Stars often appear absent in images and videos taken from space. However, stars are indeed visible from space under the correct circumstances. The perceived absence of stars in many iconic space visuals is not due to their actual invisibility but rather a combination of factors related to how light behaves, how our eyes perceive it, and the technical limitations of cameras. This article will explore these specific reasons, clarifying why stars are not always apparent and when they can be observed.
Understanding Overwhelming Brightness
A primary reason stars are not always seen from space is the overwhelming brightness of nearby light sources. The Sun, for instance, is immensely powerful, appearing roughly 10 billion times brighter to us than Sirius, the brightest star in Earth’s night sky. When sunlight illuminates a spacecraft, astronauts, or a planetary body like Earth or the Moon, this direct and reflected light is so intense that it dominates the visual field. The Sun’s apparent magnitude, a measure of its brightness as seen from Earth, is approximately -26.74, while the faintest stars visible to the naked eye have an apparent magnitude of around +6.5, illustrating the vast difference in brightness. This immense disparity in luminosity means that the faint light from distant stars is simply overpowered by the brilliance of closer, illuminated objects.
How Our Eyes Perceive Light
When exposed to intense light, such as direct sunlight or the bright reflection from Earth’s surface, the eye’s pupil constricts, becoming smaller to limit the amount of light entering. This adjustment, known as the pupillary light reflex, helps prevent overstimulation and allows us to see details in bright environments. While light adaptation occurs rapidly, within seconds, the reverse process, dark adaptation, takes much longer. When transitioning from a brightly lit environment to darkness, it can take the human eye 20 to 30 minutes to fully adapt and become significantly more sensitive to faint light. Therefore, if an astronaut is looking at a sunlit Earth or spacecraft, their eyes are adapted to those bright conditions, making it difficult or impossible to discern the much dimmer stars in the background.
The Challenges of Space Photography
Cameras face similar challenges to the human eye when capturing scenes with extreme differences in brightness. To properly expose a photograph, a camera’s settings must be carefully adjusted. When capturing brightly lit subjects in space, such as Earth, the Moon, or a spacecraft bathed in sunlight, photographers use a fast shutter speed and a small aperture to prevent overexposure. These settings allow only a brief amount of light to hit the camera’s sensor.
However, distant stars are incredibly dim and require a much longer exposure time to gather enough photons to register on the sensor. Attempting to capture both bright foregrounds and dim stars in a single shot would either result in the foreground being severely overexposed and washed out, or the stars remaining underexposed and unseen. This technical compromise explains why many iconic space photographs lack visible stars.
Conditions for Seeing Stars in Space
Despite these challenges, stars are clearly visible from space under the right conditions. Astronauts aboard the International Space Station, for example, frequently report seeing stars, especially when the station is on Earth’s night side, away from the glare of the Sun and reflected light from the planet. In such dark environments, the human eye has sufficient time to adapt to the low light levels, allowing the faint starlight to become apparent.
Similarly, deep space, far from any celestial bodies that reflect sunlight, offers an unobstructed and clear view of the cosmos. For photography, capturing stars from space requires specific techniques, such as long-exposure photography, where the camera’s shutter remains open for several seconds or even minutes. This extended exposure allows the camera to accumulate enough light from distant stars to create stunning images of the starry background.