What Am I Looking At? (A
Brief Guide to Observing Deep-Sky Objects)
One of the things I enjoy
most about astronomy, is actually being able to look through an eyepiece (or up
from my lawn chair), to see the physical universe “in action” for my very own
self! Nothing is a keener or more satisfying thrill, than to have read about
some amazing bit of physics or chemistry out in the vastness of space – and
then to actually be able to glimpse its effects directly, under the gorgeous night sky.
Unfortunately, nebulae and
galaxies are shy creatures – only sharing a faint glimmer of their true beauty
with the naked-eye observer. Still, a trained eye and a patient mind can glean
a great deal of the amazing nature of these denizens of deep space: “You can
see a lot just by looking,” as the saying goes…
To inspire (or irritate)
others into trying to see some of these fascinating features for themselves,
here is a summary list of the things that I try to look for (and to
log) in different objects, when I’m at the eyepiece:
Multiple star – How many stars appear to be together?
Is it just the primary (brightest or “A” star) and the comes (“B” or secondary star, pronounced “koh-meez”)? Or is there a
third-brightest companion (“C”), and even a fourth (“D”)? What is the separation of each companion from the
primary star? (How far apart are they in arcseconds, arcmins, or “fractions of
your eyepiece field”?) What Position Angle does each make with
the primary – PA tells a companion star’s orientation in the field, with PA 0o
meaning a companion lies due North of
its primary, PA 90o meaning due East,
etc. (Remember, you can always tell due north by “nudging” your telescope in
the direction of Polaris: where ever you see new stars entering the field, that’s
north!) Lastly, do you see any colors or contrast effects between the primary
and its brightest companions?
Variable star and “carbon
star” – these are individual stars that are mostly interesting because of
either their color, and/or the fact that their brightness can change.
Estimating the star’s magnitude (using one of the AAVSO’s approved methods, or
just by “
Planetary nebula – Can you
see a central star or “core nebula”
(central star is fuzzy)? Is the PN annular (darker or “empty” toward
the center), and/or bipolar (two or
more lobes or brighter areas are
visible on opposing sides)? How many rings
or outer shells can be glimpsed? (Remember, some PNe have an extremely
faint outer halo which may extend to 2, 3 or even 4 times the
published extent of the object!) Can you glimpse any internal structure within the inner or outer nebula – any brighter
parts, irregularities, “striations”, unusual darkenings, etc.? What Position
Angle (see Multiple stars above) does each of these features make with
the center of the nebula? Finally, how does the nebula as a whole, and each
feature you have noted (above), respond to different nebular filters? (Every PN is different – a few kinds respond best
to no filter, or a broad-band or “DeepSky” filter. Many more respond better to
a narrower-band or “Ultra-High Contrast/UHC” filter. And probably most will respond
best to an Oxygen-III (“OIII” or “O3”) filter – try them all, including a
“Hydrogen-Beta line” or “Hbeta” filter, or color filters if you have them! And remember,
different filter responses mean different
physics within that particular object… Amazing! J
“Diffuse” or “Galactic”
(non-planetary) nebula – Like Baskin-Robbins, these nebulae come in many flavors:
emission, reflection, dark nebula, supernova remnant, “Wolf-Rayet” object,
“proto-planetary” nebula, etc. Thanks to their radically different origins, and
their differing mechanisms and wavelengths of illumination or excitation, each
of these types responds differently to nebular
filters (see Planetary nebula above), and will also show its own
characteristic features or structure.
And of course, many of the most interesting GNe are a mix of two, three, or
even four of these different
“flavors”! For instance, it is not uncommon for one “object” to include an emission
component, a reflection component, and also dark
nebulae involved or in front of it. For these fascinating “smoosh-in” nebulae,
you may be able to spend hours just exploring the way different regions and
features of the GN respond to different kinds of filters and magnifications…
Now THAT is good fun. J
Galaxy – Can you see hints of
the morphology
(shape and gross features) of this galaxy: spiral, barred-spiral, Seyfert,
spindle, elliptical, irregular? How many different gradations or “brightness
steps” can you see in the galaxy? Is there a broader “outer halo”? A core –
and maybe even a smaller “inner core”?
A tiny or even “stellar” nucleus? If
it has arms, how many can you untangle with your eye? Are they loose, or
tightly wound around the core? Do they even form a complete outer or inner ring? Can you see dark
features or mottling along the arms or in the core? Brighter spots or stellarings
– or even tiny nebulae – in or near
the visible extent of the galaxy? Remember, nebular
filters – and even color filters – can sometimes be used to bring out
unique features even in the brighter galaxies! What orientation (Position
Angle) do each of the features you see make? Finally, does the
galaxy have any companion galaxies,
or does it seem to be interacting
with any other nearby galaxy (interacters sometimes have a number in the “Arp”
catalog of galaxies)? Is it part of a galaxy
group - an informal “NGC group”, or a compact “Hickson” or “Shakhbazian”
group? And/or is it part of a larger galaxy
cluster, like an “Abell cluster” (AGC)? What other tiny, faint nearby
members of that group or cluster can you glimpse? Don’t forget to try averted
vision, field “jiggling”, concentrated vision, and even deep breathing if it
helps! J
Globular star cluster – These
are some of the brightest and prettiest – and also some of the faintest and most elusive deep-sky objects. The
basics of logging a GC include: Is it tight (mostly core and little halo), or
loose (a smaller core, and then many stragglers on the periphery)? Can you
resolve its stars? Just at the edges, or right down to the center? (Some
globulars, even a few brighter ones, will not resolve at any power, because
their constituent stars are too faint
to be individually visible!) How many stars would you estimate are resolvable
total? (A trick for counting stars is to choose just one wedge or “quadrant” of
the GC in the field, to count the stars in that quadrant, and then multiply by
four!) Finally, does the GC show any unusual features – in particular, can you
see any blank areas, “cuts”, or indentations in the core of the
cluster? (These may or may not be actual physical phenomena… Whether they are
due to some trick of the eye in a crowded field, or to some obscure orbital
dynamics, or possibly even to dark material in the GC core, I have never heard
a convincing explanation – but a surprising number of GCs will show “dark
features” like this, at one power or another, and at different contrasts. For
example, sometimes these features are most visible when observing a GC in some twilight
or moonlight… Who knew! J)
Open star cluster – I don’t
often turn my attention to open clusters – but they are by far the most
numerous of the objects that appear bright in a small telescope… When logging
an OC, how many stars do you estimate are definitely visible? How many are just
on the edge of visibility? Are there any clumps of fainter, unresolved stars in
or near the OC – and what Position Angle do they make with the
cluster central area? Is this OC more or less rich (many bright stars),
and more or less concentrated (stars close together)? Also, remember that OCs
are sometimes associated or involved with a galactic nebula of one kind or another – can you see any hints of
this nebula or nebulae? Finally, do you
note any particularly pretty double stars, or strikingly colored stars in the cluster? (Many OCs of all types,
for some reason I have never understood, seem to have a nice orange or red star
near their center! And some clusters are populated mostly by very young,
“blue-white” members, while others are clearly older clusters – because many of their stars are on the yellow or
orange end of the range of spectral types.)
To learn more about deep-sky
observing techniques, or about a particular
deep-sky object, or to archive your own
observing log of any object for posterity, visit the Internet Amateur Astronomers
Catalog of Visual Deep Sky Observations (IAAC or ‘netastrocatalog’),
online at:
Clear skies!
Lew Gramer