For star trails, the key is a stable long-exposure setup rather than any special metering mode. Use a very solid tripod, and trigger the camera with a remote release if possible; otherwise use the self-timer to reduce vibration. On the D90, turn lens VR off when the camera is on a tripod, switch autofocus off once focus is set, and disable Auto ISO. Also turn off Long Exposure Noise Reduction if you want to avoid the camera spending extra time processing after each long exposure. Because long exposures are sensitive to movement, keeping the camera steady is especially important. In cold conditions, try to keep the battery warm, since batteries can lose performance in low temperatures. White balance is less critical if you shoot RAW, because you can adjust it later. Metering is also not the main concern for star trails; exposure is usually determined by experimentation based on the night sky conditions and your chosen composition.