Sky & Telescope - June 2020 - 23

Sample some of the delights in the first
comprehensive catalog of nebulae.

L

ike most astrophotographers today, my imaging adventures began on a well-trodden path of celestial showpieces, including the Andromeda Galaxy (M31), the
Hercules Cluster (M13), and the Dumbbell Nebula (M27). I
occasionally revisit these familiar landmarks to see how my
imaging techniques have improved with time. But as my experience and skills developed, I began to look beyond the popular
Messier list in search of new and exciting vistas to explore.
While the Messier and NGC catalogs guide us to many
splendors for imaging with amateur equipment, other, more
obscure compilations are available for those who want to
delve into less familiar deep-sky objects. The Sharpless catalog is one such great resource for imagers in the Northern
Hemisphere who want to embark on an "off-road" adventure.

A LL IM AG ES A RE BY THE AU THOR

A Catalog of Galactic Nebulae
Stewart Lane Sharpless (1926-2013) was a graduate student
at Yerkes Observatory who helped with the calculations used
to define the UBV photometric system that photographically
classified stars according to their colors. This later expanded
into the UBVRI system as photographic emulsion sensitivity in the mid-20th century became more responsive to red
and infrared wavelengths. Sharpless focused on determining
the true structure of the Milky Way Galaxy. In so doing, he
estimated the distances to H II regions and hot, young star
clusters. This work culminated in a paper published in 1952
with Don Osterbrock in which they conclusively established
the spiral nature of our home galaxy.
Sharpless continued his research into the structure of the
Milky Way when in 1953 he took a position at the United
States Naval Observatory Flagstaff Station in Arizona. There
he continued surveying and cataloging H II regions within the
Milky Way. But he didn't use a telescope for this task. Rather,
he pored through the available plates of the ongoing National
Geographic Society - Palomar Observatory Sky Survey (NGSPOSS). This groundbreaking survey, begun in 1949 using the
48-inch Samuel Oschin telescope at Palomar Observatory in
California, was the first to image the entire sky down to a declination of -27° in both blue and red light. Because emission
nebulosity consists primarily of ionized hydrogen, which emits
light at 656.28 nanometers, Sharpless concentrated his efforts
on identifying and cataloging emission nebulae exclusively
recorded on the red-sensitive Kodak 103a-E plates.
First published in the Astrophysical Journal Supplement in
1953, "A Catalogue of Emission Nebulae Near the Galactic
Plane" contained 142 objects. Sharpless continued to add to
his list as new plates became available. In December 1959,
the second and final edition of the catalog appeared; it was
titled "A Catalogue of H II Regions" and lists 313 entries. The
second edition supersedes the first, prefacing each object with
the designation "Sh 2."

Sharpless's goal was to produce a comprehensive catalog of
every known emission nebula north of the southerly declination of -27°, although some objects in the catalog lie farther
south. His quest focused primarily on H II regions, though
his list contains some planetary nebulae and even several
supernova remnants like the famous Crab Nebula (M1),
designated Sharpless 2-244.

Targets for Any Scope
Today, the Sharpless catalog is one of the most comprehensive
lists of emission nebulae in the sky for Northern Hemisphere
astronomers and astrophotographers. It includes both bright
and faint targets - objects suitable for most any telescope.
When I wrote previously about objects that tend to get
ignored in favor of their brighter neighbors (S&T: Oct. 2018,
p. 58), friends noted that I drew heavily from the Sharpless
catalog. They're right - the list includes all the familiar
bright nebulae visible in northern skies, as well as many faint
obscure objects. It's exciting to go after the fainter members
of this list, but you probably have unknowingly shot a few
Sharpless objects already. Here are a few of my favorites.

In the Wings of the Swan
I could spend the whole summer - maybe several - just
imaging the Sharpless objects in Cygnus. This single constellation contains no less than 26 individual targets, including
several showpieces such as the North America Nebula and
Pelican Nebula complex (NGC 7000 and IC 5070) east of
1st-magnitude Deneb that make up Sh 2-117. Just over 3° to
its west lies Sh 2-119, an equally large though faint nebula
punctuated with several small, dark nebulae.
Almost 10° south of Sh 2-117 is the Cygnus Loop,
Sh 2-103, which partly spills into Vulpecula. Sharpless
q FAMILIAR FRIENDS While the Sharpless catalog contains many faint
objects, it was meant to be a comprehensive listing of all emission nebulae visible from the Northern Hemisphere. So it includes many well-known
objects such as M1, the Crab Nebula, designated Sh 2-244 (below).

sk yandtelescope.org * JUNE 2 02 0

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Sky & Telescope - June 2020

Table of Contents for the Digital Edition of Sky & Telescope - June 2020

Contents
Sky & Telescope - June 2020 - Cover1
Sky & Telescope - June 2020 - Cover2
Sky & Telescope - June 2020 - 1
Sky & Telescope - June 2020 - Contents
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