A 300 mm lens is not so very much yet. So that you know what to expect, here is my shot of the planet Jupiter with four of its moons. My lens is a Sigma 70-300 mm cheap lens, but for this sample it does not matter, the 300 mm focal length is in question here:

^^ Jupiter and moons cropped to show 100% pixels inline.

^^ Here the full resolution photo. My camera has a 16 MP APS-C sensor.
Click here for full size photo (4912x3264).

To get closer you'll need a teleconverter in between the lens and your camera.

A 300 mm lens is not so very much yet. So that you know what to expect, here is my shot of the planet Jupiter with four of its moons. My lens is a Sigma 70-300 mm cheap lens, but for this sample it does not matter, the 300 mm focal length is in question here:

^^ Jupiter and moons cropped to show 100% pixels inline.

^^ Here the full resolution photo. Click here for full size photo (4912x3264).

My camera has a 16 MP APS-C sensor which effectively turns the focal length into equivalent of 450 mm on a full frame (or a film) camera. I only noticed after reading @MichaelClark's answer that Nikon N90 is a film camera, and while I have used my share of film I never ever took a shot at the starry sky at night :(

Jupiter is very bright, but its moons are not so. Tripod or other strudy support is a must.
Exposure settings on my digital camera: 1/5 sec - f/6.3 - ISO 400

To get as close with that N90 you'll need a teleconverter in between the 300mm lens and your camera. Then again, if you do your own developing to paper prints, you can magnify the image in your darkroom quite nicely and if exposure is right you'll end up with a better photo easily.