There are two factors that might lead to "too much":

• exposure time: In cases like the above, you want to observe something that is not bright, so you have to observe the spot for quite some time. Because of that bright stars in the surrounding will get saturated and you get the bright stars with crosses.
• false colours: If you observe in a non-visible wavelength regime, e.g. infrared or UV, you have to assign colours later. Let assume an observation is in the infrared at 1.2, 1.9 and 2.7 micrometers. You have data how much radiation at these wavelength has been detected at each point, but you do not have a picture you can see. Now you say 1.2 should be "blue", 1.9 should be "green" and 2.7 should be "red". Now you have an image. Because you want a certain feature or object to be prominent, you adjust the colour curves. And some might do too much there. This might also happen in images that were in the visible range to start with.