From the April 2017 issue

Hickson groups

By | Published: April 24, 2017
In 1877, French astronomer Édouard Stephan identified the first compact galaxy group. Astronomers now call it Stephan’s Quintet. Nearly two centuries later, Canadian astronomer Paul Hickson compiled a list of 100 such groupings while examining prints created using red-sensitive plates from the Palomar Observatory Sky Survey.

He based each group’s inclusion on its population, surface density, and isolation from other galaxies. While these gravitationally bound gems offer researchers opportunities to study galactic evolution, they also provide captivating views for backyard observers with large scopes.

For me, the real fun begins during the sketching process. The brightest members of any group are the easiest to spot, so the challenge is to locate and identify as many of the others as possible. At that point, we can study their unique shapes and draw them within the star field. I’ll break down the process with two sketches. The first shows Hickson 68, a tight, five-member group near the eastern border of the constellation Canes Venatici.

This sketch of the Hickson 68 group shows (clockwise from the top) NGC 5350, NGC 5354, NGC 5353, NGC 5358, and NGC 5355. The author drew both sketches using a 16-inch f/4.5 reflector on a non-tracking Dobsonian mount with a 12mm eyepiece for a magnification of 152x. She used graphite pencils on white paper and a blending stump. After scanning the sketch, she removed the stray markings and cleaned up the stars with Photoshop.
Erika Rix
To locate Hickson 68, sight your telescope halfway between Nekkar (Beta [β] Boötis) and Cor Caroli (Alpha [α] Canum Venaticorum). The group’s brightest member, NGC 5353 (Hickson 68A), glows at magnitude 11.0 and covers an area 1.2′ by 1.1′. It forms a close pair with similarly sized NGC 5354 (68B), a magnitude 11.4 object.

The largest of the family is NGC 5350 (68C), which glows at magnitude 11.3 and measures 3.2′ by 2.3′. The star field also features a beautiful magnitude 6.5 orange star lying farther to the southwest. NGC 5355 (68D) and NGC 5358 (68E) are the faintest, with magnitudes 13.1 and 13.9, respectively. As a bonus, you’ll be treated to magnitude 10.6 NGC 5371, a conspicuous galaxy that lies 0.5° northeast of the group.
Notice how I placed the brightest stars near the edge of my sketch. I added those first and used them as markers so that when the star field drifted out of view, I could nudge the telescope back to the exact location.

Once I completed the star field, I tackled the galaxies by marking their locations on the sketch with the tip of a blending stump that I lightly covered in graphite. Then, starting with the brightest of the group, I worked my way outward from its core to build its structure with the blending stump, lightening the pressure as I proceeded to create diffuse edges. Because each subsequent galaxy became fainter, I had to use averted vision to tease out their ghostly details.

The author’s drawing of the Hickson 61 compact group shows (clockwise from the top) NGC 4173, NGC 4169, NGC 4174, and NGC 4175. Both sketches have north at the top and west to the right.
Erika Rix
For my second example, I chose to sketch the Hickson 61 group, an alluring little quartet that lies 3° west of Gamfma (γ) Comae Berenices. Magnitude 11.7 NGC 4169 (61A) marks the western corner of the box and shines the brightest. NGC 4173 (61B), at magnitude 13.3, is more challenging to see at the northern corner due to its low surface brightness. The remaining galaxies, NGC 4175 (61C, magnitude 13.5) and NGC 4174 (61D, magnitude 13.6), form a nearly 90° angle along the eastern and southern corners.

Unfortunately, few stars were near the field’s edge for framing. So instead, I used an imaginary crosshair. Two bright stars floated just below the horizontal line and each a third of the way into the field of view from the western and eastern edges. This placed a moderately bright star at the 2 o’clock position, a double star at 3 o’clock, and a faint star at 9 o’clock. Once I had the stars in their places, I used the blending stump to draw NGC 4174 because it was conveniently placed in the center of the crosshairs. I then included the remaining family members by cross-referencing their positions with other stars or to my imaginary clock face.

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