memory card - Why does storing a long exposure photo take almost as long as the exposure time itself?

Last night I took some night sky shots, and I've noticed that with long exposure times (15s, 30s) the time it took to store the images in the camera memory card (jpeg) took roughly as long?

The actual file size is fairly small at 1 to 3MB per file.

Any idea why this is?

Answer

This is a featured called Long Exposure Noise Reduction. To cancel out noise the camera will close the shutter and take an equally long exposure again, this time capturing a black image with only the electrical noise on it. This information is then used to reduce the noise on the original exposure.

In the camera settings you can disable the Noise Reduction for long exposures. Check the manual or google to find out how to enable and disable it.