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(This article originally appeared in the Q2 2000 STAR news letter. -Ian)
Indulgence
In Obscurity Part III - The Summer Planetaries
by
Eric Honeycutt
Ever
since last October, I have been waiting in earnest for the Summer
Milky Way to present itself in an early enough hour to spend
appreciable time hunting down the many wonderful and obscure objects
that lie behind its structured and highly detailed, star-strewn belt.
Now, of course this isn't to rule out the many wonderful objects
that correspond to the other seasons - not at all! It’s just that
there is so much to observe in the Summer sky that it seems that I
never even have enough time to revisit the once-observed objects,
much less the many new and intriguing ones. I do want to establish a
criteria for this article however. That criteria is very dark skies.
Here in Raleigh area, the summertime is the worst for observing deep
sky objects. Mainly because of the haze that tends to always turn a
once-blue sky into a milky-white veil of transparent muck. For
planets, this works out pretty good because haze usually results in
steady skies - a real rarity for this area! However, there are many
great dark sky locations within a three or four hour drive that will
usually negate the hazy aspects of summertime. Thus, I encourage
everyone who reads this to seek out the dark skies of the North
Carolina mountains or better still, the darker Highlands area of
Southwest Virginia. Many times, you will get above the haze layer.
Also, the humidity is lower and the nights are much more comfortable
and quite transparent. Though the objects that I will present below
will be able to be observed form our Big Woods site, true
appreciation for these fine gems shall only be realized when a
transparent night envelops you.
There
are some really fine objects that cover the entire spectrum of deep
sky targets that grace the summer sky. However, I am going to take
this opportunity to highlight my favorite type of deep sky object
("dso" for short): PLANETARY NEBULAE! I love these stellar
tombs so much that many times I will go out and observe nothing but
these diverse dsos for the entire lunar absence. Their size, shape,
variety, and dynamics always compel me to make them a top priority on
my observing list. Therefore, I am going to offer a detailed array
of the many fine Planetaries that are tucked away behind the familiar
constellations of the Summer sky. Another reason is to try to get
you really hooked on these little buggers (sometimes big buggers) so
I can stop boring some of my RAC colleagues with my incessant
rambling about them.
First
stop: Scorpius. There are many great planetaries ("Pn" for
short) in the Scorpion that warrant recognition. This region of the
sky is home to an interesting catalogue of Pn known a the "Wray
Catalogue", among others. As a point of interest, the majority
of Wray Planetaries have actually been found to be questionable in
nature. They were mostly discovered and/or rediscovered back in 1966
while James D. Wray was working on his PhD thesis concerning the
compilation of a catalogue of H-alpha emitting objects of the
southern hemisphere. He used the objective prism spectroscopic
survey plates taken by Karl Henize, which primarily picked up a ton
of H-alpha emitting early-type stars. This is the reason that the
majority of Henize and Wray designated Pns are quite small in angular
size (typically <30"). It seems that if an H-alpha emitting
object picked up on the plates had any other odd emission features
that were not known to emission-line stars of that time, they were
automatically assumed to be Planetary Nebulae. Of course, the number
of varying types of emission line stars and symbiotic systems has
grown so much since then that the majority of these previously
misclassified Pns have been debunked. There are many good
Wray-designated targets out there however that are true Planetaries.
Wray
16-314
is a Pn that should be visible under dark skies with a low southerly
horizon. Its declination lies at -39 so you must go low for it.
However, a 10" scope and the all powerful O-III filter should
reveal a 13.8 (P) mag disk at 8.6" in diameter. I did not
glimpse detail (possibly because I was looking through so much air
since the scope was lower than horizontal) but I did "blink"
the Pn into confirmation. Remember, photographic magnitudes of Pn
are not to deter an attempt at observation. Usually the visual
magnitude will be one to even two magnitudes brighter. Also, an
observation of an object that has only recently been catalogued is a
treat. The added benefit is to also have in your notes, a record of
objects that many amateurs have never even heard of, much less
observed. It does do something for the morale of someone who may be
experiencing astronomical burn-out that I have talked about in an
earlier article. Anyhow, the coordinates of Wray 16-314 are: 17h 50m
44.3" and -39° 17' 24".
Another
Wray-designated object also happens to be a relatively well known NGC
object. Wray
16-259
(aka: NGC 6302) is also known as the "Bug Nebula) because of its
strange, unusual appearance that resembles a beetle or a butterfly
(whichever "ink blot" you prefer). High power and an O-III
should be used to observe this one. It is roughly 45" in length
and has a very bright center that should not be mistaken for a
central star (CS magnitude is at an impossible 21.1v). The bright
central core is flanked by two extensions that form a horizontal
hourglass shape. This one is easily visible in an 8" scope with
an O-III. It lies halfway between Shaula (the inner-most stinger
star) and Mu Scorpii at: 16h 51' 52.24" and -38° 02' 50.6".
Another great Wray Pn that also is a NGC object is: Wray16-267
or NGC 6337. This has also been dubbed "The Cheerio Nebula"
because of its obvious ring structure in photographs. This one is
visible in an 8" scope as a relatively bright disk with a
seemingly darker center. Dark skies, medium to high power, and an
O-III filter should reveal the annular structure with close
scrutinizing and averted vision. It is 49" in diameter and has
a photographic magnitude of 11.9(P). A magnitude 12 star is
superimposed on the NE side. Larger instruments easily reveal the
central hole at high power. You will find this one at: 17h 22' 15.6"
and -38° 29' 01".
The
last Wray Pn that I want to address is Wray
15-1607
and also has an IC designation (IC 4637). This Pn is at a
respectable 13.6 photographic magnitude and is 18.5 in size. High
power and an O-III reveal an oblong or oval disk with relatively even
brightness. A pretty starfield compliments this Pn. This Pn also has
a very bright CS at 12.5. Coordinates are: 17h 05' 10.4" and
-40° 53' 07".
Now
on to Sagittarius we go for a really nice Pn by the name of "The
Little Gem".
NGC
6445
is named this because of its nice green color, especially when viewed
with an O-III filter. Its photographic magnitude is at 13.2 and it
has a size of 33". In an 8", the Pn is roughly as bright
as the nearby globular cluster NGC 6440 which lies 20+ minutes to
the south. Medium to high power should begin to reveal a central hole
that is not truly round. Higher apertures show this object to be
elongated. At 508x in my 22", the Pn's shape takes on the shape
of a box with an elongated central hole. No CS is visible as it
shines at 19.0 visual magnitude. This one can be found at: 17h 49'
15.1" and -20° 00' 34". Another worthy Pn in Sagittarius
is NGC
6818
and it has an appropriate name of "The Little Gem" because
it is similar to the Pn listed above, just smaller. This planetary
is 48.0" and shines at a bright 9.9 photographic magnitude. An
8" scope will reveal a bright bluish disk that is nearly the
same color as the Blue Snowball (NGC 7662) in Pegasus or The Saturn
Nebula (NGC 7009) in Aquarius. You will only observe a disk about
20" in diameter. See if you can discern the slight darkening
contrast in the central region that is the central hole. A 12+ mag
star lies just to the NW and SW. You will not see the 16.9 CS. In
larger apertures a faint outer halo can be seen which justifies the
larger 48" diameter. In my 22", the Pn appeared greenish
and very bright as well as concentric in relation to the central hole
which appeared round but small. It reminded me of a bagel that had
been stepped on and flattened. The coordinates for this object is:
19h 43' and 57.8" and -14° 09' 11".
A
quick stop in Ophiuchus will lead us to a really interesting bipolar
nebula known as Minkowski's Butterfly. Minkowski
2-9
is a superb object that requires high power to observe best. If the
seeing will allow, try to use powers of 300x or more. The Pn is 30"
x 60" and has a magnitude of 13.2. The CS shines at 15.6 but is
visually much brighter. It can be seen in a 12" at high powers.
On each side of the Pn are two flattened extensions with greater
brightness toward the CS, and fading as they extend. A separation
between the CS and the two extensions are evident in larger scopes.
This one is well worth finding for the advancing amateur.
Lets
go to Scutum now for a very large and very overlooked Pn. This one
is designated as IC
1295.
I strongly recommend an O-III for this one because of its
large size (90") and its subsequent low surface brightness but
it definitely responds well to the O-III. Once you observe this one,
you might be surprised that you have never heard of this one before
as I was (that is, unless you have heard of this one before...).
This great object lies very close to globular cluster NGC 6712. An
8" will reveal a large, circular haze that is grey in color. Two
faint stars can be detected on the SW edge of the object. In larger
instruments, and medium to medium-high powers reveal a distinct
"owl-eye" pattern within the diffuse disk. It is very
impressive in apertures of 12" and larger. CS is visible at
15.5 if your scope can accommodate that magnitude, just remove the
O-III filter first! Coordinates for
this showpiece are: 18h 54' 37.0" and -08° 49' 37".
A
constellation that has a plethora of Pns lies just above Scutum. It
is the constellation
of Aquila. There are many really great planetaries here that could
make for an entire article in itself. However, since I am limited to
article-length instead of thesis length (despite what my editor
says), I will just pick out a few that really are stand-outs. The
first one is NGC
6751.
This neat little Pn has a distinct annular structure that surrounds
a 15.4 mag Central Star. However, the CS apparently is a wide
ranging variable as it has been glimpsed in 8" apertures and it
certainly was not 15.4 mag in my 22". The ring structure does
need a transparent yet steady sky (an oxymoron here, I know) but it
is possible. The color is green. What is so neat about this Pn is
that there is a fantastic red variable (V Aquilae) that
lies to the NW just under 2' away. This is a very red star that is
quite impressive for the observer who doesn't expect it to be in the
starfield. Do not forget to check it out if you have not ever
observed this one. I know that many from the RAC have observed it in
the 22" and were taken back with the intense red color. The
coordinates for NGC 6751 are: 19h 05' 55.5" and -05° 59' 32".
Now
get ready for a favorite NGC Pn of mine. This one is NGC
6781.
This Pn is a beautiful and very large 1.8' disk. The starfield is
rich and the Pn really stands out with an O-III. It seems to be
missing a "bite" that has been taken out of its northern
edge and has a central darkening that is apparent with averted vision
in a dark sky. In larger apertures the central darkening is readily
apparent and even gives the impression of a hole that is not truly
round. If you have never observed this one before, do not pass it
up! Coordinates are: 19h 18' and +6°33'.
Onto
a really cool Pn now that is far underrated in my opinion. NGC
6804
is a nice, 35" disk that stands at 12.2 (P). If your seeing
allows, try to use powers of 250x to 300x. With this power, don't
use an O-III. In my experience, you will see more detail. If you
can't get above 200x, use the O-III. At first, you will probably
only see a disk. But higher powers and concentration will begin to
show a conspicuous oblong or football shape, central hole. It
reminds me of NGC 3242 "The Ghost of Jupiter" though not as
pronounced. See if you can spot the 14th+ mag CS. If so, try to
spot the 2 faint stars that are placed on either side of the CS. If
that is obtainable, you are looking at a real treat: a bright disk,
an inner, elliptical envelope,
that has a chain of 3 stars that run right through the center of the
planetary. In larger instruments, you will see this much easier and
it is a great sight! Coordinates are: 19h 32' and +9°13'.
Lets
go to another area of the sky that is well known for its abundance of Planetary
Nebula. This is the Cygnus region. There are many diverse Pns here
that can show a wealth of appreciable detail under the right
conditions and experience level. But first let us analyze a really
neat object known as "Cambell's Hydrogen Star". The
designation is PK64+5.1.
Its visual magnitude is 11.3 and has a small diameter of 7.5".
It is a "Class 4" Pn that has a very bright and distinct
orange CS that shines at mag 12.5. The bright star makes it
difficult to observe the faint outer disk but it is discernible. I
have found that an O-III does not help a whole lot on this one but
maybe it was the orange-red star that keeps distracting me. Use the
highest usable power to see the circular haze around this unusual
object. The Pn is also designated Henize 2-438. Coordinates are:
19h 34' 45.2" and +30° 30' 59".
The
next Pn in Cygnus is one that is surrounded in controversy. Not
because of being misclassified or because of strange properties that
is has. No, it is because of its name. You see, in my 22", I
observe NGC
7008
and to me, it looks exactly like a fetus in profile (or as some
English snob tried to straighten me out on IAAC chat - "feotus").
I would not call it this if it even began to resemble anything else
but I as well as others, have totally agreed that the shape of this
Pn looks exactly like a child in the womb. I know that this isn't a
fancy name like "Minkowski's Butterfly" or "The Ghost
of Jupiter" but come by my 22" sometime this summer and I
will show you what I mean. The Pn shines at 13.3 (P) magnitude and
it is 86" in diameter. The CS is relatively bright at 13.2. It
responds very well to an O-III filter and the color is almost a
pearl-white. The CS is where the umbilical chord should be; a faint
star is there for the eye; and 2 brighter stars are where the feet
are. There is a wealth of detail at higher powers if you take some
time to observe it. The shape is well, fetus shaped! That's all I
can say. What do you think? You find this Pn at: 21h 00' 32.8"
and +54° 32' 36".
Next
we go to NGC
7048,
a 1' x 1' oval of grey-white light. You can use low to medium powers
to initially find and observe this one but pump up the power to try
to detect some structure. "Owl eyes" can be seen at higher
powers and s few faint stars touch the NNE and NNW edges. This is a
really great object. Coordinates are: 21h 14' 15.2" and +46°
17' 19".
Another
Cygnus favorite is NGC
6894.
Be sure to use an O-III on this one as its surface brightness is
low. It had a photographic magnitude of 14.4 and is 40" in
diameter. A central hole might be visible to you if you have the
dark skies and use averted vision. Annular structure really begins
to show itself pretty well at higher powers. SW and NW sectors are
slightly brighter. No CS visible as it burns at 18.1 apparent visual
magnitude. This is a great ring if you can observe it. Coordinates
are: 20h 16' 24.0" and +30° 33' 55".
My
final object for this "Indulgence" series is a neat little
Pn with the designation NGC
7027.
This is a 10.4 (P) magnitude, 14" object. With an O-III and
medium power, you should find it to be quite bright with a green
color. The Pn seems to be stretched like NGC 6543 ("The Cat's
Eye") with the western portion being much brighter than the rest
of the nebula. At high powers, see if you can detect a box-like
structure that has a distinct division by a dark lane. I have not
been able to detect the CS which shines at 16.2, but seeing was bad
when I observed this one. The coordinates are: 21h 07' 10" and
+42° 14' 10".
One
thing is for certain, you will now have many Planetary Nebulae to
observe this summer if you take the time to get out and try them.
You will be pleasantly surprised at a lot of these great dsos. It
goes without saying that there are even many other great Pns such as
NGC 6828 (The Blinking Planetary) in Cygnus, NGC 6543 (The Cat's Eye
Nebula) in Draco, Abell 50 in Draco, NGC 6210 in Hercules, Abell 46
in Lyra, Abell 62 in Cygnus, Abell 70 in Aquila, NGC 6369 (The Little
Ghost) in Ophiuchus, NGC 6905 (The Blue Flash Nebula) in Delphinus,
and IC 5148/50 in Grus. I hope
you too can find the sheer elation and indescribable enjoyment in the
death of giant red stars as I do. Good luck!
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