Question

Why are there no stars when the astronauts take pictures from space?

Asked by: Jay

Answer

When there is a full moon go outside and see how many stars you can find compared to a night when the moon is not out. You will see the difference. The stars are very faint and get washed out by the bright light of the moon.

The reason why no or very little stars can be seen is because of the Earth. The Earth, when lit by the Sun, is many thousands times brighter than the stars around it. As a result the Earth is so bright that it swamps out most if not all of the stars.

Answered by: David Latchman, B.Sc. Physics, University of the West Indies



The stars are there and the astronauts can see them if they look away from the sun. The reason that the stars do not show up on the film is that the stars are so dim that the camera cannot gather enough of their light in a short exposure. Our eyes are a lot more sensitive to light than photographic film. A good example of this is when we take a picture with a camera that is back lighted. The photographer can plainly see the features and colors of the object(usually a relative), but when the picture is developed, only the shadow outline can be seen of the person without any features.

Any picture that you may see of stars are from time-lapse photos. To take a time-lapse photo of the stars, the shutter must be left open on the camera in order for the lens to focus enough light on the film for the image to show up. Longer times allow more photons to enter the camera and record the image. The image is built over time from the total number of photons striking the film. The dimmer the object, the longer the film must be exposed because there are fewer photons per unit of time reaching the camera than for a brighter object. The brightness of an object is directly related to the number of photons that reach a recording device such as your eye or a camera. For example, to get a decent photo of the full moon, the shutter should be open for about a second or two. To record the image of a star, the shutter must be open from several minutes to several hours in order for enough photons to hit the film and make an image. Some of the spectacular photos that are made by the large telescopes, which col As for the pictures of the astronauts, the sunlight reflecting off of them is so bright that the shutter speed of the camera has to be a fraction of a second. If the exposure was longer, the film would absorb to many photons from the astronauts and they would become 'washed out' and appear as a featureless form of white, the opposite of the underexposed 'shadow'.

Answered by: Matthew Allen, B.S., Physics/Calculus Teacher Saint Scholastica Academy

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