Generally for landscapes and astrophotography you'll want a wide lens, not a long one. Longer lenses have a narrower field of view. When you're taking pictures of mountains, for example, you usually want a very wide field of view. You get a wide field of view with a shorter lens, not a longer one. In fact, with astrophotography, the shorter the lens, the longer the exposure you can take without seeing star trails. (See this post on the rule of 600 for an explanation of why.)
To let as much light into the lens as possible, you'll want to pick one that has a wide aperture setting. The lower the "f number", the wider the aperture. So f/1.8 is a wider aperture than f/3.5, for example.
I recently spent time photographing the Milky Way. I used a 16-35mm lens on a full-frame camera, and an 11-16mm lens on an APS-C crop-sensor camera (which is what the D7200 has). You can see one of the pictures here.