Some 40 years ago, Carl Sagan taught the world that there were hundreds of billions of stars in the Milky Way alone, and perhaps as many as 100 billion galaxies within the observable Universe.

Although he never said it in his famous television series, Cosmos, the phrase “billions and billions” has become synonymous with his name, and also with the number of stars we think of as being inherent to each galaxy, as well as the number of galaxies contained within the visible Universe.

A theoretical calculation from a few years ago — the first to account for galaxies too small, faint, and distant to be seen — put the estimate far higher: at 2 trillion.

If we extrapolate what we’ve seen in this tiny region as though it were “typical,” we’d find that over the entire sky, we expect there to be 170 billion galaxies contained in the observable Universe.

Although the very faint end of the galactic spectrum is the most uncertain (i.e., where the smallest, lowest mass galaxies are), this technique has been leveraged over the past few years to produce a superior estimate: that there are 2 trillion galaxies out there in the observable Universe alone.

This tells us that, in the very early stages of the Universe, these initially small, low-mass galaxies merged together very frequently, but that became both less common and less important for star-formation in the Universe as time went on.

We’re still learning which collections of stars are part of our own Milky Way and which ones are their own independent galaxies, but there may be up to 100 small, low-mass galaxies for every Milky Way-like galaxy in the Universe.

And the third thing we can do is look — both nearby and a bit farther away — at the Milky Way analogues that we can see, and attempt to measure the number of nearby small, faint galaxies found in their vicinities.

It tells us that the farther away we look, the greater the number of the smallest, lowest-mass, faintest galaxies we should expect to be out there, but we’re actually seeing even fewer of the small, low-mass, faint galaxies that ought to be present?

And that if we don’t simply rely on what either our direct observations (from the Hubble eXtreme Deep Field, for instance) or a simulation tells us, but rather use what we observe about nearby galaxies and their small, faint, low-mass satellites to inform our conclusions, we find that “billions and billions” or even two trillion galaxies is simply too low of a number.

Instead, based on what we see around nearby Milky Way analogues, there ought to be at least 6 trillion galaxies contained within the observable Universe, and it’s plausible that a number that’s more like ~20 trillion — with approximately 100 small, satellite galaxies for every Milky Way-like galaxy out there, throughout cosmic time — might be an even better estimate.

If there are somewhere between 6 and 20 trillion galaxies in the Universe, you might wonder what that means for the total number of stars in the Universe.

All told, there are still about 2 sextillion (2 × 1021) stars in the Universe; the additional galaxies only add about 0.01% to the total number of stars present.

It’s true that there are hundreds of billions of stars within the Milky Way, which is just one galaxy among trillions — likely between 6 and 20 trillion — in this enormous, expanding Universe.