Black holes are difficult to detect since they are black in color. When they pull down gas from a neighboring star or merge together, we can only guess.
How many black holes are there? Astronomers have to use theoretical calculations to answer the question. There are potentially millions of small black holes in our universe, according to a recent study. About 1% of the universe is inside black holes.
There are black holes in the universe.
The ingredients of a black hole.
Black holes come from the deaths of stars. The researchers behind the study had to take a few steps back to figure out how many black holes there are in the universe.
The model of galaxy evolution is the first step. Galaxies are the homes of stars and their evolution affects how many of them there are. New stars can be formed in some galaxies after a long time. Others may suffer merger events that cause a round of incredibly high star formation, only for them to burn out and produce nothing of note ever again.
The astronomy team took known observations of the statistics of the galaxy across time. The so-called "metallicity" of a galaxy is a measure of the amount of elements other than hydrogen and helium inside. Bigger galaxies will have more gas, which will allow them to form more stars. The cooling of gas can be aided by more metals.
The black hole has recipes.
The astronomer had a model of the stellar population within the universe, telling them how many small, medium and big stars there are.
They needed to trace the evolution and deaths of those stars. Simulations connect the properties of a star to its lifetime and eventual demise. Calculating the percentage of stars that go lights-out every year is done by simulations, and only a fraction of the very largest stars produce black holes.
Black holes can feed off of sibling stars, so the astronomer had to track the evolution of the systems. A black hole formed in a system will be larger than a black hole born solo.
The astronomer estimated that the black holes feed on any surrounding gas as they age. Sometimes black holes find each other in the dark of space and merge together. The rate of black-hole mergers within each galaxy is what the astronomer had to estimate to produce an accurate survey.
Scientists spot merger of two black holes.
The black hole census.
The astronomer were able to track the population of black holes over the course of billions of years. They created a mass function, which is a sort of astronomy census, reporting how many black holes there are at any given time.
The largest black holes are more rare than their smaller cousins. The researchers found that our universe has 50 million solar masses of black holes. 10 million black holes in the same volume would be a few times the mass of the sun.
The total amount of mass contained by black holes is less than 10% of the mass in stars. There are a lot of black holes between the stars in the night sky.
Each galaxy usually hosts only one of those monsters, and supermassive black holes are extremely rare.
Black holes account for 1% of the baryonic matter in the universe today. Most of the baryonic matter is found in the open.
Black holes are frighteningly common, and that's nothing to sneeze at.
Paul M. Sutter is an astronomer and host of "Ask a Spaceman" and "Space Radio." He is also the author of "How to Die in Space." Space.com's expert voices: op-ed and insights was written by Sutter.
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