S marks the spot
This week’s small-telescope target is the S Normae Cluster, also known as NGC 6087 and Caldwell 89. This open cluster lies in the southern constellation Norma
4.2° west-northwest of magnitude 3.8 Eta (η) Arae. You can also find it 1.3° east of the much fainter star magnitude 5.6 Iota2
If you saw this magnitude 5.4 cluster in a different part of the sky, it would be easy to see without optical aid. However, the richness of the star field, coupled with the magnitude 5.6 star SAO 243509 just 0.4 to the west, makes NGC 6087 a tough naked-eye catch.
Through a 4-inch telescope at 150x, you’ll see some three dozen stars, the brightest of which shine at 8th magnitude. Note the cluster’s rough triangular shape. It measures 12.5' across. A nice line of stars trending north-south sits at the southwest edge. You may see four, five, or six, depending on your scope’s aperture.
Near the center, the brightest star in NGC 6087 is the Cepheid variable star S Normae, which was only recently proven to be a member. It glows with an orange hue and varies between magnitudes 6.12 and 6.77 over a period of 9.75 days. You’ll easily identify this star by its color through binoculars or a finder scope.Ring around the galaxy
This week’s first large-telescope object is barred spiral galaxy NGC 6217 in Ursa Minor
. This is a target that rewards large-telescope owners. NGC 6217 is a nearly face-on, ringed galaxy that hides its details from small scopes because it floats through space some 80 million light-years away.
It lies equidistant from magnitude 4.3 Zeta (ζ) and magnitude 5.0 Eta (η) Ursae Minoris, 2.5° east-northeast of Zeta and the same distance north-northeast of Eta. Through an 8-inch telescope, you’ll see a magnitude 11.2 oval twice as long as it is wide. That’s not the galaxy’s true shape, just the 3.3'-long part you can spot through small instruments.
A 20-inch or larger scope begins to fill in the gaps. At magnifications above 300x, you’ll spot the bar and the beginnings of both spiral arms. The northernmost (and definitely brighter) arm curves eastward, and the southernmost one curves toward the west. The arms actually continue and form a complete ring around the central region, but that lies outside the realm of most amateur telescopes.Look closely
This week’s second large-scope target is planetary nebula NGC 6058 in Hercules
. You’ll find it 2.8° southeast of magnitude 4.6 Chi (χ) Herculis. Through telescopes with apertures smaller than 8 inches, you’ll see the triangle of stars that surrounds NGC 6058 before you see the planetary. Magnitude 9.0 SAO 45881 lies 6' northeast. Magnitude 9.3 SAO 45874 sits 5' northwest. And you’ll see magnitude 10.7 GSC 3064:1181 less than 4' south.
NGC 6058 is neither bright nor big. It glows softly at magnitude 13.0 and measures 42" in diameter.
Through an 8-inch scope at a magnification above 200x, the planetary appears faint and evenly illuminated. Its tiny central region appears a bit brighter.
A 14-inch instrument will reveal the magnitude 13.9 central star surrounded by a small halo. A nebula filter (especially an Oxygen-III) helps a lot.