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Creating unresolved stars

Celestial sketcher Erika Rix explains how to sketch unresolved stars in globular clusters.
RELATED TOPICS: ASTROSKETCHING | SKETCHING
Erika-Rix
As the saying goes, there’s a tool for every job. So if your next observing session involves drawing globular clusters, you might want to add a tortillon to your sketch kit. It’s a hollow cylinder of tightly wound paper with a tapered point at one end that, similar to a blending stump, you can use for drawing or smudging marks.

While sketching, you employ the chamois-like blunt tip of a blending stump to render the cluster’s soft glow. A tortillon’s firm, sharp tip, on the other hand, is ideal for stippling the unresolved stars within it. Stippling is a type of shading that uses small dots. To stipple, lightly touch or tap the tortillon to the paper as many times as necessary to produce the effect you want. I’ll demonstrate by looking at two attractive globulars in the constellation Delphinus the Dolphin.

Our first target, NGC 7006, is part of the Milky Way’s outer halo. At 135,000 light-years away, it’s one of our galaxy’s most distant globular clusters.

To find it, point your telescope 3.5° east of the 4th-magnitude double star Gamma (γ) Delphini. NGC 7006 glows at magnitude 10.6 and has a modest diameter of 2.6'.
NGC 7006
To create this sketch of NGC 7006, the author used a 16-inch f/4.5 reflector with a 12mm eyepiece that gave a magnification of 150x. For both sketches shown on this page, she used an observing template printed on white paper using a No. 2 graphite pencil, a felt-tipped superfine artist pen, a 0.5mm mechanical pencil, a tortillon, and a blending stump.
Both sketches: Erika Rix

Through a 4-inch telescope at low power, the cluster looks faint and unresolved, resembling a planetary nebula. A 10-inch scope reveals central brightening with a soft, symmetrical outer glow. The core appears mottled, still with lots of unresolved stars, when you view through a 16-inch scope at 150x.

After rendering the cluster’s glow with a blending stump, rub the tip of a 3/16-inch (5mm) tortillon through a patch of graphite you’ve created outside your sketch circle. Hold the graphite-loaded tortillon upright over the sketch, and gently stipple the unresolved stars starting at the cluster’s center. Adjust the pressure as needed for brightness.

Our second target is a cluster that sports resolved stars. Drop 4° south of magnitude 4.0 Epsilon (ε) Delphini to locate NGC 6934. It’s much closer to Earth than NGC 7006 — a mere 50,000 light-years away. It shines at magnitude 8.8 and measures 7' across. You’ll identify it easily by the magnitude 9.2 star positioned 2' west of its center.
NGC 6934
For this drawing of NGC 6934, the author used a 16-inch reflector with an 8mm eyepiece that gave a magnification of 225x. She scanned both sketches and then inverted them using Photoshop.
Seen through a 4-inch scope, NGC 6934 appears symmetrical with a bright, concentrated core. Through a 10-inch scope, the cluster extends 3' and appears mottled with unresolved stars. At 225x with a 16-inch scope, the cluster becomes slightly elongated north to south with a dozen or more resolved stars, including a strand of suns running northeast to southwest within its southern outer halo.

Because NGC 6934 has an intense spackling of both unresolved and resolved stars, you’ll want to build up the stippled layers with the tortillon before plotting the resolved stars with a graphite pencil.

Tortillons are available in several sizes, and they work well when you create detailed areas in your sketches. For the best results, sharpen and clean them often with a block of sandpaper, and practice your stippling technique before sketching live at the eyepiece.

Do you have a sketching question? Contact me at erikarix1@gmail.com.
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