The finest detail your aperture can resolve: Dawes limit 116/D and Rayleigh criterion 138/D, in arcseconds, from your aperture.
Dawes limit = 116 ÷ aperture (mm) arcseconds, an empirical figure for splitting equal double stars; the Rayleigh criterion 138 ÷ D is the slightly more conservative optical-theory version (at ~550 nm). A 150 mm scope resolves about 0.77″. In practice atmospheric seeing (often 1-3″) is the real limit for larger apertures.
An empirical rule for the closest equal double star a telescope can split: 116 ÷ aperture in mm, in arcseconds. A 150 mm scope resolves about 0.77″, a 250 mm about 0.46″.
Rayleigh (138 ÷ D at green light) comes from diffraction theory and is slightly more conservative; Dawes came from William Dawes' practical double-star observations. Real performance sits between them and depends on brightness and contrast.
Because the atmosphere blurs stars to 1-3″ on typical nights, worse than the optical limit of anything above ~100-150 mm. Aperture still wins on brightness, and on rare steady nights the full resolution shows.