How this photo was made
Why take hundreds of photos?
Faint things in space are buried in noise. Taking lots of short photos and averaging them makes the noise fade away while the nebula stays.
Hover or drag on the photo to magnify
1 photo
20 seconds of light
One 20-second photo. The speckles are noise from the camera sensor, and the nebula is almost hidden.
Why does averaging work?
Noise is random, so it's different in every photo. The nebula is the same in every photo. When you average them, the random parts cancel out and the real signal stays.
The noise shrinks with the square root of the number of photos: 4 photos halves it, 16 photos quarters it.
Each version was lined up star by star and brightened with exactly the same settings, so the comparison is fair.