Messier 97 also known as the Owl Nebula, is a famous planetary nebula in Ursa Major’s constellation. The nebula lies at a distance of 2,030 light-years from Earth and has an apparent magnitude of 9.9. It has the designation NGC 3587 in the New General Catalogue.
Messier 97 occupies an area of 3.4 by 3.3 arc minutes in apparent size, corresponding to a spatial diameter of 1.82 light years. It was named the Owl Nebula because of its appearance in larger telescopes, which reveal two dark patches that look like the eyes of an owl. William Parsons, 3rd Earl of Rosse, first sketched these in 1848.
Owl Nebula
The Owl Nebula can be seen in 20×80 binoculars and small telescopes, but only under excellent conditions, and it only appears as a fuzzy ball of light. The owl-like eyes can be seen in 10-inch and larger telescopes. It can be found about 2.5 degrees southeast of Merak, Beta Ursae Majoris. The star marks the southwest corner of the Big Dipper’s bowl and, together with Dubhe, Alpha Ursae Majoris, the star at the northwest corner, points the way to Polaris. From Merak, M97 lies just over 2.5 degrees in the direction of Phecda (Phad), the other star at the bottom of the Dipper’s bowl.
- Owl Nebula
Nebula’s owl-like appearance
Another Messier object, the barred spiral galaxy M108, can be seen 50 arc minutes northwest of the Owl Nebula. The two objects appear in the same wide-field view. The best time of year to observe M97 is during the spring, but observers north of latitude 35N can see the nebula throughout the year, as it never sets below the horizon for them.
Like most planetary nebulae, M97 appears brighter visually than photographically because it emits most of its light in one green spectral line. Its outer halo was not detected until 1991. The nebula formed when a dying Sun-like star ran out of hydrogen fuel, collapsed from a red giant to a white dwarf, and ejected its outer envelope. The expelled material is now heated by the radiation of the central white dwarf, producing the nebula’s glow. The nebula has been gradually expanding and will completely disperse into space over the next several thousand years. The white dwarf will cool and fade over several billion years.
Nebula’s Estimated Age
The estimated age of the Owl Nebula is about 8,000 years. The nebula is expanding at a range of 27 to 39 km/s. It contains about 0.13 solar masses of material, including hydrogen, helium, nitrogen, oxygen, and sulfur. The matter within the nebula has a density of fewer than 100 particles per cubic centimeter.
The 16th magnitude central white dwarf has about 0.7 solar masses and is between 41 and 148 times brighter than the Sun. The star’s estimated temperature is about 123,000 K.
When it collapsed, the central star expelled a large amount of matter in two opposing directions, which resulted in the nebula’s owl-like appearance. The two jets of matter are almost aligned with our line of sight but just slightly off. The dust in the jets blocks some of the light of the expanding nebula. Giving it the appearance of two dark eyes. The jet pointing in our direction appears as the darker eye. While the one pointing away from us seems slightly fainter.
The M97 nebula is arranged in three concentric shells. The outermost shell is about 20 to 30 percent larger than the inner shell. It is not circularly symmetric and forms a barrel-like structure that gives M97 its owl-like appearance when seen from Earth. The barrel-like structure is aligned at a 45-degree angle to our line of sight.
The Owl Nebula’s appearance is typical. The Southern Owl Nebula (ESO 378-1), located in the constellation Hydra, resembles it.
Messier 97 is one of only four planetary nebulae in the Messier catalog. The other three are the Dumbbell Nebula (M27) in Vulpecula. The Ring Nebula (M57) in Lyra. And the Little Dumbbell Nebula (M76) in Perseus constellation.
French Astronomer Pierre Méchain
French astronomer Pierre Méchain discovered the Owl Nebula on February 16, 1781. Méchain reported the discovery to his friend and colleague Charles Messier, who added the nebula to his catalog.
In his notes about M97, Messier also mentions two other objects he observed simultaneously. He determined the positions for both objects but still needs to include them in his catalog. From his descriptions, it is clear that he observed the objects we now know as Messier 108 and Messier 109. The two galaxies were given Messier names by the American astronomer Owen Gingerich in 1953.
Describing M97, Messier wrote:
Nebula in the Great Bear, near Beta: It is difficult to see, reports M. Méchain, especially when one illuminates the micrometer wires: its light is faint, without a star. M. Méchain saw it the first time on February 16, 1781, & the position is that given by him. Near this nebula, he has seen another one, [the position of] which has not yet been determined [Messier 108], and also a third which is near Gamma of the Great Bear [Messier 109].
William Herschel described M97
As a “very bright, round nebula of about three ′ in diameter,” adding that “it is nearly of equal light throughout, with an ill-defined margin of no great extent.”
Admiral William Henry Smyth was the first to classify M97 as a planetary nebula. After observing the object in March 1837, he wrote:
Globular Collection of Nebulous Matter
A large planetary nebula, or globular collection of nebulous matter, was found by Messier in 1781, on the Great Bear’s flank. With several stars in the field, one of which is pretty close. It lies about 2 deg to the southeast of Beta, Merak, and just south of an imaginary line from Beta to Gamma. This very singular object is circular and uniform. After a long inspection, looks like a condensed mass of attenuated light, seemingly of the size of Jupiter.
The mean apparent place was obtained by differentiation with that of Psi Ursae Majoris, and this diagram was sketched. Sir William Herschel discovered this orb in 1789 and found it a globular body of equal light throughout: he also said. “From the observation of the 20-foot telescope, it appears that the profundity of this object is beyond the gauging power of that instrument. And as it must be sufficiently distant to be ambiguous, it cannot well be less than of the 980th order.”
He described it as “very remarkable; planetary nebula; very bright; very large; round; very, very gradually, then very suddenly brighter toward the middle of this planetary nebula; 19.0s [of time; about 150″] diameter.”
Lord Rosse, who noticed the owl-like appearance of Messier 97, wrote, “Two stars considerably apart in the central region, dark penumbra around each spiral arrangement, with stars as apparent centers of attraction. Stars sparkling in it; resolvable.”
https://www.messier-objects.com/messier-97-owl-nebula/
