Stellar neighbors

Bright stars can drown out nearby faint objects, making them a challenge to see. But when push comes to shove, those luminous balls of gas act as locating beacons and can provide engaging views when they pair with their neighbors through an eyepiece. I’ve chosen two globular clusters in the constellation Scorpius to illustrate how they do it.

Red supergiant Antares shines brilliantly at 1st magnitude near the head of the scorpion. Lying low in the sky, the star provides a distinct marker to pinpoint the location of NGC 6144. The 9th-magnitude globular lies just 38' farther northwest, reaching 9' across with a low surface brightness. If you find NGC 6144 elusive, nudge your telescope northwest until Antares is just out of view to minimize the star’s glare and make NGC 6144 pop out.

When I observed it with a 4-inch scope at 125x, the globular cluster appeared as a dim glow with a 12th-magnitude star on its western edge. When I used averted vision, half a dozen faint stars flickered in and out of view across its face with hints of granulation. An 8th-magnitude star floated 14' to the south near the edge of my field of view. Depending on your latitude, larger telescopes may reveal nearly a dozen resolved stars against the cluster’s hazy backdrop.

I was at a slight advantage in that my observing site lies at 31° north latitude. But wherever your location, take advantage of clear, dark skies and clean optics (which help reduce light scattering) to optimize contrast. While you’re in the area, make sure to grab a view of globular cluster M4. You can spot it 1° southwest of NGC 6144.

Trailing behind the scorpion’s tail, 3rd-magnitude G Scorpii makes a striking pair with the second globular cluster, NGC 6441. (Note the subtle naming distinction from the previous cluster!) Located a mere 4' east of its neighboring orange-colored star, the cluster shines brightly at magnitude 7.2 and reaches 7.8' in diameter.

Even through a 4-inch telescope, NGC 6441 was an easy catch 2.5° south-southwest of naked-eye open cluster M7, also known as Ptolemy’s Cluster. At 125x, a soft 1' halo surrounded its bright, concentrated core. The halo extended nearly a degree when I moved G Scorpii just outside the field of view. A 10th-magnitude star was visible 1.5' southwest from its center. Through a 10-inch scope, the globular takes on a mottled, slightly irregular shape, with the halo reaching 3' in diameter.

Some optical designs produce coma (distortion) near the edge of the lens. Rather than nudging G Scorpii out of view to reveal more of the cluster’s structure, you can block out the star and reduce its glare by attaching a thin strip of foil at the focal point of your eyepiece, creating an occulting bar.

The purpose of my sketch was to capture both objects as they appeared together with their bright neighbor stars. But another option is to include any additional details of NGC 6441 once G Scorpii is hidden from sight. Whichever method you choose, be sure to note the view your sketch represents.

Questions or suggestions? Feel free to contact me at erikarix1@gmail.com.