NGC 2903 — ししの銀河

NGC 2903 is a large, bright barred spiral galaxy in the constellation Leo, about 30 million light-years away. At magnitude 8.9 and spanning nearly 12 arcminutes, it is one of the brightest galaxies not included in Charles Messier's catalog — an oversight that has earned it the informal title of 'the galaxy Messier missed.' It has a well-defined central bar, a bright core, and loosely wound spiral arms studded with bright H II regions. Its luminosity is comparable to the Milky Way's, making it a near-twin in many respects: a spiral of similar size, similar stellar content, and active ongoing star formation in its nuclear region.

A splendid galaxy target. In binoculars it is visible from dark skies as a faint elongated smudge; a 4-inch telescope at 80-120x shows a clear central concentration with a surrounding diffuse halo. An 8-inch at 150-200x under dark skies shows the bar clearly, with hints of the inner spiral arms curling outward. A 12-inch or larger begins to reveal individual H II regions as tiny brighter knots. The galaxy lies about 1.5 degrees south of Lambda Leonis, an easy naked-eye star marking the lion's head. Best observed February through May.

Discovered by William Herschel on November 16, 1784. Why Messier missed it — given its brightness and Northern Hemisphere location — is a classic amateur-astronomy question. The most plausible answer is simply that Messier's catalog was driven by the search for comets, and the region around NGC 2903 was not where he happened to be looking at the right time. Herschel described it as 'very brilliant' and noted its strongly elongated shape. Modern observations have confirmed its barred spiral classification and its active nuclear starburst.