Messier 73 — Asterism in Aquarius

M73 is a small asterism of four unrelated stars in the constellation Aquarius, appearing as a Y-shaped or triangular grouping spanning about 2.8 arcminutes. Unlike most Messier objects, M73 is not a true deep-sky object — it is simply a chance alignment of four stars at vastly different distances ranging from roughly 1,000 to 2,500 light-years. The component stars range from about magnitude 10.5 to 12. For decades, there was debate over whether M73 constituted a genuine sparse open cluster, but modern proper motion and radial velocity studies have conclusively shown that the stars are unrelated and moving in different directions through space. M73 remains in the Messier catalog as a historical curiosity and is one of the least impressive entries in the entire list.

Located in Aquarius about 1.5 degrees east of M72. In a small telescope at moderate magnification (around 80-100x), M73 appears as a tiny group of four faint stars forming a rough Y or triangle shape. It is easy to overlook or mistake for a random star grouping, which is essentially what it is. Use at least a 4-inch telescope to clearly see all four component stars. The challenge with M73 is not in finding it but in recognizing it — star-hop from M72 or use the star Nu Aquarii as a guide. Best observed from August through October when Aquarius is well-placed in the evening sky.

Cataloged by Charles Messier on October 4, 1781, who described it as 'a cluster of three or four small stars, which resembles a nebula at first sight.' Messier likely included it because at low magnification in his small telescope, the tight grouping of faint stars mimicked the appearance of a nebula or comet — exactly the type of object his catalog was designed to help observers avoid. The debate over its nature persisted for over two centuries until Bassino et al. (2000) used proper motion data to definitively prove the stars are unrelated.