From the February 2020 issue

Observe winter’s twin treats

You’ll get twice the bang for your buck when you point your scope at these celestial pairings.
By | Published: February 6, 2020 | Last updated on May 18, 2023
The Medusa Nebula (Abell 21) in Gemini pairs with open cluster NGC 2395, which lies to the upper left, out of this image’s field of view. 
Adam Block/Mount Lemmon Skycenter/University of Arizona
On frigid February evenings in the Northern Hemisphere, we get crystalline views of Gemini the Twins riding high in the sky. The celestial Twins represent this month’s deep-sky survey theme: namely, objects in the night sky found in pairs or in such close proximity that we see them as twin delights. We’ll begin in a region likely avoided by star-hopping amateur astronomers armed with equatorial mounts: the area around the North Celestial Pole.
The face-on spiral galaxy NGC 2276 (left of center) makes a striking pair with NGC 2300, the lenticular galaxy to its right.
Adam Block/Mount Lemmon SkyCenter/University of Arizona

Way up north

Alpha (α) Ursae Minoris, popularly known as Polaris, Stella Polaris, Lodestar, or the North Star, also has a deep-sky moniker: Struve 93 (Σ93), because it is a magnificent double star. Visible in a 2.4-inch refractor at 60x under excellent seeing conditions, Polaris AB is a celestial wonder that can be shared by all. The 2nd-magnitude golden primary has a 9th-magnitude secondary 19″ to the southwest. The secondary is white, but a contrast illusion may make it appear green or blue. In his and Wil Tirion’s beautiful The Cambridge Double Star Atlas, James Mullaney notes, with wit, that the pair displays a “24-hour orbital period” caused by Earth’s rotation.

Polaris is also part of a magnificent binocular asterism popularly known as the Engagement Ring. Look for a semicircle of eight conspicuous stars (6th magnitude and fainter) just southeast of Polaris, which not only completes the ring but represents its dazzling gem.

Struve 93, or Polaris (Alpha [α] Ursae Minoris), the bright star to the left in this image, is an easy double star to split. The Integrated Flux Nebula also shows up nicely in this photo.
Scott Rosen
Look about 4¼° southeast of Polaris for the starburst spiral galaxy NGC 2276, which is a prodigious supernova factory in Cepheus. It may be interacting with the lenticular galaxy NGC 2300. It’s unknown whether the asymmetrical appearance of NGC 2276 is due to tidal interactions with NGC 2300 (or other members of the sparse NGC 2300 galaxy group), or due to the galaxy’s motion through a massive cloud of gas.

German astronomer Friedrich Winnecke discovered the pair in 1876 with a 6.5-inch refractor, although French astronomer Alphonse Borrelly had already found NGC 2300 some five years earlier with a 7-inch comet-seeker. Glowing at 11th magnitude, NGC 2300 is not only a full magnitude brighter than NGC 2276, but also 1′ smaller, making it more readily visible in backyard telescopes. The galaxy pair lies about 120 million light-years away and is separated in the sky by only 6′.

Wasat (Delta [δ] Geminorum) is an easy split through just about any telescope. What colors do the stars look to you?
Jeremy Perez
Moving 12° farther southeast, we meet another interacting pair of small (2′) and moderately dim (12th magnitude) galaxies: ellipticals NGC 2633 and NGC 2634 in Camelopardalis. Discovered by German-born Wilhelm Tempel in 1882 with an 11-inch refractor, the pair is oriented roughly north-south and separated by 8′. NGC 2633 is the northern component.

For those with larger telescopes, the non-interacting, magnitude 13.5 edge-on galaxy NGC 2634A lies 2′ south of NGC 2634. Anyone with a large-aperture, wide-field telescope might also challenge themselves to detect the wisps of Integrated Flux Nebula (IFN) — dust clouds illuminated by the background glow of the Milky Way. These clouds are most apparent southeast of NGC 2634 and run east to west. See the image of Polaris below for a good look at the IFN.

M35 is the larger, looser cluster at center, while much more compact NGC 2158 lies to its lower left.
Chuck Kimball

Auriga, Gemini, and Taurus

Third-magnitude Theta (θ) Aurigae, otherwise known as OΣ545 (the O stands for Otto), is a beautiful double star with a 7th-magnitude companion 4″ to the west-northwest. Don’t be fooled by the separation; the primary can visually subjugate the secondary under poor atmospheric seeing. It’s a great challenge for 4- to 5-inch telescopes at 250x.

Open cluster M38 is the rich cluster to the right of center. Smaller NGC 1907 sits to its left within a pocket of red emission nebulosity. Both clusters lie in the constellation Auriga.
Dean Salman

Through an 8-inch, catching a glimpse of the aqua secondary — a contrast illusion because the star is white — hugging its yellow luminary lends a warming impression of a distant view of our Earth and Sun. For added enjoyment, check out Theta’s magnitude 11 C (line-of-sight) companion roughly 50″ farther to the west-northwest.

Nearly 6° west-southwest of Theta Aurigae lies a yin-yang “double cluster”: M38 and NGC 1907. Shining at magnitude 6.5, M38 is a wonder through any instrument, including binoculars. And some even can see it with the unaided eye. All powers reveal the 15′-wide cluster’s cross-shaped core (whose center is wreathed by scintillating starlight) and starfish arms. NGC 1907 lies in the same field of view 30′ to the south-southwest and appears merely as a compressed phantom glow at low power. At high magnification, it resolves into a rectangle of 30-odd suns crisscrossed with dark veins. Once considered to be physically related, the two clusters are only “flying by” one another, some 1,200 light-years apart.

M38 and NGC 1907 are a dimmer doppelgänger of M35 and NGC 2158 in northwestern Gemini. Only, in this case, M35 is the one with the rectangular shape. At 5th magnitude, M35 is visible to unaided eyes even from some suburban locations as a pale glow rivaling the apparent size of the Full Moon.

The Double Bubble Nebula (NGC 2371–2) is a bipolar planetary nebula whose discoverer thought it was two objects, thus the double NGC designation.
Dietmar Hager
An exquisite cluster through binoculars and telescopes, M35 is one of the richest and most compact open clusters located in the direction away from the galactic center. NGC 2158, on the other hand, is a magnitude 8.5 pipsqueak (5′) cluster about 0.5° southwest of M35. Like NGC 1907, it appears at low power through a 4-inch scope as a milky splotch with some stars hovering near the limit of vision. It resolves well, however, with higher magnifications and larger apertures.

Just north of Orion’s Shield, we find another twin cluster treat: NGC 1807 and NGC 1817 in Taurus. At 33x in a 5-inch, these objects appear as two magnitude 7.5 knots of starlight separated by 20′. While NGC 1807 is 0.7 magnitude brighter than NGC 1817, it is less visually appealing, containing some two dozen stars in an area 12′ across and looking much like a stick figure.

NGC 1817 is a more dynamic grouping of irregularly bright suns — a lightning bolt of brighter members with direct vision that swells into a 20′-wide ball of noisy starlight with averted vision. These dueling objects may truly be twins — a single cluster nearly 1° across in the sky, with dual cores lying nearly 6,000 light-years distant.

Next, slip over to Castor (Alpha Geminorum or Σ1110), which is Gemini’s most famous twin wonder and one of the night sky’s most celebrated double stars. The 2nd-magnitude white primary has a hint of green, while its 3rd-magnitude secondary (5″ to the northeast) shines with a pale pumpkin light. Resolving the close pair requires medium to high magnifications on a night of steady seeing.

The artist sketched Castor (Alpha [α] Geminorum) while using an 8-inch f/6 reflector and an eyepiece that gave a magnification of 200x.
Jeremy Perez
The Double Bubble Nebula (NGC 2371-2) is an 11th-magnitude bipolar planetary nebula 1½° north of 4th-magnitude Iota (ι) Geminorum. When William Herschel discovered these objects, he saw them as independent, thus the two NGC numbers. The planetary’s southwestern lobe is NGC 2371; NGC 2372 is the northeastern one. Through a 5-inch refractor at 33x, the nebula is a mere 1′-fleck of fuzz. Seeing the binary nature requires magnifications of 100x and greater. Though I have yet to see it, the magnitude 14.8 central star has been sighted at 225x through a 12-inch telescope.
NGC 1807 and NGC 1817 make a striking pair when seen through a wide-field telescope. NGC 1817 is the loose group of stars left of center; NGC 1807 has fewer but brighter members to the lower right.
Bernhard Hubl
Wasat (Delta [δ] Geminorum) holds a special place in the hearts of those who knew the late American astronomer Clyde Tombaugh. In 1930, Tombaugh discovered Pluto a mere ½° east of this star. To observers, it is another prize double star: Σ1066. The magnitude 3.5 bleached-yellow primary has an 8th-magnitude red dwarf companion about 7″ to the northwest. The 19th-century observer William Henry Smyth saw this companion shining with a purple light, though rose-lavender comes to my mind.

Near Gemini’s southern border, about 1¾° northwest of the magnitude 4.5 star 6 Canis Minoris, is the 8th-magnitude, low-surface brightness open star cluster NGC 2395. This loose and scattered aggregation of about 30 dim suns that form an ellipse lies in the same field of view as a planetary nebula just 30′ to the southeast: Abell 21 — more popularly known as the Medusa Nebula.

NGC 2183 and NGC 2185 are a pair of blue reflection nebulae in the constellation Monoceros. NGC 2185 is the larger, brighter one just to the right of center. Smaller NGC 2183 lies to its left.
Adam Block/Mount Lemmon SkyCenter/University of Arizona
I’ve spied this large (10′-wide) waif of nebulous tendrils through an 8-inch reflector at its lowest magnification on the darkest of nights. Sweeping was required to pick out the brightest arc of its ancient light (without a filter), which appeared as a phantasm. However, repeated hits secured its impression. Most people have success with an Oxygen-III filter.


Next, move southwest into Monoceros. Just 1° west of Gamma (γ) Monocerotis, you’ll find a double “dwarf nebula”: NGC 2183 and NGC 2185. Through a 4-inch refractor at 72x, they appear as a tiny nebula glowing with a double nucleus. Actually, the double nucleus is the two nebulae, which appear to be hugging. NGC 2185 is the brighter and more easterly of the pair.

NGC 2207 and IC 2163 are interacting galaxies in the constellation Canis Major the Big Dog. NGC 2207 is the larger galaxy to the left in this image.
Thalia and Norman Terrell/Adam Block/NOAO/AURA/NSF
We’ll end with a bang, of sorts. Look 3¾° southwest of 2nd-magnitude Mirzam (Beta [β] Canis Majoris) for a pair of colliding spiral galaxies, NGC 2207 and IC 2163, some 80 million light-years distant. Strong tidal forces from NGC 2207 have distorted the shape of IC 2163, flinging stars and gas into long streamers stretching out 100,000 light-years. This near collision is one of the most dynamic visible through backyard telescopes. Through an 8-inch at 300x, magnitude 10.8 NGC 2207 appears as a 5′-wide mass of light with a bright core, while IC 2163 is a conspicuous 12th-magnitude bullet of light 2′ across with a soft central concentration.

I hope you enjoyed this celestial soirée with these select twins of the night. The deep sky is filled with wonders like these, which help us better appreciate the depth, grandeur, and beauty of our infinite universe.