NGC 581

Number of stars per data source

	Reference   25 :   10 stars
	Reference   50 :    2 stars

25: Zug R.S.; (1933) Lick Obs. Bull. 16, 119; An investigation of color excess in galactic star clusters
50: Cannon A.J., Pickering E.C.; (1918-1924), Ann. Obs. Harvard Coll. Obs. 91 to 99 and 100; Henry Draper catalogue and extension