Ah Pegasus -
For naked eye observers, the great square of Pegasus is the dominant
feature in northern hemisphere skies in the fall. Ironically,
the stars that make up the "Great Square of Pegasus" are
not all in Pegasus - the NE star is Alpha Andromedae. Alpha
Andromedae
was - at one point - shared between the two constellations, but when
the official constellation boundaries were drawn up the great square
lost one of it's cornerstone to Andromeda - Alpheraz.
No matter -
Andromeda is
happy to
lend it back as needed.
Pegasus covers 1,121 square
degrees of sky - the seventh largest constellation overall - yet
because of it's position overlooking intergalactic space, for small
scope owners, it's relatively devoid of bright objects. It
you are
lucky enough to own a moderately large scope (8 inches and up), then
Pegasus will keep you busy for
a very long time.
The winged horse
Pegasus is a fairly content creature
- especially
considering only his front half made it into the night sky - and upside
down at that... If you have problems seeing a horse
in this constellation, trust me, you won't be alone. But give the
following a try. First, if you are in
the northern hemisphere, stand on your head. Then reference
the picture to the left. Note that south is now up (and should
match your inverted view).
The green is his neck and head, the red - his body (sans wings)
and the blue - his front legs out stretched in
full gallop.
Historically, we can trace Pegasus back to the ancient greeks - Pegasus
was the offspring of Medusa and sprung
from her neck after she was slain by Perseus. And we think
sci-fi / fantasy is a "new invention" - Gotta love those
ancients...
I first was introduced to the concept of Pegasus as a test for
naked eye limiting magnitude (NELM) by Walter
Scott Houston
(Deep Sky
Wonders).
Houston notes that by
counting the stars visible inside the great square you can determine
your limiting magnitude. He writes that at mag 6, you should be
able to see 13 stars inside the square. I thought it might be fun
to take that a little further. I'd suspect that many readers of
this column probably have skies worse than mag six.
|
NELM
for # of Visible Stars In
The
Great Square of Pegasus
30
|
Mag 6.5
|
17
|
Mag 6.25
|
13
|
Mag 6
|
9
|
Mag 5.75
|
7
|
Mag 5.5
|
5
|
Mag 5.25
|
4
|
Mag 5
|
3
|
Mag 4.75
|
1
|
Mag 4.5
|
0
|
Mag 4 (or
worse)
|
|
Now you might want to check your NELM when Pegasus is in several
different locations - if your
site
is like most, the NELM will increase as Pegasus gets closer and closer
to the meridian - the imaginary line that passes from north to south
directly through the zenith (zenith being a point directly
overhead). Depending on sky glow, humidity, local light pollution
and other effects, your sky may actually be much darker in certain
areas than others - something good to be aware of when planning
starhops.
Lets take a quick look at the three cornerstones of the Great Square
that actually lie in Pegasus. Starting with the SE corner and
proceeding counterclockwise around the square we have:
Gamma Pegasi (Algenib)
at mag 2.83, it's the third brightest star in the constellation
pegasus. Hipparcos satellite data places it at 333.15 light years
away. Thus
the light you see tonight was leaving this star around 1670. To
give
you a taste of current events in 1670: The Hudson's bay company was
formed, the Kings of England and France met in secret to sign a treaty
ending hostilities between their nations (at least for a while),
Phosphorus was discovered, Senegal was settled by the French, a Cossack
rebellion in the Ukraine was crushed, and Henry Morgan captured Panama.
Alpha Pegasi (Markab or Marchab)
is mag 2.49, and is placed at a mere 139 light years away (again
Hipparcos) - the closest
of the three. This means the photons entering your eyes tonight
left around
the year 1864. This year saw the continuation of the American
civil war, Montana becoming a territory, 70,000 people killed as
a cyclone swept through Calcutta, James Clerk Maxwell discovered
microwaves, Emil Nobel was killed while experimenting with
nitroglycerin, and the birth of both Jacob Astor and Toulouse-Lautrec.
Beta Pegasi (Scheat) is
around magnitude 2.4 and lies 199 light years away. In 1804,
world events saw the end of French rule in Haiti, the start of the
first Barbary War, the first steam locomotive, the Louisiana purchase,
the beginning of the Louis and Clark expedition, Boneaparte crown
himself emperor (the first emperor of France in 1000 years),
Spain declared war on Britain, and the discovery of the asteroid
Juno by German astronomer K. L. Harding. Ponder the the state of
humanity as you gaze into the past at the deep orange of Scheat this
evening.
Astronomy can be your own personal time machine - but these are among
the closest targets we will visit tonight. With one
exception, the rest of our stops will be in the depths of intergalactic
space.
- Trivia - Alpha Andromedae (Alpheraz) was also known as
Delta Pegasi until Pegasus lost that celestial tug of war...
Not totally devoid of bright objects, Pegasus contains three Caldwells
and one Messier. While these objects are visible in
fairly small scopes (especially M15), you're really in luck if you've
already succumbed to that dreaded disease of both the mind and the
pocket book - aperture fever and purchased a scope 8
inches or larger.
Now without further adieu, let's continue our tour....
NGC 7814 (also Caldwell 43)
First stop
lies about two degrees west north west of Gamma Pegasi - the 10.5
magnitude galaxy NGC 7814. While this is a spectacular galaxy in
pictures, small scope owners should content themselves with
looking for a small tuft of cotton floating in intergalactic space.
If you have a larger scope, be careful that you don't stop at NGC
14. This mag 12 galaxy might confuse you for a minute as it lies
almost exactly between 7814 and gamma, but at mag 12.2 (compared to mag
10.8) it's a fair amount dimmer.
While it's been seen in telescopes as small as 4" from very very dark
sites (I should note not by yours author - at least of this writing), I
find that I usually need something
more along the lines of 6-8 inches to pull it out. Ironically,
the Night Sky Users Guide lists 7814 as a binocular object. I
have my doubts, but must confess I've never actually tried.
Perhaps someone out there would be so kind as to let me know the
smallest aperture you can grab this faint galaxy through - can you
indeed
spot it in binoculars? As with all faint galaxies, try to
minimize light pollution when searching - even the moon can seriously
affect your ability to see it. Once you find it, try alternating
between high and low powers and note the results.
One should be aware, that a galaxies listed magnitude is not
necessarily the best indicator of it's
visibility. The magnitudes listed
may actually be, and often are photographic magnitudes; a film emulsion
has
different sensitivities than the human eye. Additionally, the
magnitude describes the brightness of the object as if it were
collapsed into a point - like a star. Galaxies have their
light spread out over a larger area of the sky. A better
indicator of visibility is a galaxies surface brightness - usually
measured in magnitudes per square arc second - a brightness per unit
area
indicator. Still, don't ever let an objects low magnitude or
surface brightness ever stop you from looking for it - you just may
surprise yourself!
NGC 7479 (also Caldwell 44)
And now onto one of
my favorite galaxies in Pegasus - NGC 7479
Continue along the bottom of the great square until you hit Alpha, then
move about three degrees south.
This is a very nice barred spiral galaxy, and serves as an excellent
example of what larger aperture and darker skies can do - showing a
progressive increase in detail with each size.. Smaller scopes
should look for a straight wisp of light that may resemble an edge on
or lenticular galaxy - larger scopes will begin to show
progressively more detail. The view to the left greatly resembles
the view through my 18" at ~200x from a dark site - a
star like core, an obvious bar, and two well defined arms.
Steven James O'Meara showcases a drawing in The Caldwell Objects that
also
looks remarkably like the DSS image shown above. Amazingly he was
observing with a "lowly" 4" scope. Granted
it was from the pristine skies of Hawaii, but still it goes to show
just what dark skies (and excellent well trained eyes) can achieve.
NGC 7619 and
the Pegasus I Galaxy Cluster
Right off the bat,
I'll tell you this target's not really all that fair. It really
should have been a
challenge object, but I wanted to reserve this months challenge for
something special.
So don't be dismayed if you don't find it, but if you do - let
your
mind encompass exactly what you are seeing. This is a fairly
loose galaxy cluster located about 250 million light years away.
That's right - 7619 is the center of the Pegasus I *galaxy* cluster.
If you look carefully, you may find galaxies scattered across a couple
of fields of view, but NGC 7619 (center) and NGC 7626 (left) are two of
the brightest members and thus you have the greatest chance of picking
them up..
While I've not quite managed them in my 4" scope, my good friend
Ron
B(ee) has grabbed both of them and one other -
NGC 7611. An impressive feat for a 4" scope for sure.
Turn as
much aperture as you have to this galaxy cluster and see what you can
see. Take notes and then compare to the DSS picture above.
Be sure to take time the time to let your eyes fully dark adapt.
You might try viewing
with your head under a dark towel or sheet to shield all extraneous
light, and use averted vision while scanning the field and moving the
scope very
slowly - (we tend to pick up dim objects slightly better if they appear
to be in motion). Who cares what the neighbors think?
M15
I frequently like
to start my sessions with a
spectacular object like M15, move on to the faint fuzzies, then after
my eyes are used to searching for faint targets - finish up with a
couple of bright showpieces. After all those dim faint
galaxies, M15 comes across as a sensation to the eyes.
From a dark site, M15 is a naked eye object, and an easy catch in
almost any size binocular. With my 4" refractor, I find
100x and just
under a degree field to be the perfect view. As I increase
in aperture, I like to increase power. When I get to larger
scopes, I find I'm fond of busting this globular apart with 400 and 500
power!
How much power does it take with your scope before the cluster appears
resolved across the face? Does it ever?
M15 contains a hidden prize for the careful observer - it's very own
planetary nebula! Pease 1, found in 1928 by Francis
Pease, was the first planetary nebula ever discovered in a
globular cluster. Pease 1 is so far away it appears stellar at
any power in a amateur telescope - the only real way to identify it is
to star hop to the
location indicated and then pop in an OIII filter. Because
Planetary Nebulae are high in OIII, the rest of the stars in the field
dim, and the one that remains bright - is the planetary nebula.
While most observers who record Pease 1 are looking for it with large
aperture (12" and up), there are reports of it being caught with scopes
as small as
8". If Pease 1 interests you, then be sure to visit the site
listed in "Additional Reading".
NGC 7331 -
The Deer Lick Group (also Caldwell 30)
Welcome to one of the best galaxies in the fall sky (and incidentally
another galaxy group). This galaxy has
been used in astronomy texts as
an image of what the Milky Way would like like from a similar vantage
point.
NGC 7331 is without doubt, the dominate member of the "Deer Lick group"
- which apparently received it's name in commemoration of an excellent
night
of
observing at Deer Lick Gap just off the Blue Ridge Parkway in the North
Carolina mountains.
An easy catch in my 4" scope at 40x, I note "..a fantastic needle that
showed some apparent mottling..." with higher powers. While 7331
is spectacular in and of itself with any aperture you throw at it,
larger
scopes provide more of a glimpse into its surroundings. With
sufficient aperture, it becomes a Gulliver surrounded by an army of
Lilliputians. When viewed through a 15" scope, I've noted:
"This SA(s)b m9.5 galaxy is simply superb... 99x does a wonderful
job of framing the galaxy in it's surroundings, and even allows for
occasional glimpses of what appears to be a dust lane on the western
side. The core itself is non-stellar and perhaps even a bit
mottled. Increasing the magnification to 243x darkens the sky
further and allows a glimpse of a featureless NGC 7335 and NGC
7337. One should be sure to look for NGC 7336 which can
masquerade as a double. It's "companion", a mag 10 star, lies a
short distance to the north."
Although it fits
easily into any telescope, O'Meara's
book -
The Caldwell Objects
tells us that
NGC7331 is among the largest galaxies known. It's diameter (for
an accepted distance of 47 million light years) is 130,000 light years,
and that it equals in both size and mass the great galaxy in
Andromeda - M31.
While it's a fairly bright galaxy, and should be easily picked up by
sweeping, it might be a little hard to find the initial area.
Fortunately, there's a solution. If you can see Pi and Eta naked
eye the imagine the right triangle pictured. 7331 lies
right
at the 90 degree angle. If you can't see these two stars
naked
eye, you may wish to start a star hop at beta and "come round the
corner" so to speak. Which ever method you choose - don't give
up, 7331 is worth the effort.
Although it's a beautiful object in it's own right, 7331 is a jumping
off point for
our final and challenge object this month.
Challenge
Object: 7320 and Stephans Quintet
Stephan's Quintet -
5 galaxies squeezed into 3.5' of sky, Stephan's is on the must view
list for any serious deep sky observer.
Unfortunately, the brightest of the bunch - NGC 7320 is also
fairly faint
- coming in at mag 12.5. From a moderately dark site, I've seen
all
5 in a 10" scope, and 7320 should be visible in an even smaller scope
under decent skies. The bottom line is, you'll never know till ya
try.
- Trivia - Another early
movie star, Stephan's Quintet was used to portray a group of angels in
the 1946 holiday classic - "It's a Wonderful Life"
While some might think this group a bit much for the typical small
scope owner, I've been constantly surprised by communications I've
had with various folks - some seem to be hitting the challenge objects
with little difficulty - so, I present the following:
- What's the smallest scope you can make out at least one of
the members of Stephan's with?
- What's the smallest scope you can make out all 5 members
with?
When you view all 5 members, you've ascended one of the coveted peaks
of amateur astronomy.
Congratulations!
Additional Reading:
Historical events obtained from:
http://encyclopedia.thefreedictionary.com/
Pease 1 finder charts and observing reports can be found at
- Doug Snyder's Planetary Nebula Web
Site:
http://www.blackskies.com/
Hubble Pics of Stephan's Quintet
http://hubblesite.org/newscenter/newsdesk/archive/releases/2001/22/
Hipparcos Space Astrometry Mission
http://astro.estec.esa.nl/Hipparcos/
Recommended Books:
The Caldwell Objects -
Stephen James O'Meara
The Night Sky Observers Guide
-
Kepple and Sanner
