Here is a brief explanation about why you get noise on some images, particularly on night shots with long exposures.
Noise is an extraneous electrical signal that is picked up by your camera’s sensor which translates into a pattern of color and graininess on your image. Look at the image above, you’ll see specs and small streaks of magenta against the dark sky; this is the most common symptom of noise. Heat is the primary culprit that generates the extraneous electrical signal.
Heat is generated in one of 2 ways: high ISO settings and long exposures. The pixels on your camera sensor are very tiny. The longer you hold the shutter open, the more electricity gets fed to your sensor which causes heat to build up in your sensor. Increasing ISO is like turning up the volume on your radio. When you turn up the volume on your radio, you are amplifying or increasing the strength of the signal to your speakers by pumping in more electricity. Likewise, when you increase ISO, you are increasing the strength of the signal to your camera’s sensor. The more electricity you feed it, the more the heat builds up and the more extraneous electric signals are picked up.
Each camera model has different noise characteristics. Camera manufacturers are always trying to improve their noise reduction. But, noise is a reality of digital cameras. If you are planning to photograph night shots, experiment with your camera ahead of time to find your best noise performance so you can judge how to set up your camera when you are shooting for real.
Software such as Photo Ninja, Photoshop and Adobe Lightroom have ways to reduce noise. They work well but need to be used in moderation. As you apply more noise reduction the more you loose sharpness in your finished image.