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Transcript
Late Summer Messier Objects
There are 24 Messier objects between RA 18 and RA 21, the second-richest octal of the
Messier list after the one that includes the Virgo Cluster. The abundance of Messier objects is
due to the fact that this part of the sky contains the widest, richest, and brightest part of the
Milky Way, with some of the finest star clusters and nebulae in the sky.
These late-summer Messier objects are both a blessing and a curse for urban and suburban
observers north of latitude 30N or so. The blessing is that unlike the faint galaxies of spring,
many of these are bright showpiece objects that are spectacular even through heavy light
pollution. The curse is that they are heavily concentrated at southerly declinations, where they
never really climb out of the soup of light pollution along the horizon.
In addition, they are well-placed only during the summer, when the nights are at their shortest
and the logistics of observing are most problematic. But they are not quite as bad in this regard
as the objects between RA 15 and RA 18, at least not for people who do most of their
observing in the evening rather than in the pre-dawn. These objects become well-placed in the
evening sky just as the nights start to become significantly longer, so although they are slowly
slipping out of grasp, the ever-earlier evenings give you a chance to admire these showpiece
objects well into autumn, if you start as soon as the sky gets dark.
For instance, M8, the quintessential late-summer object, is at its highest at 10PM daylight time
around August 1, which is around when the sky becomes fully dark at my latitude of 42N. One
month later, the sky is fully dark at 9PM, and M8 is only an hour past its prime. On October 1,
the sky is fully dark at 8PM, and although M8 is then 2 hours past its prime, it is still moderately
well placed, just 5 degrees lower than optimal.
For convenience, I have included M23 in this section, despite the fact that its RA is three
minutes shy of 18:00, because it clusters naturally with the other objects in Sagittarius. I have
transferred the three easternmost objects (M29, M72 and M73) to the early-fall section to
augment the measly five objects that actually fall properly in that section, between RA 21 and
RA 23:59. Here, then, are the late-summer Messier objects:
In the unlikely event that you tire of the objects in this section, M13 and M92 remain
reasonably well-placed in the evening sky throughout the summer for observers in the North
Temperate Zone, and several of the objects in the early- autumn section lie quite far north, and
rise reasonably high fairly early in the summer. As for galaxies, the king of them all, M31,
begins to rise in the eastern sky just as M51 is disappearing in the western sky.
M23
M23 is a delightful open cluster in its own right, but its primary significance for me is as the
forerunner of the magnificent Sagittarius Messier objects, and of the summer Milky Way in
general. I love observing the distant galaxies of spring, but I am always glad when they yield
place to our own galaxy. Lying on my back on a warm summer night, I can see far more detail
in the Milky Way with my unaided eyes than I can see in any other galaxy with the largest
telescope I have ever used.
To locate the Sagittarius Messier objects, you must first locate Sagittarius itself, which is not as
easy as it should be under bright skies at my latitude of 42N, due to its southerly declination
and the fact that many of its signature stars are only third magnitude. Although Sagittarius
means "archer", most modern astronomers prefer to picture it as a teapot, with the handle on
the east and the Milky Way issuing out of the open spout on the northwest. The only genuinely
bright stars in the constellation are mag 2.1 Sigma in the handle and mag 1.8 Epsilon at the
SW edge of the base. The key stars for locating the Messier objects are Lambda, Delta, and
Gamma on the top of the kettle, all mag 2.7 - 3.0.
M23 lies a little under 2/3 of the way between Lambda Sgr and Xi Serpentis, but that is a long
enough stretch to make it hard to locate M23 by the point-and-shoot method. It helps a lot if
you can see Mu Sgr (mag 3.8) partway between Lambda and M23. Failing that, it is a long 6degree starhop from Xi Ser or a very long 10-degree starhop from Theta Oph or Lambda Sgr.
In my 7x35 binoculars, M23 is visible as a faint, subtle cloud of light in under a suburban sky,
but invisible from the city. In my 70mm scope, it is barely visible at low power but quite
attractive at 60X both in the city and in the suburbs, showing a few stars with direct vision in
the city and about a dozen in the suburbs. In both cases, the directly visible stars are
supported by a much richer background of stars that can be seen only with averted vision.
In my 178mm scope, M23 is fairly well resolved at all powers both in the city and in the
suburbs, showing several dozen stars in a 25' circle. Because extra magnification is not
needed to bring out the faint stars, as it is in the smaller scope, M23 shows best at modest
power, around 40X, where enough of the background is visible to set the cluster off well.
M8
M8, the Lagoon Nebula, is a close rival of M42 and M45 as my very favorite Messier object to
view with small instruments under dark skies. Unfortunately, the nebulosity that makes M8 so
magnificent is hurt badly by light pollution, especially at my latitude of 42N, where M8 never
rises higher than 24 degrees off the horizon.
M8 is the same kind of object as M42, a dense cloud of gas which is actively giving rise to new
stars. M8 appears somewhat fainter than M42, but that is due entirely to the fact that it is much
farther from us, some 5000 light years distant as opposed to 1500 for M42. In fact, M8 is
considerably larger and more active than M42.
M8 is a very large object, listed as 90' by 40' by many sources, but the portion that is visually
observable is much smaller; the bright part of the nebulosity is no more than 30' in the E-W
direction and 20' N-S. Somewhat E of center is a cluster of about two dozen stars, mostly ninth
and tenth magnitude, arranged in a crude rectangle about 10' from NE to SW and 7' in the
narrower axis. This cluster has the separate designation NGC 6530.
Encircling NGC 6530 is a ring of stars considerably brighter than any in the cluster proper, the
brightest of which is the mag 6.0 star Sagittarii directly W of the cluster. This star, and its
companion Herschel 36 to the WSW, are the primary illuminators of M8, just as star C in the
Trapezium is the primary illuminator of M42. Like the Trapezium stars, 9 Sgr and Herschel 36
are O-type stars emitting the great majority of their energy in the ultraviolet spectrum, and
causing the hydrogen and oxygen in the gas cloud to fluoresce in a few specific wavelengths
of visible light.
Even brighter than 9 Sgr is mag 5.4 7 Sgr 15' to the WNW. 7 Sgr is flanked by a few other
brightish stars which appear to form a sparse mini-cluster in their own right.
The nebulosity is brightest around 9 Sgr and Herschel 36, especially in very small and intense
patch about 1' in diameter around the latter star. The star cluster NGC 6530 is also suffused in
a somewhat fainter nebulosity, and those two patches of nebulosity are connected by an arch
to the N. In the middle of all of that is the famous dark patch, like a patch of water surrounded
by a coral reef, which gives the Lagoon Nebula its name. There is also a very faint nebulosity
surrounding the whole object, but it is quite invisible under urban or suburban skies, even with
a nebula filter. A few isolated bright patches do show through under good conditions, notably
near the mag 7 star E of NGC 6530 and around 9 Sgr.
M8 is very obvious to the naked eye under dark skies, looking like a small disjoint piece of the
Milky Way some distance W of the main body, and somewhat brighter. Even under suburban
skies, I can see M8 fairly easily with my unaided eyes, although not in the city. I have never
been sure whether I am actually seeing the nebulosity or the cluster of stars contained within.
If you cannot see M8 directly, which is likely, it is a moderate 4-degree star hop from mag 3.8
Mu Sagittarii, or you can locate it by the fact that Mu Sgr and M8 are equidistant from Lambda
Sgr. If you cannot see Mu, you might try extending the line from Phi Sgr through Lambda and
bending slightly S, or you can star hop 5.5 degrees from Lambda.
Under urban skies at 42N without a nebula filter, the view of M8 is dominated by the stars
rather than the nebulosity. Even my 7x35 binoculars show the brighter stars easily, such as 7
Sgr and 9 Sgr, and they show NGC 6530 as a fairly bright cloud with a few stars peeking
through. In my 70mm scope at 60X, NGC 6530 is resolved into about a dozen stars, and the
nebulosity near 9 Sgr and surrounding the cluster is faintly visible. NGC 6530 is well resolved
at all powers in my 178mm scope, with higher powers showing more detail and lower powers
giving a better overview. 60X may be the best compromise. Again, the embedded nebulosity is
visible, still faint but much bolder than in the 70mm scope.
Adding a nebula filter under urban skies changes the view completely. The stars become much
less evident, and the star cluster NGC 6530 nearly disappears in my 70mm scope. The
nebulosity, however, becomes very much bolder and brighter, and the Lagoon shape becomes
quite obvious, which is not true without the filter.
M8 does much better under suburban skies; the nebulosity is quite obvious even in my 7x35
binoculars, and is prominent in both my scopes even without a filter. Using the 178mm scope
with the filter, the nebulosity is overpowering in the brighter sections, and shows a wealth of
fine detail throughout.
M20 and M21
Moving north along the axis of the Sagittarius Milky Way, the next Messier objects after M8 are
M20, another bright nebula with embedded stars, and M21, a rather unremarkable open
cluster. M20 is a magnificent object under dark skies, especially in large telescopes, but its
nebulosity is much more subdued than M8's, and it suffers very badly from even modest light
pollution.
M20 and M21 are mostly easily reached by star hopping from the much more prominent M8, or
simply by moving the telescope north two degrees. All three objects (M8, M20, and M21) are
visible simultaneously in a rich-field telescope at low power, forming what many people
consider to be the single finest wide telescopic field in the whole sky.
M20 and M21 lie at opposite ends of Webb' Cross, a wonderful asterism of stars from mag 6 to
mag 8 arranged like a cross with the long arm faintly curved and the short arm, consisting of
the three brightest stars, an arc severely concave to the S. The whole formation is about 40'
long, and M21 surrounds the northernmost star, while M20 surrounds the whole S section. The
cross is quite obvious in my 7x35 binoculars, but M21 and M20 show only as vague glows in
that instrument under suburban skies, and not at all under urban skies.
M21 is rather small and sparse, and the stars have a wide range of brightness, from the central
mag 7 star down to mag 12 or 13. It is also set against a fairly rich background, which makes it
look even less prominent. Many of the stars are arranged in a nearly perfect circle about 3' in
diameter with the mag 7 star on the S and a complete void in the middle, which gives the
cluster a rather unusual appearance once it has been detected. The remaining stars are
scattered loosely in a circle some 10' to 15', and it is quite impossible to say where the cluster
ends.
M21 is reasonably well resolved in my 178mm scope at high power, even under urban skies,
and enough stars show in the 70mm scope under suburban skies to make the flesh the cluster
out. Only half a dozen stars are visible in the 70mm scope under urban skies, and they do not
add up to a convincing cluster.
Under urban skies, M20 is quite invisible in my 70mm scope, and I am not 100% sure that I
have seen it in my 178mm scope either. The best place to look is around the lovely double star
Herschel 40, which forms the S end of Webb's Cross. If I look hard, I see just a hint of
nebulosity, maybe 5' across, surrounding the star. The nebulosity -- real or imagined -- remains
but is not improved by my nebula filter.
M20 is much more definite under suburban skies, but is still only a pale shadow of its true self
as seen under dark skies. Again, it appears as a 5' circle around Herschel 40, faintly present in
my 70mm scope and fairly obvious in my 178mm scope. In both scopes, a nebula filter makes
the nebulosity much more definite, but it does not make it seem any larger or show any more
detail.
I have never been able to see under suburban skies any of the wonderful detail that makes this
nebula famous, notably the 15' circle around Herschel 40 split into three or four lobes by
striking dark lanes, which gives this object its popular name the Trifid Nebula, famous from
science-fiction horror novel and movie. I have also never had a hint in city or suburbs of the
secondary nebulosity some 15' to the N, separated by a wide dark lane from the primary
nebulosity.
M24
Moving 5 degrees NNE along the galactic axis from M20 and M21, we encounter M24, one of
the most unusual objects in the Messier list. Indeed, this is technically not an object at all.
Messier called it a star cluster, but that implies a collection of stars which are physically related
and bound to each other by gravity, which is not true for M24. Instead, M24 is a star cloud, a
rich portion of the inner Milky Way which happens to be visible to us because of a gap in the
dust clouds that normally prevent us from seeing far along the galactic plane, particularly
towards the center, where stars, gas, and dust are all most dense. If not for the dust clouds,
this whole quadrant of the sky would be ablaze with light, as M24 is.
M24 is sometimes called the Small Sagittarius Star Cloud. The Great Sagittarius Star Cloud is
the "steam" that issues from the spout of the teapot, N of Gamma and Delta Sagittarii, some
ten degrees S of M24. The Great Sagittarius Star Cloud is even more remarkable than the
lesser cloud, being a portion of our galaxy's central bulge, again showing through a gap in the
celestial clouds, whereas M24 is part of one of the inner spiral arms. The actual center of the
Milky Way lies some distance WSW of the Great Sagittarius Star Cloud, but it is completely
blocked from us in the visual spectrum by dense clouds of dust. It does shine through in X-ray
and radio frequencies, where it is called Sagittarius A.
Like M8, M24 is very obvious to the naked eye under dark skies, and visible but somewhat
subtle in the suburbs at 42N. Again like M8, it appears like a piece of the Milky Way, not as
clearly separated from the main body as M8, but much larger than M8, and much brighter than
most of the Milky Way.
If M24 is invisible to the naked eye, it can be found some 2.5 degrees N of Mu Sagitarii,
assuming that that mag 3.8 star is visible. Failing that, it can be found 2/5 of the way from
Lambda Sagittarii to Nu Ophiuchi
M24 is huge by telescopic standards, some 2 degrees along the long axis, from NE to SW, and
30' to 45' across. It is best seen at very low power under dark skies in an instrument large
enough to fit the whole thing lengthwise. But it can still be appreciated well in a scope whose
maximum FOV is in the 1 - 2 degree range by scanning the object lengthwise. A 1-degree FOV
fits just enough of the relatively sparse background on either side of M24 so that one can
appreciate M24's richness and separate identity.
Unfortunately, under urban skies, the background is filled with light pollution rather than
darkness, and enough of M24's stars are blotted out to detract greatly from the overall
impression. M24 does appear as a markedly rich star field in my 7x35 binoculars, in my 70mm
scope at 16X, and in my 178mm scope at 28X, but the true grandness of this star cloud is lost.
M24 fares much better under suburban skies, where even my 7x35 binoculars are big enough
to resolve a fair number of stars, and where the contrast between M24 and the background is
much more prominent. Again, it looks best at the lowest possible power in both of my scopes.
M16, M17 and M18
Just N of M24 lie three closely spaced Messier objects: M18, M17, and M16, moving S to N.
M16 happens to be just across the constellation boundary into Serpens, but I think of all three
objects as part of the Sagittarius Milky Way The three objects all fit together with M24 in the
field of most hand-held binoculars, and my 70mm F/6.9 scope at lowest power is just able to fit
the three objects without M24.
M18 is by far the least interesting of the three objects. It is a perfectly respectable open cluster,
and might even seem rather attractive in other parts of the sky, but it is totally overwhelmed by
its neighbors.
M17, the Swan or Omega Nebula, is an emission nebula like M8 or M42, and every bit a match
for those two. Its surface brightness is not as high as the Hughenian area of M42 or the
intensely bright patch in M8 near 9 Sgr, but the bright part of M17 is much more extensive than
the bright part of either of those other nebulae. Like M8 and M42, M17 is giving birth to a
population of brand new stars, but these are entirely shrouded by gas and dust, and cannot be
seen in the visible spectrum. Infrared photographs, however, reveal a budding star cluster
within the nebula.
M16, sometimes called the Eagle Nebula, is yet another star cluster embedded in nebulosity,
but it is at the opposite end of the spectrum from M17. In M17, the nebulosity is bright and
bold, but the cluster is invisible. In M16, the cluster is prominent and attractive, but the
nebulosity is subtle even under dark skies, let alone in city or the suburbs.
Once any member of this triplet has been found, the other two are easy to locate by moving
the scope slightly S or N. They are also all easy to find off the N end of M24. I generally find
M16 the easiest of the three to pick up in binoculars or a finderscope, especially under heavy
light pollution, but M17 is also quite easy to see with even the most modest optical aid under
all but the worst skies. Both objects show at low power as unresolved patches of light.
Like the other objects of the Sagittarius Milky Way, these are all surprisingly far from any truly
bright star despite being located in the one of the richest parts of the sky as measured by the
density of stars mag 6 and fainter. When looking for them under skies to faint to see M24
directly, your best bet may be to star hop from Nu Ophiuchi, but the hop is long and arduous
and the anchor star is not terribly bright, at mag 3.3.
M18, the southernmost of the three, has two different aspects under suburban skies or darker.
At very low power, as in 7X binoculars or at low power in my 70mm scope, it shows as a small,
faintish cloud of light, with only the central mag 8.7 star possibly visible. At 60X in my 70mm
scope, I can see half a dozen stars around that central star in a rough 7' circle, and high power
in my 178mm scope brings in perhaps another dozen fainter stars.
Under urban skies, the cloud-of-light view is lost entirely, making M18 invisible in my 7x35
binoculars or in my 70mm scope at low power. At 60X in my 70mm scope, I can make out the
brighter stars with averted vision, and M18 is adequate although unimpressive at 120X in my
178mm scope.
Under urban skies, M17 is readily visible in both my scopes without a filter, but far more
impressive and detailed with the aid of a nebula filter. The same is true under suburban skies,
but of course both the filtered and unfiltered views are improved by the darker skies.
The most prominent feature of M17 is a broad, bright bar of light running some 10' from WNW
to ESE, and about 3' wide. This is what shows in the smallest instruments and in the worst
conditions. A second, fainter bar of nebulosity running roughly N-S joins the first at the WNW
end, making a striking checkmark shape. Under darker skies, you can see that the vertical bar
bends to the W, forming the neck of the Swan as seen in an inverting telescope, and you can
also see some much fainter nebulosity surrounding the whole, but I have never seen these in
city or suburbs even with the aid of a nebula filter.
In the city, without a nebula filter, M16 shows as a star cluster -- a very attractive cluster, but
utterly without nebulosity. This fact is actually more apparent in my 178mm scope than in my
70mm scope, because the former scope resolves M16 well, whereas the latter shows enough
stars near the edge of resolution to add up to some false sense of nebulosity. The cluster
consists of five mag 8 and mag 9 stars, two very close to each other on the W edge, with some
twenty fainter stars filling the intervening space, all in a 6' circle.
Under suburban skies, the star clusters shows much as in the city, but a faint nebulosity is
visible in my 70mm scope without a nebula filter, and more prominent, but only slightly so, in
the 178mm scope. The nebulosity is only really visible around the mag 9.5 star in the NW
corner and its mag 11 companion.
Adding a nebula filter changes M16 dramatically. The cluster becomes much more subdued,
and pretty much disappears in my 70mm scope under urban skies. In the city, the filter gives
much the same view of the nebulosity that one gets without the filter in the suburbs, a faintish
cloud in the NW corner. In the suburbs with the filter, the nebulosity in the NW corner becomes
quite bright and prominent, and fainter tendrils stretch out from there to suffuse the entire
cluster.
Even under the darkest skies, the nebulosity in M16 is a difficult subject for the visual observer.
But for astrophotographers and imagers, this is one of the most spectacular subjects in the
sky. Most famous is the famous Pillars of Creation image from the Hubble Space Telescope.
M25
M25 frames the Sagittarius Milky Way on the E as M23 frames it on the W. M25 is another fine
open cluster, but whereas M23 is rich in moderate and faint stars, M25 is a fairly coarse
agglomeration of bright stars.
M25 is a 4.5-degree star-hop from Mu Sagittarii, or if you are already pointing at M24, you may
be able to find it by moving due E. It may also be visible to the naked eye under good
suburban skies.
M25 shows easily in my 7x35 binoculars as a bright patch, with several stars resolved,
especially under suburban skies. The telescopic view is best in my 178mm scope, but the
70mm scope does nearly as well, and the cluster is relatively impervious to light pollution,
because most of the stars are quite bright (mag 7 - 9). In all cases, the view is best at roughly
60X. Higher powers constrict the field of view too much to set this fairly large and coarse
cluster off from the rich background, and lower powers fail to do justice to the tight clump of
faintish stars near the center of the cluster.
M25 contains a little knot of ten stars near the center in an area about 5' x 2' elongated E-W
with a pretty arc of five or six stars just S of that, and a looser arc of stars near the S border of
the cluster.
M22 and M28
Just as Ophiuchus contains a wealth of globular clusters W of the main axis of the Milky Way,
so Sagittarius contains a fine collection of globular clusters trailing the Milky Way to the E, and
the finest of these by far is M22. Nearby M28 is another fine globular cluster, but very much
eclipsed by its magnificent neighbor.
Let us start with M28, the westernmost and lesser of these clusters. M28 is very easy to find,
just one degree NW of mag 2.8 Lambda Sagittarii.
Under urban skies at latitude 42N, M28 is not exactly hard to see in my 70mm scope, but it is
not at all prominent, possibly requiring averted vision. In my 178mm scope under urban skies,
it is obvious at all powers, showing at 120X as a small circle of light about 1.5' to 2' in diameter,
strongly concentrated towards the center.
Under suburban skies, M28 is bright and concentrated, almost starlike in my 70mm scope at
40X or 60X. In the 178mm scope, in shows a bright 2' core inside a halo perhaps 4' across,
fading out gently at the edges.
M22 is a little harder to locate but much brighter, bigger, and easier to see. If you take a line
from Gamma Sagittarii through Lambda Sgr and continue it another 2.5 degrees ENE of
Lambda, you reach M22. M22 is fairly easy to see in my 7x3 binoculars, although it is
somewhat washed out but light pollution at my latitude of 42N, especially under urban skies.
In my 70mm scope, M22 is obvious at all powers under all skies, showing about 4' - 5' across
under urban skies and more than 6' across under suburban skies at 60X. The brightness is
fairly uniform across the face of the cluster, but moderately concentrated towards the center. It
is distinctly grainy under suburban skies, and resolves a few stars with averted vision under
dark skies.
In my 178mm scope, M22 is magnificent at all powers under all skies. Several stars are
resolved at 120X under urban skies and many more peek through intermittently. Under
suburban skies at 120X, M22 shows at least 7' across, but fades out vaguely to hint at a much
larger disk. The central area is very bright, glowing as if on fire. I can pin down the location of
two dozen stars, and far more are visible intermittently.
M54, M69 and M70
Three Messier globular clusters lie along the S edge of the Teapot asterism. Moving W to E
they are M69, M70, and M54. None of them can be resolved easily in medium-sized scopes
even under dark skies, let alone under urban or suburban skies, but they are all interesting and
attractive objects. M54 also has the distinction of being the only globular cluster in the Messier
list which arguably belongs to a galaxy other than the Milky Way.
Epsilon Sagittarii, the star at the SW corner of the Teapot, is the brightest star in the
constellation at mag 1.8, making it quite prominent even under urban skies at latitude 42N,
where it never rises more than 14 degrees off the horizon. M69 lies 2.5 degrees to the NE, in
the direction of Phi Sagittarii (mag 3.1), and M70 lies 4.5 degrees ENE, nearly halfway to Zeta
Sagittarii (mag 2.6). M54 lies 1.5 degrees WSW of Zeta, just N of the line connecting that star
to Epsilon.
Nominally, M70 is slightly fainter and very slightly larger than M69, but according to my visual
impressions, M70 is quite a bit larger, with significantly lower surface brightness, and quite a
bit harder to see under heavy light pollution, especially at low magnifications.
In my 70mm scope at 60X, M69 is fairly easy to see, but M70 is elusive, especially under
urban skies, where it requires concentrated effort with averted vision. M69 is 3' or under, with a
concentrated core, while M70 is a vague, about 4', uniformly faint across the disk.
Both clusters are much easier to see in my 178mm scope at 80X to 120X, even fairly bold and
attractive, but they do not show much more detail than in the smaller scope.
M54 is much more remarkable in every way. This globular cluster is inherently very bright, but
it is by far the most distant of all the Messier globulars, some 70,000 light years distant, on the
far side of the galactic core. Recent analysis indicates that M54's motion, and the motion of
many nearby stars, is radically different from the prevailing motion of that portion of the Milky
Way. This is attributed to the fact that those stars, and M54, are actually part of a small galaxy,
the Sagittarius Dwarf, which is currently colliding with the Milky Way. In all probability, the
Sagittarius Dwarf will not survive the collision; its stars will be swallowed up in the Milky Way's
disk, while M54 will be added to the halo of globular clusters circling the Milky Way's core.
M54's great distance makes even its brightest stars appear quite faint, so M54 is quite hard to
resolve even in fairly large amateur telescopes under good conditions, and quite impossible
with medium-sized scopes under urban or suburban skies. However, M54 is quite bright and
concentrated, giving it a high central surface brightness which makes it quite easy to see even
under heavy light pollution.
In both my 70mm scope and my 178mm scope, M54 is readily visible at all powers under all
skies, but it is quite small, nearly starlike. The best views are at fairly high powers, 60X in the
70mm scope or 80X to 120X in the 178mm scope. It shows a vague disk about 1' across under
urban skies, and maybe 2' across under suburban skies.
M55
Trailing far E of the Milky Way and the Teapot, in a part of the sky otherwise devoid of
interesting deep-sky objects, lies the remarkable globular cluster M55. M55 appears almost as
large as M4, and has even lower surface brightmess, making it quite prominent under dark
skies but difficult to see under heavy light pollution, especially considering its low maximum
altitude above the horizon for northern observers.
Finding M55 is a major nuisance even under dark skies; it is nowhere near any plausibly bright
star. I find the best strategy to be the arduous 8-degree starhop from Zeta Sagittarii.
I can resolve M55 fairly easily in my 178mm scope under dark skies, but I have never seen
any hint of resolution under urban or suburban skies. Due to its immense size and low surface
brightness, M55 under light pollution shows best at fairly modest magnifications, around 30X in
my 70mm scope and 60X in my 178mm scope. As with other large, diffuse objects, the view
varies more due to sky brightness than to aperture.
In the city, M55 is difficult to see even with averted vision in both of my scopes, showing up
best when I pan around the area so that M55's apparent motion catches my eye. It appears
enormous (over 7') and vague.
M55 is much easier to see under suburban skies, but it still appears both vague and faint.
Again, it is enormous, around 10' across, nearly as big as the distance between the prominent
pair of mag 8 stars 45' to the NNW.
M11 and M26
Returning to the main axis of the Milky Way, we move north in the Scutum star cloud, an area
full of magnificent deep-sky objects including two open clusters catalogued by Messier: M11
and M26. M26 is rather attractive under dark skies, but it is the faintest open cluster in the
Messier list, which makes it disappointing under bright skies and/or in small instruments. M11,
by contrast, is extremely bright and attractive in all instruments and under all skies. Indeed,
many people consider it to be the most beautiful open cluster in the sky.
For all of its wonderful deep-sky objects, the constellation of Scutum is a sorry affair,
composed of four fourth-magnitude stars in a nondescript pattern. It is unconvincing under
dark skies and may be entirely invisible under urban skies. In general, I find it easier to locate
objects within the Scutum star cloud off of Auqila to the NE. Aquila can be spotted immediately
by brilliant first-magnitude Altair with its attendants Tarazed (Gamma Aquilae, mag 2.7) and
Alshain (Beta Aquilae, mag 3.7) to the NNW and SSE respectively. The four third-magnitude
stars making up the rest of the outline of Aquila (the Eagle) are less prominent, but they make
a distinctive and attractive pattern.
The tail of Aquila is composed of three stars arcing SSW: Lamdba Aquilae (mag 3.4), 12
Aquilae (mag 4.0) and Eta Scuti (mag 4.8) just over the border into Scutum. One or more of
them may be invisible to the naked eye under urban skies or poor suburban skies, but the arc
should be striking in binoculars or a finderscope. If you continue from Lambda Aql through Eta
Sct and then on for another 1.5 degrees, you reach M11. Continuing from Lambda through 12
Aql and on for another 5.5 degrees, you reach M26. Alternatively, M26 is an easy starhop from
Eta Sct via the line of 5th and 6th magnitude stars S and SW of Beta Sct, and then back to
M26 via bright Epsilon and Delta Scuti.
M11 in some ways resembles a globular cluster more than an open cluster. It contains
hundreds of stars packed very tightly, but all of the stars are fairly faint, and the range of
brightness among the stars is fairly narrow. Therefore, like globular clusters, M11 appears as
an intense but unresolved patch of light in very small instruments and at low magnifications.
Indeed, M11 is visible as a patch of light under virtually any skies with almost any optical aid. It
is quite obvious in my 7x35 binoculars under urban skies.
Only three stars are clearly resolvable in my 70mm scope at 60X: a mag 8.6 star near the
center and a pair of 9th magnitude stars near the SE edge. Under suburban skies, a few other
stars occasionally pop out with averted vision, particularly on the S side. All of this is set
against a lovely bright glow of unresolved stars about 8' square.
M11 is breath-taking in my 178mm scope. At low power, it shows as a fan of light stretching W
of the bright central star, scattered with faintish stars, and with a few other stars set around. At
120X, the fan of light resolves into a huge number of stars, too many to count. Despite the
superficial similarity to a globular cluster, M11 is much easier to resolve, and lacks the classic
circular symmetry.
M26 is like a radically scaled-down version of M11. Like M11, it has a bright central star, but it
is a magnitude fainter than M11's. Like M11, M26 contains a scattering of medium-bright stars
and a dense swarm of faint stars, but the latter are genuinely faint in M26 -- near the limit of my
178mm scope under dark skies -- and nowhere near as dense as in M11.
With a total magnitude of only 8.0, M26 is quite invisible both in my 7x35 binoculars and in my
70mm refractor under urban skies. The central star is indeed visible in the small scope, and
perhaps one or two of the surrounding stars, but that does not add up to a cluster. The bright
sky background camouflages the haze of light that would otherwise result from the unresolved
stars.
Much to my surprise, M26 is visible in my 7x35 binoculars under suburban skies, showing as a
tiny but fairly bright patch of light forming a nearly perfect isoceles right triangle with Delta and
Epsilon Scuti. The view is quite similar at 16X in my 70mm scope, but using averted vision at
60X, I can resolve several stars in addition to the central star.
M26 does far better in my 178mm scope at 120X, which is enough aperture to resolve about a
dozen stars under urban skies, and somewhat more under suburban skies. Nonetheless, the
stars are a little too scattered to form a completely convincing cluster, especially under urban
skies.
M27 and M71
Following the galactic axis far to the north of the objects we have discussed before, we come
to two more Messier objects: the faint and unusually sparse globular cluster M71 in Sagittae,
and big, bright planetary nebula M27 in Vulpecula. Although these objects are far E of
Sagittarius, they are also far enough N so that they reach a reasonable altitude earlier for most
observers in the North Temperate Zone, and they linger well into autumn.
These objects are placed in a rather inscrutable part of the sky. Vulpecula, in particular, is one
of the sorriest constellations in the sky, composed entirely of faint stars arranged in no
recognizable pattern. Sagittae is compact, and very shapely if you can see it, but two of its four
essential stars are only mag 4.5 and the brightest is only mag 3.5, making it a challenge for
urban and suburban observers. Fortunately, it is small enough to fit completely into the field of
7X or 10X binoculars, and it is well worth a look.
It is easy to locate the general area of the sky between brilliant first-magnitude Altair on the S
and Albireo (Beta Cygni, mag 3.1) on the N. Albireo, by the way, is generally considered to be
the most spectacular double star in the sky, readily split at 15X or higher, with wonderful
contrast between the golden primary star and the blue secondary star. Having done that, look
hard for Sagittae halfway between Altair and Albireo, and slightly E. Once this is done, M71 is
very easy to locate halfway from Delta Sagittae to Gammma Sagittae, and just 20' from
(telescopically) bright 9 Sagittae. Delta Sge, Epsilon Sge, and M27 form a very obtuse, nearly
isoceles triangle with Gamma at its apex.
M71 is unusually sparse for a globular cluster; in fact, it was usually considered to be an open
cluster until the last few decades. The small number of stars gives M71 one of the lowest total
brightnesses of any Messier globular cluster, but the individual stars are fairly easy to resolve.
In my 70mm scope, M71 shows best at 40X, where it appears as a faint cloud of light, about 3'
- 4' across, best with averted vision but still perceptible with direct vision, even under urban
skies. It is slightly extended NE - SW, with a faint extension to the N.
In my 178mm scope, M71 shows much as in the 70mm scope, but there are hints of resolution
at 120X under urban skies, and at least a dozen stars are clearly perceptible with averted
vision under suburban skies.
M27 is a completely different animal, the brightest planetary nebula in the sky by a fair margin.
M27 also has high surface brightness, allowing it to show fairly well under heavy light pollution.
M27 is readily visible, although surprisingly small, in my 7x35 binoculars under suburban skies
but quite difficult under urban skies.
Like most planetary nebulae, M27 responds well to a narrowband nebula filter, but even if you
own such a filter, M27 should be observed both with it and without it. The filter brings out
certain aspects of the nebula, but others are best observed without it.
In my 70mm scope, M27 shows best at 60X, where it appears as a rectangle with rounded
corners, extended NNE - SSW, with a hint of scalloping along the long sides.
The 178mm scope brings out the scalloped shape much more strongly, making it obvious why
this is sometimes called the Dumbell Nebula. To me, it is more reminiscent of an apple with
large bites taken out of the sides. The view is best at 80X to 120X. Using my narrowband filter,
the bites on the sides cease to be hollow, and instead become filled with a faint nebulosity,
especially under suburban skies. In fact, the faint nebulosity extends farther from the center
than the bright apple part, so whereas without the filter M27 is about 5x3 extended NNE SSW, with the filter it is about 8x5 extended WNW - ESE.
M56 and M57
The small constellation of Lyra contains two Messier objects: M57, the Ring Nebula, which
may be the most famous of all planetary nebulae, and the modest globular cluster M56. These
are the northernmost of the late-summer Messier objects, and also placed fairly far W, so that
they reach reasonable altitude quite early in the summer, or even in the late spring. Few
people go out of their way to observe M56, but M57 is one of the great showpieces of the
heavens, and with the highest surface brightness of any Messier object, it is a particularly
attractive target for urban and suburban observers.
The constellation of Lyra is one of the most prominent and attractive star patterns in the sky,
although some of the stars may be difficult or invisible under urban skies. The eye is
immediately drawn to Lyra by dazzling blue-white Vega, fifth brightest star in the sky, at
magnitude 0.03. Native American legend has it that Vega and Altair are lovers from warring
tribes, immortalized in the sky but perpetually separated from each other by the band of the
Milky Way.
Vega forms a tight equilateral triangle with two fourth- magnitude stars, which attaches to a
nearly perfect, elongated parallelogram to the S. Under very poor skies, only Vega and the two
3rd-magnitude stars at the far end of the parallelogram are likely to be visible, and the latter
may be hard. Each of the six signature stars of Lyra is remarkable in some way; refer to a
more general guidebook like Burnham's for more information. For now, I will only comment that
Epsilon Lyrae, Vega's partner at the far N tip of Lyra, is the famous Double Double, resolvable
into two tight pairs of stars at 100X or higher.
M57 is extremely easy to locate 40% of the way from Beta Lyrae at the SW end of the
parallelogram to Gamma Lyrae at the SE end. But for a deep-sky object whose position is so
tightly defined by bright stars, M57 proves surprisingly hard for many novices to find. The
reason is that M57 is very small, the smallest of all the Messier objects. At low or even medium
power, it may be hard to distinguish M57 from a star. Look for a star that appears slightly hairy.
In my 70mm scope under urban skies, M57 appears merely as a tiny but bright patch of light,
slightly elliptical ENE - WSW. Under suburban skies at 60X, it begins to become apparent that
the center of the disk is hollow, especially using averted vision.
In my 178mm scope at 120X, it is immediately obvious why M57 is called the Ring Nebula; the
doughnut shape is evident even under poor urban skies.
M56 lies almost directly between Alberio (Beta Cygni) and Gamma Lyrae, 45% of the way from
the former to the latter. But this is a fairly large stretch of sky, and M56 is fairly faint, so it may
be hard to locate M56 just using the point-and-shoot method. If that fails, M56 is a somewhat
easier starhop from Gamma Lyrae than from Alberio, due to a convenient chain of reasonably
bright connecting stars.
M56 has fairly low surface brightness, making it a little hard to see under urban skies,
especially in my 70mm scope, where it is barely perceptible with averted vision. It shows a little
better in my 178mm scope at 120X, as a small cloud about 3' across, almost evenly bright, but
fading slightly at the edges.
M56 is considerably better under suburban skies, where it is reasonably easy to see even in
my 70mm scope. In my 178mm scope at 120X, it shows as a bright core around 1.5' - 2',
surrounded by a very faint halo about 4' across. The whole thing seems slightly elliptical ENE WSW, and the core seems to be subtly flattened on the S side. I can resolve a few stars in
M56 using the 178mm scope under dark skies, but not under suburban skies.
M75
Like M55, M75 floats all on its own, far from any other notable deep-sky object or reasonably
bright star. But where M55 is enormous and vague, M75 is tiny and highly concentrated.
I have a habitual strategy for locating M75 which seems rather round-about but turns out to be
quite efficient in practice. It relies on the fact that the handsome pair of mag 5 stars Rho and Pi
Capricorni, close enough to fit easily in a wide telescopic field, point almost directly at M75.
Take a line from Rho to Pi, bending slightly N, and you pass through mag 5 Sigma Cap. Half a
degree S of Sigma, a line of three mag 8 stars starts a sweeping arc of other mag 8 stars
leading W and S to M75.
Unfortunately, Rho and Pi Cap are usually too faint to be seen directly under urban or
suburban skies. So I find them (very easily) by extending a line from the lovely star pair of
Alpha and Beta Capricorni which dominates this part of the sky. Alpha is actually a pair of
stars, mag 3.6 and 4.2, separated by 6.5', which I can sometimes but not always separate
naked-eye.
M75 shows best at highish power in both my scopes, around 60X in the 70mm scope and
120X in the 178mm. In all cases, it shows readily as a tiny, nearly starlike core surrounded by
a small halo. Both the core and the halo appear to grow with darker skies, with larger
apertures, and with higher magnifications.