The cluster Terzan 5 has many older, lower-mass stars present within (faint, and in red), but also hotter, younger, higher-mass stars, some of which will generate iron and even heavier elements. It contains a mix of Population I and Population II stars, indicating that this cluster underwent multiple episodes of star formation. The different properties of different generations can lead us to draw conclusions about the initial abundances of the light elements, and holds clues as to the star-formation history of our cosmos. (Credit: NASA/ESA/Hubble/F. Ferraro)

How many stars are in the Universe?

There are ~400 billion stars in the Milky Way, and ~2 trillion galaxies in the visible Universe. But what if we aren’t typical?

Everywhere we look, in all directions in space, we see that the Universe is filled with stars and galaxies to the limits of our observations. On a clear, dark night, the naked human eye can see about 6000 of them, but that’s just a tiny fraction of all that’s out there. Our Milky Way galaxy — our cosmic home in the Universe — spans over 100,000 light-years in diameter and contains approximately 400 billion stars all on its own. There are about 60 galaxies, total, making up our Local Group, and one of them, Andromeda, contains even more stars than we do.

If we look out across cosmic time and extrapolate what must be out there, based on both what we can see and what we know about the Universe that lies beyond our current capabilities to uncover, we find that there are a total of about ~2 trillion galaxies in the Universe. Very simply, you might think to multiply the number of stars in our own galaxy by the number of galaxies in the Universe to estimate the total number of stars that we could potentially see.

Only, if you do this, you won’t just get the wrong answer, you’ll wind up overestimating the number of stars by a…