During the day, we cannot see stars because the brightness of sunlight overwhelms the relatively faint light emitted by stars. There are two main reasons for this:
Sunlight Intensity: Sunlight is incredibly bright compared to the light emitted by stars. The Sun is our closest star, and its proximity combined with its size and energy output makes it the dominant source of light in our daytime sky. The intensity of sunlight drowns out the relatively dim light of stars, making them virtually invisible to our eyes.
Scattering of Light: The Earth's atmosphere scatters sunlight, causing the sky to appear bright blue during the day. This scattering phenomenon, known as Rayleigh scattering, preferentially scatters shorter wavelengths of light (blue and violet) more than longer wavelengths (red and orange). As a result, the blue light from the Sun's rays fills the sky, making it even more challenging to spot the faint light from distant stars.
It is worth noting that it is theoretically possible to observe stars during the day under certain circumstances. During a total solar eclipse, when the Moon blocks the direct sunlight, the darkened sky allows stars to become visible. Additionally, in extremely clear and dark locations, such as remote areas with minimal light pollution, it may be possible to see some of the brightest stars during daylight hours, particularly when they are close to the horizon. However, under typical conditions, the combination of sunlight's intensity and atmospheric scattering makes it difficult to see stars during the day.