Why is astrophotography exposed for so long?

Because celestial photography usually needs to be done at night, and the brightness at night is very low. Brightness is the degree of brightness seen by human eyes in the direction of the luminous surface or reflective surface of the scene illuminated by the light source. The brightness at night is several hundred thousand times different from that during the day. Therefore, the night shooting speed of a scene that only needs to be exposed for1125 seconds during the day needs to be slowed down, usually tens of minutes. One advantage of night shooting is that you can adjust the aperture value at will, usually to the minimum aperture, because the shutter can be extended at will. For such a long time, some bright stars in the sky will produce beautiful trajectories. Some photographers even have 8 hours of exposure time in order to shoot the circular trajectory of the stars around Polaris, and it should be noted that this long exposure must be sufficient, otherwise it will give up halfway.