Catch sight of a celestial owl flying overhead in the spring sky

By Ade Ashford

The readily identifiable constellation of Ursa Major, the Great Bear, glides overhead as night falls in the spring as seen from the British Isles. For scale, the seven-star asterism forming the Plough or Big Dipper is about the span of an outstretched hand at arm’s length wide. Ursa Major and adjoining constellation Canes Venatici (the Hunting Dogs) are littered with Messier deep-sky objects. Galaxy M108 and the Owl Nebula (M97) lie 1½° and 2¼°, respectively, southeast of magnitude +2.3 star Merak, the southernmost of the two pointer stars (the other being Dubhe) to the Pole Star. Field stars are shown to magnitude +6.5. AN graphic by Ade Ashford.
If you are fortunate to observe from a rural area or have woodland nearby, one of the joys of setting up your ‘scope and waiting for darkness to fall on spring nights is the sound of owl calls. Listen for the sharp ‘kiew-kiew’ or brief, repetitive ‘woop’ of the little owl, the screech of the barn owl, or the ‘ke-wick’ and wavering ‘hoo-hoo’ of the tawny owl.

The Owl Nebula (M97, or NGC 3587) is a tenth-magnitude planetary nebula in Ursa Major. M97 is visible as a dim 3.3-arcminute diameter glow in large binoculars and small telescopes on moonless nights, but it takes at least a 10-inch (25-cm) aperture telescope to see the owl-like ‘eyes’. Image credit: HyperStar-enabled Celestron C11 at f/2 with an Ultrastar C camera by Ade Ashford.
Owls may be scarce near your favourite viewing spot, but the Northern Hemisphere spring sky contains one celestial owl that you can track down in small telescopes – Messier 97 (NGC 3587). Commonly called the Owl Nebula, M97 is a planetary nebula discovered by Pierre Méchain in 1781 that is currently ideally placed for observation in the constellation of Ursa Major, the Great Bear.

Almost circular in appearance with a diameter of about 3.3 arcminutes (about one-tenth that of the full Moon) and slightly brighter than magnitude +10, M97 gets its name from two dark patches in the nebula’s soft glow lending it the appearance of a barn owl’s face in sketches by William Parsons, 3rd Earl of Rosse who viewed it through his 72-inch (1.83-metre) aperture reflecting telescope in 1848.

Locating and viewing the Owl Nebula
At this time of year, Ursa Major lies overhead as darkness falls in the UK. The Owl Nebula lies just 2¼ degrees to the southeast of magnitude +2.3 star beta (β) Ursae Majoris, better known by its proper name, Merak. The nebula’s dim, circular glow is visible in higher magnification, larger aperture binoculars (though the current overhead aspect is hardly conducive to binocular viewing) or small telescopes.

If viewing with an undriven telescope equipped with a well-aligned typical 6 or 8× finder, M97 lies almost half of the finder’s field of view to the southeast of Merak. Recall that your telescope’s field of view will be about one degree (or two full Moons side by side) at 40 to 50× magnification. Be sure to use the lowest magnification eyepiece in your collection and allow enough time (at least 15 minutes) to get your eyes dark-adapted.

Once you have the soft, round glow of M97 in your eyepiece, can you see any structure? (Remember to use averted vision, or let your eye wander around the field of view.) Seeing the owl’s ‘eyes’ calls for a clear, moonless night and a 10-inch (25-cm) aperture telescope, preferably equipped with a UHC or OIII filter. That said, I recall one notable night at a really dark location when I saw the eyes with averted vision while using an unfiltered 8-inch (20-cm) Schmidt-Cassegrain.

Located just 48 arcminutes (0.8°) northwest of the Owl Nebula in Ursa Major, Messier 108 (NGC 3556) is a magnitude +10.7 barred-spiral galaxy. In telescopes magnifying 50× or less it’s possible to see the Owl Nebula and M108 in the same field of view. Image credit: HyperStar-enabled Celestron C11 at f/2 with an Ultrastar C camera by Ade Ashford.
Bonus object: galaxy Messier 108
If you search for the Owl Nebula by offsetting southeast from β Ursae Majoris (Merak), it’s highly that you will first encounter magnitude +10.7 barred-spiral galaxy Messier 108 (NGC 3556). Visually, there’s no mistaking the two as M108 is fainter and more elongated than M97. What’s more, if you are using a telescope magnifying 50× or less it’s possible to see the Owl Nebula and M108 in the same field of view since they are just 48 arcminutes (0.8°) apart.

If your telescope has an accurately aligned computerised GoTo mount then it’s a simple matter to select M97 or M108 from the Messier object sub-menu, but if you wish to jump to the mid-point of the pair then use the following J2000 coordinates: α = 11h 13.0m, δ = +55° 21′. If your GoTo hand control or digital setting circles (DSCs) require JNow coordinates, then you should use: α = 11h 14.1m, δ = +55° 15′ (J2019.3).

If you succeed in viewing M97 and M108 together, contemplate for a moment that the Owl Nebula is about 8,000 years old and lies some 2,500 light-years distant. The nebula represents the expelled outer layers of the elderly Sun-like magnitude +14 central star, which is condensing to form a white dwarf. Also discovered by Pierre Méchain around 1782, galaxy M108 is around 45 million light-years from Earth – around 18,000 times farther than the Owl Nebula. M108 is an outlying member of the Ursa Major Cluster of galaxies in the Virgo Supercluster.

When to look for the Owl Nebula and M108
New Moon next occurs on 4 May 2019, so from the end of April into early May the evening skies will be free from moonlight. By the end of this month, the last vestiges of astronomical twilight fade to black around 11:30pm BST for the heart of the British Isles, so deep-sky observers should make the most of any viewing opportunities.