Why do photos of space taken from space never show any stars? (Intermediate)

I have an above average knowledge of physics, astronomy, and science in general, having made these my hobbies in the past. However, when my colleague asked me this question it didn't make sense so I first told him he was mistaken.

However, when looking at any photo of any man-made object (such as satellites, stations, Shuttle etc) taken in space by astronauts, although the foreground object is in sharp focus, the background is devoid of any light (Including pin pricks) at all. I figured that even if the stars were out of focus, there should be some light registering from the black "space" areas in the photo.

Why is this? If I too am now mistaken, can you please show me a non-doctored picture taken in space without a telescopic lens that shows stars and the foreground object?

The pictures of human-made objects in space that you speak of all suffer from one fatal flaw: they lack what astronomers call "integration time". Even in space, stars are very faint. If you use a camera to take a picture of an object in space, then, you have to illuminate it using some kind of flash (just like on Earth). The flash is bright enough that the time over which the camera film is being exposed is, like on Earth, only a fraction of a second. This short time is more than sufficient to get a picture of the man-made object that your flash illuminates, but way too short to capture the stars. The fundamental difference between pictures of the stars themselves taken by telescopes and the pictures of things in space with stars in the background is the exposure time, or integration time: in fact, astronomers do everything they can to avoid "doctoring" images they obtain, since this might hide the very science that they are trying to get at.

I bet you can see how this works for yourself. The next time you are out on a clear night with some friends, take a picture of them with a starry sky in the background. When you develop the pictures, have a hard look for the stars that you know were there when you took the picture. Just like in space, a flash on Earth that allows you to photograph your friends obscures the stars (the effect should be more pronounced on Earth than it is in space because of our light-scattering atmosphere). To photograph the sky from Earth, you need a long-exposure camera, just like in space.

Page last updated on June 22, 2015.

About the Author

Kristine Spekkens

Kristine Spekkens

Kristine studies the dynamics of galaxies and what they can teach us about dark matter in the universe. She got her Ph.D from Cornell in August 2005, was a Jansky post-doctoral fellow at Rutgers University from 2005-2008, and is now a faculty member at the Royal Military College of Canada and at Queen's University.

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http://www.astro.queensu.ca/people/Kristine_Spekkens/main.php
http://www.rmc.ca/aca/phy/per/spekkens-k-eng.php

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