You know, after decades of pointing optics toward the sky, I consider myself a knowledgeable observer. But while researching the Pleiades a few months back, I was surprised to learn that it wasn’t the brightest star cluster. And there were other surprises when I started comparing overall brightnesses of deep-sky objects. For instance, you might think some easy winners would be galaxies, housing billions of stars, but when I started ranking objects, only two galaxies made the Top 25; most of the targets I’ve listed here are either open clusters, stellar associations (looser, poorly populated open clusters), or asterisms.
While you may have looked near many of these targets before, you probably didn’t really observe them, looking past the forest for the fainter trees. Well, here’s your chance to take a step back and take in the most light the night sky has to offer.
Top of the heap
You may have guessed that the brightest deep-sky object is the first galaxy on our list, the Large Magellanic Cloud. Straddling the border between the constellations Dorado and Mensa, it glows at a worthy magnitude 0.1. And it’s huge: It covers an area greater than 9° by 11°, or 400 times that of the Full Moon. Visible just south of latitude 20° north, the Large Magellanic Cloud is best seen in the Southern Hemisphere summer (that is, December through February).
Second on the list is the open cluster Hyades and the neighboring bright star Aldebaran (Alpha [α] Tauri). Together, they form a V-shaped group that marks the head of Taurus the Bull. Although Aldebaran is counted as part of the Hyades, giving that group a worthy magnitude 0.5, it’s not part of the star cluster — it actually lies nearly 90 light-years closer to us.
The third-brightest deep-sky object, glowing at magnitude 0.6, is Collinder 70, an open cluster of 100 or so hot young stars. You may know it by the more famous name of its three brightest members: Orion’s Belt. The Belt is probably tied with the Big Dipper as the most well-known asterism in the night sky. But through even modest binoculars, the Belt looks to be embroidered with a slew of fainter, still bright companions.
Tied for third place on our list — also at magnitude 0.6 — is Collinder 173, a loose (6°-wide) open cluster on the border between Puppis and Vela. It lies about 1,400 light-years away. Such a bright cluster should be easy to find, but it’s tough to identify the members. Look roughly 10° north-northwest of Alsephina (Delta [δ] Velorum), and use 7×50 binoculars to frame it best.
Coming in at No. 5 with a magnitude of 1.0 is the Upper Scorpius association — an association rather than an official open cluster. Also called the Antares Cluster, this 8°-wide group surrounds the star Antares (Alpha Scorpii). Even low-power, wide-field 7×50 binoculars can’t take in the entire group, so you’ll have to scan around. You’ve probably seen these stars — the four front stars of Scorpius (β, δ, π, ρ) plus a few others — lots of times, but perhaps never knew that they were an association.
No. 6 on our list is the Alpha Persei Cluster, an open cluster named after its brightest member: magnitude 1.8 Mirfak (Alpha Persei). With a total magnitude of 1.2 and a diameter of 6°, it’s easy to find and just small enough to frame through 7×50 binoculars.
One of the more famous objects on this list, coming in seventh place with a magnitude of 1.6, is the Pleiades (M45). In ancient times, this star pattern was so highly regarded that many Greek philosophers considered it a separate constellation. Often called the Seven Sisters, a reference to a Greek myth, most sharp-eyed observers can only spot six stars. High-quality binoculars will raise that number to 50.
The eighth-brightest deep-sky object is the Coma Star Cluster (Melotte 111), an open cluster that lies some 280 light-years away. Although its combined magnitude is 1.8, that brightness is spread across an area 4½° wide. The cluster contains fewer than 100 members, and only a dozen or so are bright enough to see without optics. So, use binoculars.
The Southern Pleiades (IC 2602), a gorgeous open cluster that surrounds the star Theta (θ) Carinae, comes in ninth on our list. It glows at magnitude 1.9 and spans nearly 1°. Binoculars that magnify 10x or more do a good job revealing this cluster, but if you have an eyepiece in your telescope that has a field of view of 1° or more, you’re in for a real treat.
Second class
The 10th object on our list is the first to fall into the 2nd-magnitude class. It’s the Pi Puppis Cluster (Collinder 135), which is centered around the magnitude 2.7 star Ahadi (Pi [π] Puppis). This star sits alone about 7° east of the midpoint of a line between the sky’s two brightest stars, Sirius (Alpha Canis Majoris) and Canopus (Alpha Carinae). The cluster has a total magnitude of 2.1, but Pi is the standout for two reasons. First, it’s the brightest star, and second, it’s a red supergiant. All the other stars in the field of view are blue.
The Omicron Velorum Cluster (IC 2391) registers at magnitude 2.5, and is tied for 11th on our list. This isn’t a small object, measuring just less than a degree in diameter. That means its area is more than twice that of the Full Moon. The best views through any scope will come when you use an eyepiece that gives a 1° field of view.
The next object is one I’m sure you’ve seen hundreds of times but may not have known it was an open cluster. The Lost Jewel of Orion, otherwise known as NGC 1980, glows at magnitude 2.5. It’s centered around the magnitude 2.8 star Iota (ι) Orionis, and is part of the Hunter’s Sword. It lies just south of the Orion Nebula (M42).
Yet another pair ties for lucky 13, with first up the open cluster Collinder 121 in Canis Major. You’ll find it less than ½° south of Omicron1 (ο1) Canis Majoris. It glows nicely at magnitude 2.6 and has a diameter of 30′. It lies some 3,500 light-years away, but there’s another, more widely spread, group of stars just 2,000 light-years distant, between Collinder 121 and us. No worries, though. The closer stars will simply appear as field stars through your eyepiece. Tied with Col 121 is the open cluster NGC 6231 in Scorpius. Often called the Baby Scorpion Cluster, it also glows at magnitude 2.6 and spans 15′. Along with Zeta (ζ) and Mu (μ) Scorpii, it forms an asterism British astronomer Sir John Herschel called the False Comet. He named it this for two reasons: 1) The group looks like a comet; and 2) Herschel, who had traveled on a ship to South Africa to view the southern sky, landed at False Bay.
The other galaxy on our list, and the 15th-brightest deep-sky object, is the Small Magellanic Cloud, which lies mainly in Tucana with a small part in Hydrus. Its combined brightness equates to a 2.7 magnitude star. It lies some 200,000 light-years away, but still covers an area of sky a bit greater than 5° by 3°. That’s a space more than 70 times as large as the Full Moon.
The Lambda Orionis Cluster in Orion makes our list at number 16. It’s easy to find because its brightest member is Meissa (Lambda [λ] Orionis), the star that marks Orion’s head. The cluster glows at magnitude 2.8 and spans a bit less than 1°.
Tied for 16th place, also at magnitude 2.8, is open cluster NGC 2451 in Puppis, sometimes called the Stinging Scorpion Cluster. In the 1990s, astronomers discovered that this object was a combination of two unrelated open clusters (now known as NGC 2451A and NGC 2451B) that align from our point of view.
Bottom of the top
Eighteenth on our list is open cluster Collinder 65 in Orion. In fact, it lies about as far north in that constellation as you can go, right on the border with Taurus. It glows at magnitude 3.0, and it’s large: It boasts a diameter of 3½°. Wide-field binoculars will show the cluster best.
Tied for 18th place is the Firefly Party Cluster (NGC 3532) in Carina. Point a 4-inch scope at this beauty, and you’ll understand how it got its whimsical name. Look for it roughly 4.7° south-southwest of 4th-magnitude Pi Centaurus. Its 3rd magnitude brightness is spread out over nearly a 1° diameter, so even though you’ll see it as a fuzzy patch without optical aid, a telescope will bring out its best features.
Also tied for 18th place is Melotte 186, a loose open cluster in Ophiuchus. And when I say loose, I mean it. Although the combined brightness of its stars equals magnitude 3.0, the diameter of this group is a whopping 4°. Use binoculars or scan around with the lowest-power eyepiece in your telescope to view the whole group.
You’ve probably seen No. 21 on our list. It’s the Beehive Cluster, also known as the Praesepe, M44, NGC 2632, and more. This open cluster lies in the center of Cancer the Crab. Sharp-eyed observers have no problem seeing M44 with naked eyes, even if there’s some scattered moonlight. It glows at magnitude 3.1 and has a width of more than three Full Moons.
The 22nd object lies near the bottom of the figure of Scorpius. It’s Ptolemy’s Cluster (M7), and is the most southerly Messier object. This open cluster is an easy naked-eye catch, glowing at magnitude 3.3, even though it lies centered within the Milky Way.
Collinder 140, an open cluster in Canis Major, shines 23rd on our list. It glows at magnitude 3.5 and spans 42′. To find it, look 3° due south of Aludra (Eta [η] Canis Majoris). Through an 8-inch scope at low power you should be able to count some three dozen member stars.
Sharing the 23rd slot is Trumpler 37, an open cluster in Cepheus. Longtime amateur astronomers, especially astroimagers, recognize this as the cluster in the heart of the Elephant Trunk Nebula (IC 1396). An 8-inch scope will reveal roughly 50 member stars.
The 25th and final object on our list is Collinder 132, an open cluster in Canis Major. To find it, look not quite 3° southwest of Aludra. It glows at magnitude 3.6 and spreads out over an area 1½° across.
OK, I lied. Collinder 132 isn’t the last object because another group of stars is tied with it in brightness: the Coathanger, an asterism in Vulpecula. This object carries a variety of names, including Al Sufi’s Cluster, Brocchi’s Cluster, and Collinder 399. Although it’s a loose glow to naked eyes, 7-power or greater binoculars show it as a 1¼° line of stars with a separate curve on the southern side, giving it the appearance of a coathanger.
And there you have it. Next time you step out into the darkness, take a minute to appreciate these not-so-hidden gems, and bask in all their glorious light.
