Galaxian is the game that took Space Invaders and made the aliens fly. Namco released it in 1979, and instead of a formation that only shuffled sideways, its aliens peel out of the grid individually and dive at your ship in swooping arcs, firing as they come. It was also one of the first arcade games with a full colour RGB display and an animated starfield.
Everything that made Galaga great two years later starts here: the diving attacks, the escort formations, and the risk-reward of shooting a diver mid-flight for extra points. It's harder than it looks because the divers don't follow straight lines.
Galaxian's aliens break formation and dive at you individually in curving flight paths, and it has a colour display and a scrolling starfield. Space Invaders' aliens only move as a block. Galaxian is the game that introduced diving attacks to the genre.
Galaga is the 1981 sequel. It keeps the diving formations and adds the tractor beam and dual-fighter mechanic, the Challenging Stages and dual shots.
Yes — a tiny arcade remake that runs in the browser with nothing to install.