The Milky Way is our cosmic home, and like any dwelling we inhabit, we naturally feel like we ought to know it inside out. Stargazers have studied it for thousands of years, but it wasn't until as recently as the last few centuries, after the invention of the telescope in 1608, that astronomers could confidently understand its true nature. A galaxy is an enormous ensemble of stars - most of which are star systems containing their own exoplanets - bound together by their mutual gravity, as well as gravitationally interactive substance: dark matter. The stars are formed from clouds of gas - nebulae - and in most galaxies, a significant quantity of the material remains present. The gas clouds in our galaxy glow with the energy of stellar radiation, or appear dark in silhouette against the myriad stars behind them.

From the ground, the immense cumulative fury of all those stars appears muted by distance to a faint river of light, that stretches from one horizon to the other, and only a few thousand of the brightest nearby stars are individually resolved to the unaided eye. We can sense that the diffuse Milky Way must be home to millions, perhaps billions of stars, but the true figure is still quite surprising. Counting the stars in our galaxy isn't as trivial as we would like it to be. The nebulae and other material in the interstellar medium make some regions of the Milky Way difficult, or perhaps impossible to observe. Moreover, at great distances, calculating the density of stars accurately is challenging. The precise size and mass of the Galaxy are also elusive to measure, and yet knowing these three quantities is the key to 'counting' the members of this great star city.

As it stands, we can only make broad estimates, and the value you choose to settle on will depend quite sensitively on your preferred model of the Milky Way. Measurements with very high confidence have yet to emerge, although several missions and programmes are underway to refine them. But for now, our estimate for the number of stars in the Milky Way is between 100-400 billion. Literally hundreds of billions of star systems drift through space with us, each as real as our own, and all practically on our doorstep, cosmically speaking. To put this into perspective, scientists estimate that roughly 110 billion humans have lived in all of history, meaning that everyone who has ever been born could likely have their own star system - possibly more than one. And that's just one galaxy. The Milky Way is an island universe among many, drifting in a cosmic expanse of unimaginable scale.

Pioneering astronomers like Galileo, who first proved that the Milky Way was made up of distant stars, would probably never have imagined that our galaxy was so gigantic, or that we would one day be closing in on an accurate measurement of its population. It's a testament to the power of astronomical science that we can describe something so far out of reach. In the years and decades ahead, astronomers will undoubtedly bring us much closer to the correct figure. For now, we can simply say that there are more than enough to go around!

Image credits: Tom Kerss