A.N. — The wiki has only, "The Gryffindor first years have an Astronomy Class at midnight". I went with a well-known space theme to tie into that, though I seemingly cannot escape this family thanks to one damnable tapestry in Order of the Phoenix...
324. Pollux Black
Pollux used to hold his family's stellar naming tradition in high regard. But they'd really missed an opportunity with his name. He wasn't a twin, and had no Castor to partner with. He adored his younger siblings: Cassiopeia, Dorea, even squib Marius. But he still felt alone, forever denied a Gemini connection.
So when it came time for Pollux to start having offspring of his own, his first-born's name eschewed the stars. Walburga inherited all of Pollux's bitterness, and then some. Second-born Alphard got Pollux's sense of caring. Cygnus, the youngest, embraced the Black legacies Pollux had grown to loathe.