When stars appear to randomly appear in the sky, it is usually due to a phenomenon known as "dark adaptation" or "dark-adaptive vision." Dark adaptation refers to the process by which the human eye adjusts to low light conditions.
In well-lit environments, our pupils constrict, allowing less light to enter the eye. This is to protect the retina from excessive brightness. However, when we move from a well-lit area to a darker one, such as going outside at night, it takes time for the eye to adapt to the lower light levels.
As you enter a dark environment, your pupils dilate, allowing more light to enter the eye. At the same time, your retina undergoes chemical changes that enhance its sensitivity to light. This process takes time, typically around 20 to 30 minutes, for your eyes to reach their maximum sensitivity to low light.
During this dark adaptation period, your eyes become more capable of detecting faint sources of light, including stars in the night sky. Initially, you may not notice many stars because your eyes are still adapting to the darkness. However, as your dark adaptation progresses, you become more sensitive to dimmer objects, and stars that were previously invisible start to become visible.
The perception of stars "randomly appearing" is an effect of this dark adaptation process. It can create the impression that stars are suddenly appearing in the sky, even though they were present all along. As your eyes continue to adapt to the darkness, more and more stars become visible, revealing the vastness and beauty of the night sky.