Messier 48 (NGC 2548) is a conspicuous open cluster in Hydra, visible to the naked eye under good atmospheric conditions. Despite this, Messier 48 has long been thought to be one of the "missing" Messier objects.
Discovery and History
This open cluster was catalogued by Charles Messier in 1771. However, there is actually no cluster in the position indicated by Messier. The value that he gave for the right ascension matches that of NGC 2548; however, his declination is off by five degrees. The object remained missing until identified by Oswald Thomas in 1934, and independently by T.F. Morris in 1959. The identification of M 48 by Oswald Thomas confused some historians, who have claimed erroneously that he had instead identified M 47.
While M 48 was lost, two independent rediscoveries occurred. First, Johann Elert Bode apparently found it on or before 1782; second, Caroline Herschel independently rediscovered M 84 in 1783, and added it as No. 5 to her small list. This latter discovery was published by Caroline's famous brother, William Herschel, who included it in his catalog as H VI.22 in 1786. Thus credit for M 48's discovery is sometimes given to Caroline Herschel in 1783 instead of Messier.
The thinking today is that Messier was really looking at NGC 2548, and had made an error in reporting his observation. The cluster NGC 2548 fits his description of Messier 48, though it is 5° south of the position Messier reported in 1771.
Appearance and Properties
Located in the head of Hydra, almost on its western border with Monoceros, M 48 is quite a conspicuous object. With a total visual magnitude of 5.5, it should be a naked-eye object under good conditions. It is an ideal target for high power binoculars or the smallest of telescopes. The cluster contains about 80 stars, of which about 50 are brighter than magnitude 13. The more concentrated core extends over 30', while the outskirts reach out to about 54'.
M 48 is around 1,500 light years away, and about 24 light years in diameter. Its age is estimated at around 300 million years. M 48's hottest star is of spectral type A2 and magnitude 8.8; its luminosity is about 70 Suns. M 48 also contains three yellow giants of spectral types G and K.