Most space photos taken by astronauts have no stars in the background. Why?

By comparing how many stars can be seen with and without a full moon, we can understand the difference. At the full moon, the starlight becomes very weak in the moonlight.

The reason why we can't see the stars or can only see a few stars is because we are above the earth. When the earth is illuminated by sunlight, its contribution is ten million times that of the surrounding stars. Because the earth is so bright, most of the starlight is covered.

All photos that can see the stars are time-lapse photography. When shooting stars with time-lapse photography, the shutter of the camera should always be open, so that the lens can collect enough light and display the image on the negative. Prolonging the exposure time allows more photons to enter the camera to display images. As time goes on, all the photons that hit the negative form the final image.

The darker the object, the longer the exposure time, because the number of photons collected per unit time is much less than that of bright objects. The brightness of an object depends on the number of photons received by a camera or human eyes. For example, to take a good picture of the full moon, the shutter only needs to be opened for a second or two. But if you shoot a star, the shutter will open for a few minutes or even hours, so that enough photons can hit the negative to get an image of the star. Some very spectacular photos were taken with a telescope.

When taking pictures of astronauts, sunlight reflects from them and enters the camera. The light is so strong that the exposure time of the camera must be as short as a fraction of a second. If the exposure time is too long and the negative film receives too many photons, the astronaut will become an uncharacteristic white object, contrary to the dark shadow shown by low exposure.

By 2002, astronauts were not only trained and funded by the government, but also included military and civil aviation institutions. With the successful flight of Spaceship 1 in suborbital space in 2004, a new type of astronaut-commercial astronaut was born.

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