Tweedledee Cluster — Ammasso aperto in Ofiuco

NGC 6633 is a large, bright open cluster in the constellation Ophiuchus, about 1,200 light-years from Earth. Commonly known as the Tweedledee Cluster, it is roughly the size of the full Moon and contains about 30 bright blue stars strewn across a rich Milky Way background — a splendid target for any optical aid from binoculars upward. It forms a famous visual pair with the nearby cluster IC 4756 (sometimes called Tweedledum or Graff's Cluster), which lies just 3 degrees to the east; the two are often framed together in low-power surveys along the summer Milky Way. Both clusters share similar age (~700 million years) though they lie at different distances.

An ideal binocular target. NGC 6633 is visible to the naked eye from dark sites as a small misty patch. In 7x50 or 10x50 binoculars it resolves into a striking elongated chain of bright blue stars scattered across a rich Milky Way field — a textbook 'binocular cluster.' A small telescope at 20-30x frames the cluster beautifully but high magnification dilutes its spread-out character. Pair the view with IC 4756 three degrees east for the full 'Tweedledee/Tweedledum' experience. Best observed from June through September when Ophiuchus is high in the evening.

Cataloged in 1702 by the German astronomer Philippe Loys de Chéseaux's predecessor Johannes Hevelius, though William Herschel is more commonly credited with its rediscovery in the 1780s. The cluster appears in most 19th-century observing guides. The 'Tweedledee' nickname — a whimsical pair with IC 4756's 'Tweedledum' — appears to have originated in amateur astronomy literature of the 20th century, a nod to Lewis Carroll's characters. The name is informal but widely recognized in modern observing handbooks.