Space
Galaxies vary in size and shape. Each is a gigantic collection of billions of stars, usually orbiting a central body such as a supermassive black hole. There are billions upon billions of galaxies in the universe. The Greeks called our galaxy “galaxias”, which means “milky one” because of the light band across the sky; from these terms we now call our “galaxy” the “Milky Way”. That galaxy is about 100,000 light years (30,000 parsecs) in diameter, with a supermasive black hole called Sagittarius A at its center.
Of course, your setting doesn’t have to be the Milky Way galaxy. It doesn’t even have to be our universe; however, our own neighborhood is used as an example throughout this section.
Galaxies are gravitationally bound into groups. The Milky Way is part of a 10 megalightyear diameter group of 50+ galaxies, known as the Local Group. The Milky Way and Andromeda are the two largest galaxies in the group and are also headed for collision several billion years in the future. The Triangulum Galaxy is the next largest. Andromeda is about 780,000 parsecs away, while the Large Magellanic Cloud – one of the Milky Way’s orbiting dwarf galaxies - is about 50,000 parsecs distant.
Travel between galaxies is a very different scale of achievement to that between stars. Even at speeds which make crossing a galaxy a feasible endeavour, reaching another galaxy requires speeds an order of magnitude higher, absent some kind of shortcut. As a point of reference, the distance to Andromeda is roughly 25 times the diameter of the Milky Way.
Galactic groups such as the Local Group of galaxies are found in superclusters; the Local Group is part of the Virgo Supercluster of at least 100 similar groups, which in turn is merely an appendage of the Laniakea Supercluster, a structure 500 million light years (153 million parsecs) across and containing some 100,000 galaxies.
Distance
Interstellar distances are typically measured in parsecs. One parsec is 3.26 light years.
Object Parsecs
Distance to Proxima Centauri 1
Large Magellanic Cloud diameter 4,300
Distance from Sol to galactic center 8,300
Milky Way diameter 30,000
Distance to Large Magellanic Cloud 50,000
M87 galaxy diameter 300,000
Hercules A galaxy diameter 460,000
Distance to Andromeda 780,000
1 day at FTL-10.5 1