JPEG quality alone usually isn’t the cause. A more likely reason is that Darktable’s preview and final export can use different rendering/downsampling behavior, so tiny bright points like stars may look sharper on screen than in the exported file.

From the community suggestions, the main things to check are:

• Evaluate sharpness at 100% zoom, not in a fit-to-screen preview.
• Review the profiled denoise module: its default settings can be too strong and may suppress small star detail.
• If needed, reduce denoising and tune it while viewing at 100%.
• Try a different demosaicing method such as VNG.
• Add a final sharpening step after your other edits, especially if you are not doing further post-processing.

Also remember that when an exported image is viewed at reduced size, the viewer’s own scaling algorithm can make stars look softer. Compare the export at 100% magnification before judging the result.

So the fix is usually not “higher JPEG quality,” but adjusting denoise/demosaic/sharpening and checking the image at 1:1 zoom.