[Artwork depicting the Milky Way and Virgo I, more or less to scale. Credit: NASA/JPL-Caltech/Robert Hurt]
Galaxies are immense structures. Composed of gas, dust, dark matter, and billions of stars, big ones can be a hundred thousand light-years across — a million trillion kilometers — and have masses equaling trillions of Suns.
Our own Milky Way fits those numbers pretty well. It formed not long after the Universe, itself, did, probably a billion or so years after the Big Bang, collapsing from a vast cloud of hydrogen and helium gas. It wasn’t alone, though: Two other big galaxies were born along with it (the Andromeda Galaxy and Triangulum), and a handful of smaller ones that are all bound by their mutual gravity, forming what we call the Local Group.
Some of these galaxies are actually satellites of the Milky Way, in a similar way that the Moon is a…