For sharp, non-trailed astrophotography, the 400 Rule is used to estimate the maximum shutter speed before stars begin to appear as streaks.
The rule is simple: divide 400 by the focal length of your lens (in millimeters) to calculate the maximum exposure time in seconds.

Make sure to check if your camera has a full-frame sensor or crop sensor before applying the rule.

Full-frame sensor
400 ÷ focal length (mm) = maximum shutter speed (seconds)
Example
400 ÷ 200 mm = 2,5 seconds

Crop sensor
400 ÷ focal length (mm) x crop factor = maximum shutter speed (seconds)
Example
400 ÷ 200 mm x 1,6 (= 320 mm) = 1,25 seconds

For sharp star photos without trails, shoot in Manual mode (M) and use manual focus,
carefully focusing on a bright star using live view at maximum zoom.

Set your aperture wide open (around f/1.8 to f/4), choose an ISO between 1600 and 3200 depending on light pollution,
and use a shutter speed based on the 400 rule to prevent star movement.

Turn off image stabilization, shoot in RAW for maximum editing flexibility,
use a sturdy tripod, and trigger the camera with a timer or remote to avoid vibrations.

Star trail Photography
Star trail photography captures the movement of stars.
Choose a dark location with an interesting foreground and point your camera toward Polaris for circular trails or east/west for flowing lines.

Mount the camera on a sturdy tripod, use manual focus on a bright star,
turn off image stabilization, and shoot in RAW.

Capture a sequence of shots over 1 to 2 hours. Set the shutter speed to 20 or 30 seconds at f/2.8 to f/5.6 and ISO 400 to 1600. 
Later, combine the images in software like StarStaX to create smooth,
continuous trails, optionally blending with a sharp foreground for added detail.