The density of black holes

Black holes are the most extremely compact objects of the Universe. In fact, at their center lies a singularity: a point with zero volume, where all the mass of the black hole is compressed. The singularity is a region of infinite densitysomething that is not really conceivable by the human mind.

Something we can instead ask ourselves is: “What is the average density inside the Event Horizon of a black hole?” And we might enjoy the answers that Nature gives us quite a lot.

Density is a measure of how much mass is enclosed in a volume. The larger the mass in a given volume, the denser the object is. The smaller the volume of a given mass, the denser the object is.

For example, we can think to the density of citizens as a measure of the number of people in a city. Here the number of people represents the mass, while the size of the city represents the volume. If we add more and more people in a given city, the density of citizens will increase. Also, if we fix the number of people and we make the city smaller and smaller, the density of citizens will increase. Generally speaking then, a larger number of people and/or a smaller city gives a higher density of citizens (it would then be very easy for citizens to stumble upon each other!); while a smaller number of people and/or a larger city, gives a smaller density of citizens (you could easily walk around without seeing any other citizen for a while).

Now, (one of)the funny thing about black holes is: the size of their Event Horizon is proportional to their mass. In other words: the more massive is a black hole, the larger it is. In particular, if we double the mass of a black hole, we get a black hole volume eight times larger; if we increase the mass by a factor of three, the volume will get 27 times larger; and if we increase the mass of the black hole by a factor of ten, its volume will increase by a factor of 1000.

Think about it. If you double the number of people in your black hole city, the black hole city automatically becomes eight times larger. If you triple the number of people in your black hole city, the black hole city becomes automatically almost thirty times larger. If for any person of your black hole city you add 9 more people, your black hole city will become 1000 times bigger. What does this mean? The more people you add to the black hole city, the smaller density of citizens you get! For a black hole in the Universe: the larger its mass, the lower its average density will be.

Massive black holes are less dense than small mass black holes.

A black hole with the mass of a star like our Sun is extremely dense: about 1,000,000,000,000,000 times denser than the densest material on Earth.

A black hole of a thousand times the mass of our Sun is “only” 1,000,000,000 times denser than the densest material on Earth, while a supermassive black hole with a million times the mass of our Sun is just 1,000 times denser than the densest material on Earth.

But a black hole of a hundred millions times the mass of our Sun, is as dense as… water.

The largest black holes that we know have masses of about ten thousand millions times the mass of our Sun: they are in fact, on average, less dense than the Earth’s atmosphere.
The largest black holes we know would not only float in water, but they would also float in the air that humans breathe.

Our Universe is amazing, isn’t it?

What is a Black Hole?

Before telling you the fascinating story of what humans understand about black hole winds and galaxies and their cosmic energy exchange, let me try to explain each part of the story; and let me start with the main characters:

Black holes.

Black holes went from being a purely mathematical abstract in a few human minds, to something routinely observed and monitored by humans, day by day, in the local and distant Universe — in just a couple of centuries! And it’s been only a couple of decades since humans realized that a supermassive black hole, with a mass of about three or four millions the mass of our star the Sun, resides at the center of the galaxy where they all live in: the Milky Way.

Actually, nowadays humans suspect that a supermassive black hole might reside at the center of every galaxy, and might be intimately related to her formation and evolution. This would make black holes fundamental cosmic players. But first of all: what is a black hole?

So, what is a Black Hole?

Discover it in the first entry of “Our Universe“: Black Holes.

Welcome! Bienvenid@s! Bem-vind@s! Benvenuti!

[Aquí en español Qui in italianoAqui en português]

Welcome to!

This page was born from the need to share the riches that my occupation gives me (I will tell about the torments only if they contain a remarkable and immediate didactic side). I am a lucky human, spending my life in one of the most beautiful and oldest occupations on this planet: I am an astronomer.

Good power, when shared, multiplies. Like knowledge.

So: what is our “cosmic address”? I would like to tell you about it in “our Universe“.

And also: how beautiful is our planet, with her unique geometries and diffraction and refraction patterns? I would like to show you something about her in “our planet Earth“.

Curious about my personal history as human as-all-peculiar? You can have a look at “about me“.

I study the winds originating close to supermassive black holes in the center of distant galaxies: but what does all this mean? What are black hole winds, and why are they important? Indeed, first of all: what are black holes? And how do we observe them, if they are black holes? And what do they teach us? I would like to explain it in the section “Research“. The idea of ​​this section is to build a triple exposure level: for everyone (no jargon at all), for Ph.D. students (jargon at minimum), and for astronomers (jargon like hot cakes).

While completing the main page, I will post updates here on the blog. With the existence of “” I will not disappear from Facebook or Twitter or Instagram or LinkedIn; nor from the real world! — but this page might serve as a repository for those who might want to read without having to go through the aforementioned social media — and with an extra touch of solid science.
I hope you enjoy it as I do 🙂!