Messier 97, the Owl Nebula, is a planetary nebula in Ursa Major. It is one of the four planetary nebulae in Messier's catalog, and the third to be discovered. The nebula gets it name from the appearance of owl-like "eyes", visible in telescopes.
M 97 was discovered by Pierre Mechain in 1781, and Messier added it to his catalogue that same year. Admiral William H. Smyth first classified it as planetary nebulae in 1844. The name "Owl Nebula" goes back to Lord Rosse, who first used it in 1848. In 1866, William Huggins recognized its gaseous nature from his observation of its spectrum.
Observing Messier 97
The Owl Nebula is one of the fainter objects in Messier's catalog. As is often the case for planetary nebulae, the Owl is significantly brighter visually (mag 9.9) than photographically (about mag 12), as most of its light is emitted in one green spectral line. The planetary's disk has dimensions of 3.3' x 3.4' - rather larger than Jupiter's - but medium aperture telescopes are necessary to glimpse the two internal ovals which suggest owl eyes.
When viewed through an 8-inch or larger telescope under dark sky conditions with the aid of a nebula filter, the Owl Nebula is a large, faint, circular disk with a bluish tint an diffuse edges. The grayish halo is brighter along its northern and southern periphery and seems slightly elongated N-S. The core area is slightly brighter, though the very difficult central star reveals itself only by an occasional twinkle. The core is flanked by the two bean-shaped darker ellipses which form the Owl's eyes.
The "eyes" are also easily visible through photographs taken of the nebula. Images of M 97 also reveal several small nebulous objects, probably very distant galaxies, in the background of this nebula.
Properties and Structure
The distance to M 97 is uncertain; estimates range from 1,300 to 12,000 light years, with 2,600 being the most likely value. At this distance, it would have a diameter of about 2 light years.
The mass of the nebula has been estimated at 0.15 solar masses, while its 16th-magnitude central star contains about 0.7 solar masses. The nebula formed roughly 6,000 years ago.The Owl is one of the more complex planetary nebulae known. Its appearance has been interpreted as that of a cylindrical torus shell (or globe without poles), viewed obliquely, so that the projected matter-poor ends of the cylinder correspond with the Owl's eyes. This shell is enveloped by a fainter area of lower ionization.