(setq gnus-sum-thread-tree-root "●") (setq gnus-sum-thread-tree-false-root "☆")
thanks.
If you know either their Unicode code point, or their official Unicode name, you can use C-x 8 RET.
C-x 8 RET
Turn on Egg input method (M-x toggle-input-method), which is usually bound to "C-\". Then, type "z!" for "●", "z6" for "☆".