Constellation of Cygnus (its Northern Cross asterism part; this star connection pattern) and Draco (its head part).

Now that I think about it, is there some deeper meaning to it? Draco the Dragon is 竜 (ryuu) in Japanese, which is part of Ryouma's name, definitely connected. And this depiction of Cygnus the Swan looks fairy similar to Grus the Crane. Was this intentional? Yet it's definitely Cygnus: its position relative to Draco corresponds to the star map; you can see a faint part of Cygnus's wing; Grus's horizontal lines have mirrored angles. But at the same time, cranes have deep connection to Japanese culture while swans don't, I'd say they're rather symbols of Western cultures.

Is this crane-like swan constellation an allusion to Saigo Takamori and his role in the Meiji Restoration? Oversimplified: Saigo Takamori played one of the key roles in overthrowing the Tokugawa shogunate and later in the Restoration and westernization (the swan symbolism). But after a reform that stripped the samurai of their privileges he rebelled against Meiji government and died. This last act of his is often (mis)interpreted as the act of a "traditionalist defending the old ways" (the crane symbolism).