Sky & Telescope - September 2021 - 56

BRAD EHRHORN / ADAM BLOCK / NOAO / AURA / NSF
SEPTEMBER 2021 OBSERVING
Suburban Stargazer
When Charles Messier scrutinized
this " non-comet " in 1780, he considered
it a faint nebula. No wonder. M72
is a class IX (very loose) globular whose
brightest members are 14th magnitude.
In my 10-inch at 64×, M72 is a pale
blob next to the above-noted guardian
star, HD 198885. However, 169× reveals
a larger patch that's teasingly granular.
At 218×, my averted vision picks up
several peripheral pinpoints of light.
Not bad!
The Y in the Sky
A mere 78′ east of M72 is the somewhat
mysterious and hugely underwhelming
M73. Often designated an open cluster,
M73 is actually just a Y-shaped quartet
of unrelated stars. The four feeble
components have magnitudes of 10.4,
11.3, 11.7, and 11.9. The Y spans 1′ and
at low magnification resembles an outof-focus,
9th-magnitude star. M73 is
arguably the least inspiring entry in the
Messier canon.
To capture the wee Y, I start at M72
- not the greatest signpost, but still
useful. I throttle down the magnification
to 64×, nudge the scope over to the
guardian star, HD 198885, then head
directly eastward 1° to 9.3-magnitude
HD 199524. I stop there, rev up to 169×,
and creep cautiously an additional ¼°
to the paltry prize.
Watery Treats
Object
Type
β Cap
α1+2 Cap
α1 Cap
α2 Cap
M72
M73
NGC 7009
Double star
Double star
Double star
Double star
Globular cluster
Asterism
Planetary nebula
like protrusions, or ansae, extending
east-west from its 35″-wide disc. The
dim and delicate ansae were discovered
in 1850 by Irish astronomer William
Parsons, who awarded NGC 7009 its
evocative moniker.
I approach NGC 7009 from
FUZZY SATURN English astronomer William
Herschel discovered the remarkable Saturn
Nebula, NGC 7009, in 1782. NGC 7009's
threadlike ansae earned the nebula its popular
name. The object is roughly 3,000 light-years
from Earth.
Messier described his 73rd object
as a cluster containing nebulosity. My
backyard 10-inch at 169× guarantees a
fuzz-free Y, but M73 is a tougher catch
in smaller optics. My little refractor
collects precisely one of the four stars.
My 7.1-inch (180-mm) f/15 MaksutovCassegrain
operating at 113× coughs
up two stars. Doubling to 226× delivers
the full letter. I recommend high power
in city conditions to darken the sky
background and improve the resolution
of this intricate " cluster. "
Deep-sky dud? Meager celestial
curiosity? Obscure multiple star? Whatever
- ya gotta get M73 to conquer the
Messier catalog!
The Nebula Called Saturn
Less than 2° northeast of the Messier
set is the aptly named Saturn Nebula,
NGC 7009. This superb 8.0-magnitude
planetary nebula features thread4.5-magnitude
Nu (ν) Aquarii, just
above the Capricornus wedge. The
nebula is situated 11
/3° west of Nu.
Using my Mak-Cass at 90×, I sweep
westward from Nu until I spot a tiny
bluish disk. At 113×, the Saturn Nebula
becomes oval like its namesake planet,
though to me the aqua object invokes
Neptune more than Saturn. The Neptune
impression is even stronger in my
10-inch Dob at 128×. The luminous
disk fades noticeably at higher power,
yet I still can't perceive the 12th-magnitude
central star lurking inside.
What about the ansae? Detecting
them requires steady seeing, high
magnification, averted vision, and likely
a narrowband nebula filter. Employing
the 10-inch at 218×, I can occasionally
glimpse the ansae - particularly
the slightly thicker thread west of the
disk - with the aid of an Ultra-High
Contrast (UHC) filter. A doubly ionized
oxygen (O III) filter produces greater
contrast but no extra detail.
In truth, filters aren't needed to
Mag(v)
3.2, 6.1
3.7, 4.2
4.2, 9.6
3.7, 10.5
9.2
8.9
8.0
Size/Sep
205″
381″
47″
153″
6.6′
1.4′
35″
RA
20h 21.0m
20h 18.0m
20h 17.6m
20h 18.0m
20h 53.5m
20h 58.9m
21h 04.2m
Dec.
-14° 47′
-12° 32′
-12° 30′
-12° 32′
-12° 32′
-12° 38′
-11° 22′
Angular sizes and separations are from recent catalogs. Visually, an object's size is often smaller than
the cataloged value and varies according to the aperture and magnification of the viewing instrument.
Right ascension and declination are for equinox 2000.0.
appreciate NGC 7009. Indeed, an O III
nullifies the nebula's lovely aqua color
and extinguishes the surrounding field
stars. Adding (or changing) a filter
might also alter the focus. A good way
to achieve a sharp filtered image at high
power is to retreat to Nu Aquarii and
focus on that bright beacon. One more
reason to know your Nu.
My smallish, low-in-the-south tour
region is forever submerged in a " milky
way " of urban light pollution. Those
of you south of the 49th parallel will
do better, provided your city sky isn't
milkier than mine. So, knock off the
Sea Goat doubles and snap up those
minuscule Messiers.
¢ When he was a teenager, budding
deep-sky observer KEN HEWITT-WHITE
finished second in a Messier contest.
The objects he missed? M72 and M73.
56 SEPTEMBER 2021 * SKY & TELESCOPE

Sky & Telescope - September 2021

Table of Contents for the Digital Edition of Sky & Telescope - September 2021