On your player turn, you may choose to either Observe or Rest.
The Observe action allows you to spend Stardust to discover stars on the Constellation cards using your dry erase marker. You may only mark stars in a straight continuous line, but you may spend Telescope tokens to start new observations. As soon as you mark the last star in a Constellation, you take the card from the Night Sky, granting you a powerful reusable ability for the rest of the game. But other players also benefit from the discovery: based on how many stars they marked, they may choose one of the card’s instant benefits (Boons) at the time it is discovered.
The Rest action allows you to not only refill your Stardust up to your capacity, but also to reactivate all of your card abilities that match the currently active Sphere. Your card abilities are very powerful and strategy-enabling, so a well-timed Rest action can give you a huge advantage.