Two Pretty Spring Clusters

There are quite a few deep-sky objects near the limit of normal naked-eye vision that serve as guides to atmospheric conditions overhead. One of the oldest and best known involves the Beehive, M44, in Cancer. With some experience it’s possible to estimate the limiting magnitude of a telescope from a naked-eye view of M44. As early as several centuries BC, the invisibility of M44 was considered an omen of coming rain. Today amateurs can use the same observation to judge the night sky’s transparency.

Its true nature was first discerned by Galileo, who described it in his 1610 astronomy pamphlet Sidereus Nuncius as “the nebulae called Praesepe, which is not only star only, but a mass of more than 40 small ones.” Indeed, only the slightest optical aid is needed to resolve the cluster. In low-power fields, finders, and binoculars, M44 is a brilliant show object. It has no sharp boundary. No one can say for sure where the cluster’s faint glow merges into the placid sky background. And the center is hardly brighter than the edge. The cluster appears as a ghostly sheen on cobwebs at least a degree in diameter, sometimes maybe two. Through a large telescope the view is not particularly impressive, but the cluster is an exciting object for binoculars and rich-field telescopes.

The open star cluster M48 (in Hydra) was long believed to be a "missing" object until Harvard astronomer Owen Gingerich linked it with NGC 2548, which Caroline Herschel discovered in 1783. If Gingerich is correct, the original published position for M48 was about 5° in error. Seemingly, Messier made a mistake of 5° in declination, but his right ascension is correct. But the identification seems pretty certain since there is no other nearby candidate matching Messier's visual description of M48.

This sparse sprinkling of stars has roughly the angular size of the Moon. Because it's a very loose group, it's best viewed with low magnification or the finder. W. H. Smyth with his 6-inch refractor saw this cluster as "a splendid group, in a rich splashy region of stragglers, which fills the field of view, and has several small pairs, chiefly of the 9th magnitude." In my 5-inch binoculars, its shape appears distinctly triangular. This cluster contains about 60 members brighter than 13th magnitude. The total magnitude is about 5.8, and the English author and observer Kenneth Glyn Jones notes that many people can see the cluster's glow with the naked eye. Being so bright and large, it would seem that M48 would be easy to find. Experience suggests otherwise. The cluster is sparse and the background rich. I've never been convinced that it's visible to the naked eye, but it does show nicely in small telescopes.