simulation, which corresponds to a sphere with a diameter of around 90 billion light years. The computer then simulated the formation of different stars: isolated specimens, binary star systems and star clusters in the galaxies.
But the researchers found something else: About 1 percent of all ordinary matter in the universe is hidden in black holes. Larger stellar black holes, ranging in mass from a few to tens of times the mass of the Sun, are mainly formed by dynamic events in star clusters. Here such objects can also merge and become an even larger black hole where the masses add up. Astronomers have often been able to follow such events with gravitational wave observations.
When the black holes that Albert Einstein predicted in his General Theory of Relativity form, the stellar fuel is gone. Nuclear fusion inside dies out, and the star's complex balance is shaken. After huge cosmic explosions, so-called supernovae, stellar black holes can form. If the mass gets too close from other cosmic objects or even light, it will disappear like a waterfall inside the holes and never come back.
In a further work, the scientists also want to deal with supermassive black holes.
Little is known about their origin.
Researchers suspect that such gravitational giants, some with billions of solar masses, are often found in the centers of galaxies.
The core of the Messier 87 galaxy, for example, is known, a giant elliptical galaxy 55 million light-years from the Milky Way, which astronomers like to focus on because it shines brightly and has a center that is easy to observe.
Medium-sized black holes, which are said to have a maximum of several thousand solar masses, also pose a mystery.
So far, however, there are no indications of these links between small and very large black holes - only a few candidates for such objects are known.
In a study last year, NASA researchers examined the globular star cluster NGC 6397, which is around 7,800 light-years away from our galaxy, the Milky Way.
But instead of such a suspected medium-mass black hole, the scientists discovered a cluster of smaller black holes in the center.
Instead of a dot that would have indicated a black hole, the mass seemed to spread out over a larger area.
