We turn our gaze to the southern reaches of the constellation Puppis, south and east of the bright star Sirius and Canis Major, to examine two stunning star groups in a rich field of the Milky Way.
The first stop is the star cluster NGC 2477. Discovered by Nicolas de Lacaille (the ‘father of southern astronomy’) in 1752, this is a glorious star cluster, bright enough to be visible without optics from southern latitudes. It’s a fantastic binocular object, but it’s best viewed at low-power with a small telescope where it fits in the same field of view as an adjacent star cluster, NGC 2451. At a distance of 3,700 light years, NGC 2477 is one of the richest and densest of open star clusters and looks a little like the loose globular cluster M71 in the constellation Sagitta. The cluster has an impressive 1,900 members and spans about 37 light years. It’s also an ancient cluster, about 1 billion years old, and likely has lost many members since its birth to gravitational perturbations from other stars and star clusters.
NGC 2477 is great fun to observe. Aside from its intrinsic beauty, it tricks the human brain into detecting many shapes and patterns. To some observers, the cluster appears to have arms sticking out from its center, almost like a glittering celestial spider. Others have noted its shape strongly resembles that of an electric guitar.
Just 1.5º northwest of NGC 2477 lies the splendid star cluster NGC 2451. This group of stars has been observed since the 17th century, but astronomers were never sure if it was a true cluster of associated stars. A definitive study in 1994 revealed it is not: these stars are moving in different directions. But astronomers did find 24 stars within 4o of NGC 2451 that are moving together and which likely formed at the same time. These stars, which are 700 light years away, form what’s now called the Puppis Moving Group, which is not the same as NGC 2451, although four of of the group’s members belong to NGC 2451.
While NGC 2451 has been demoted from its status as a star cluster, it remains a delight to observe. The cluster surrounds a single bright yellow-orange star of magnitude 3.6. The star is visible with the unaided eye, and the related cluster forms a faint haze around the star. Binoculars reveal about 15-20 stars spread across 1.5o, some three times wider than the full Moon. The smallest scope shows 20 more stars arranged in the shape of a scorpion with outstretched claws and a tail curving towards the north.
NGC 2451 and NGC 2477 are about 3o northwest of ζ (zeta) Puppis and some 10o southeast of η (eta) Canis Majoris in the dog’s tail. They both lie in the same low-power telescopic field of view as NGC 2451. The two clusters are easily as beautiful, if not more so, than the more famous pair of open clusters M46 and M47, also in Puppis but well to the north. But they are difficult for stargazers north of latitude 45ºN to see well because they are very low, at best, above the southern horizon. The further south you go, the better NGC 2477 and NGC 2451 will appear.
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