Back to: Astronomy & the Cosmos
Every star you can see with your naked eye belongs to the Milky Way, our home galaxy: a spiral disk of a few hundred billion stars about 100,000 light-years across, with our solar system parked in a quiet suburb roughly 26,000 light-years from the center. That milky band across a dark summer sky, the feature that named the galaxy, is the disk seen edge-on from inside: the combined glow of billions of stars too distant to separate by eye.
For most of history we assumed our galaxy was the whole universe. Then in the 1920s Edwin Hubble proved that certain ‘spiral nebulae’ were in fact separate galaxies at staggering distances, and the universe grew a million-fold overnight. Today’s estimates suggest the observable universe contains on the order of two trillion galaxies, in spiral, elliptical, and irregular varieties, strung along vast filaments of a cosmic web with enormous voids between.
Galaxies also hide a mystery. Stars orbit galactic centers too fast for the visible matter’s gravity to hold them; something unseen adds the missing pull. This dark matter outweighs normal matter about five to one, and we still don’t know what it is, one of the biggest open questions in physics. A practical postscript: from a dark site, the Andromeda galaxy is visible to your naked eye, 2.5 million light-years away, the most distant thing most humans will ever see unaided. Go find it.