system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] How long was the Death Star over Alderaan? Didnt the population panic when it showed up?
gpt
You're presuming they would comprehend what it was, and what it was going to do. The second it showed up, it was almost certainly recognized as an impossibly large Imperial battle-station - a good reason for rebel-sympathetic elements on the planet to flee and everyone else to hunker down for an occupation, but no reason to suspect that the planet was about to be destroyed! Most citizens of Alderaan - those who might have tacitly supported the Rebellion, but not actively taken part - might have been loathe to flee their old homes and lives without an explicit reason to do so.

Even if they had known - what could have been done? It seems unlikely the Death Star arrived more than few hours beforehand at absolute maximum; that's not enough time to get everyone off a well-populated island, let alone off a planet.

Fun side note: Alderaan's planetary shield was up when the Death Star fired, a presumable recognition of implied hostile intent. Given the trouble planetary shields can give even moderately sized fleets, Alderaan may have assumed it would protect them long enough for an evacuation force to be assembled or other action taken to protect the planet. Certainly they didn't expect to have the entire planet shot out from under them.