What is the stationary object in the window that the astronauts use to hold course during the burn in APOLLO 13?

In "Apollo 13," the astronauts use a fixed star, specifically the star Canopus, visible through a window as a stationary reference point to help maintain their course during the burn. By aligning their spacecraft with Canopus, they can accurately perform the necessary maneuvers for trajectory correction. This method was crucial for ensuring their safe return to Earth after the mission's critical failure.