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Look into the night sky on a clear night, and you’ll see thousands of stars spanning
across your view.
These stars are a beautiful reminder of the cosmos we live in.
But curiously, when you look at the many different images released by space agencies of objects
in space, the background looks mysteriously void of anything.
Why is that?
Surely, with nothing in the way like an atmosphere, we should be able to see all the stars that
would be in that view.
In fact, even Apollo astronauts reported not being able to see any stars on the moon, where
no atmosphere was in the way.
So, what’s up with that?
Well this is all to do with how eyes and cameras work.
When we look at a bright object, our iris shrinks in order to block out some of the
light.
This is what happened with the Apollo astronauts.
Even though there was no atmosphere above them, the reflection of the Sun’s light
from the Moon’s surface made their eyes adjust to as if it were day.
One astronaut reported only being able to see some of the brightest stars when he was
in the shadow of the Lunar Module, looking directly up with his protective visor raised.
I imagine though, if astronauts ever land on the night side of the moon and have their
eyes adjust to near pitch-black conditions, they would get a stunning view of the stars
in our galaxy.
When a camera onboard a spacecraft images a planet, the shutter speed, or the amount
of light a shutter allows into the camera before it closes again, is set for the equivalent
of daytime levels, because what we are usually looking at is the day side of the object.
This shutter speed is too quick for background stars to be picked up.
But every so often, we do want to image the night side of the object, which means we must
increase the exposure time to allow more light into the camera.
When this happens, suddenly the background becomes full of stars.
This is a time lapse of raw images taken by the Cassini spacecraft.
As you can see, Cassini takes lots of images with different exposures.
In some images, there are no background stars visible, and in others, there are.
But this leads on to another interesting point about the images space agencies release to
the public.
You may have seen some of these stunning images of Saturn before, but you may not realise
that this isn’t one image, but it's rather a mosaic of several images stitched together.
In such mosaics, you definitely won’t see background stars because the sections of black
around the image won’t have been added to the mosaic.
Lots of spacecraft take images this way, the reason being it can dramatically increase
the resolution of an image and reduce the amount of data that has to be transmitted
back, bandwidth coming at quite a premium where data speed can be reduced to 2kbps at
the farthest distances.
So, there you go, know you know why you don’t tend to see stars in the background of space
images.
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