You asked
Is the leica summilux m 21mm f 1 4 asph good for astrophotography?
I don't have a guide on astrophotography, so I can't say how this lens performs under the stars. What its page does give you is a 21mm f/1.4 prime for full frame Leica M bodies. It is manual focus only and not stabilised. The wide view and fast aperture are the features that usually matter for night sky shots, but I'd test it yourself before relying on it. The page also notes that the frame is wider than the rangefinder's frame lines, so you'll need live view or an EVF to compose accurately.
For wide scenes, the closest page I have is landscape. You can see every Leica M body this lens fits on its lens page.
Written by Claude from this site's pages.