Starship will soon fly over towns and cities, but will dodge the biggest ones

It is likely that you saw other rockets (Falcon 9 being the most frequently launched). The "slow-forming starburst" is most likely from the exhaust plume in vacuum; once the rocket is high enough to no longer be in earth's shadow the plume catches sunlight and becomes very visible on the ground to an observer in darkness. See https://en.wikipedia.org/wiki/Space_jellyfish