Reference: Marsalkova, P.: 1975, Astrophysics and Space Science 273. A
comparison catalogue of HII-regions.

Mu Cep 
   The Mu Cep type of variable star is an older,  cooler
star that has a fairly regular period of variation of 30
to several thousand days in length,  with some irregular
changes from time to time.  This type of variable usually
varies by about one magnitude in brightness.

   The Bright Star catalog sometimes will remark on
names commonly used for brighter stars.

   A nanometer is one-billionth (.000000001) of a meter.
("Nano" always indicates a billionth.  It's the step
beyond "micro",  or one-millionth.)  The wavelength
of light is usually measured in either nanometers or
Angstroms.  For example,  visible light has wavelengths
between about 400 to 700 nanometers.  An Angstrom is
one-tenth of a nanometer.

Nearby star 
   Many of the nearer stars have special names given by their
discoverers,  such as Barnard's Star,  Proxima Centauri,
Kruger 60,  Lalande 21185,  and so forth.  These stars are
not likely to appear in standardized catalogs such as the
   The Go to Star menu contains an entry,  "Nearby Star",
listing a few dozen of these odd designations.  Click on
one of them,  and Guide will recenter on that star.

   Scattered throughout the sky are vast clouds of gas
called nebulae.  These clouds come in many forms.  Some
are gas clouds in the process of condensing into a star.
Others are the remains of stars that have exploded.  Some
are bright,  because there is a star inside them that
keeps them glowing.  Some are dark,  and only seen as dark
patches against a background of stars.  This program makes
the distinction between planetary nebulae,  symbolized by
a circle with four rays,  and diffuse nebulae,  symbolized
by a square.  Some are reflection nebulae,  where light
from a star inside reflects from dust particles,  and some are
emission nebulae,  where light from a nearby star ionizes
or excites atoms in the nebula until it glows.
   You can find a particular nebula using the Go to Nebula
menu in the Go To menu.

Nebula Databank 
   The Nebula Databank was compiled by Eric-Sven Vesting to evade
the problems that came with earlier bright nebula databases.  For
example, previous versions of Guide gathered nebula data from five
separate catalogs.  There were few cross-indexes from one catalog to
another, and no way to indicate that one designation applied to a
part of larger area with a different designation.  Also,  brightness
levels were applied in an inconsistent manner at best.
   The Nebula Databank contains explicit links between the various
nebula catalogs,  enabling Guide to show all designations for a
given object and to avoid drawing some objects twice (if they appeared
in separate catalogs under different names).  Also,  Eric-Sven Vesting
created the nebula isophotes used by Guide to indicate the shapes of
most prominent nebulae.  Better positional data was generated,  usually
by comparing catalog positions to actual RealSky images.

   Neptune is the eighth planet from the Sun.  It is usually
visible at about 8th magnitude through small telescopes.
Physically,  it is very similar to Uranus in its composition.
   Neptune was originally located mathematically.  By
1845,  Uranus had been observed for over sixty years,  and
it was quite apparent from its motion that the gravity of
an unseen object was pulling and pushing it in its orbit.
Analysis of the motion made it possible to calculate where
the unseen object must be,  and two people,  John Couch
Adams,  an undergraduate at Cambridge,  and Urbain
Leverrier,  a French mathematician,  did this
independently.  After the math was done,  an hour or so of
telescopic searching was enough to pin Neptune down.
   Neptune has several satellites,  one of which,  Triton,
is roughly the size of our Moon.  Triton is unique in that
its orbit is backwards,  or "retrograde",  compared to
that of all other large objects in the Solar System.  If
you looked at the Solar System from above,  all the
planets would seem to go counterclockwise around the Sun,
and most of the satellites counterclockwise around their
planets.  Triton's motion would be clockwise.
   Much of what we know about Neptune and its satellites
comes from the Voyager 2 flyby in 1989,  which collected
pictures and other data on these objects.

neutral hydrogen 
   Neutral hydrogen is,  logically enough,  hydrogen with
no electrical charge.  Among other properties,  it tends
to emit microwaves with a 21-cm wavelength.  This happens
because a hydrogen atom consists of an electron combined
with a proton.  Each of these has a quantum mechanical
property called "spin";  for purposes of this discussion,
think of "spin" as resembling magnetic poles.  If the
spins of the two particles are the same,  then you can
add a little energy (perhaps from a collision with another
hydrogen atom) and the spins will be opposite.  Later,
they will tend to fall back to their normal state;  the
energy is released as a photon of 21-cm microwave energy.
   A cloud of neutral hydrogen can be detected by this
radiation,  and its speed and other properties can be
determined by examination of the 21-cm emissions.

neutron star 
   A neutron star is a more compressed version of a white
dwarf.  In these objects,  the pressure is high enough to
force electrons and protons together into neutrons.  An
object consisting of nothing but neutrons can collapse to
a density of about 100 million tons per cubic centimeter.
   Neutron stars are usually around ten to twenty
kilometers across,  yet are more massive than the Sun.
They generate tremendous magnetic fields and spin
rapidly,  generating radio pulses that sweep around
line the search beam on a lighthouse.  Sometimes these
beams intersect the Earth,  and we see a pulsar.  One
example is in the Crab Nebula (M-1).  This object was a
star that went supernova in 1054,  and was visible in
daylight for some time.  Now there is a nebula and a
pulsar in its middle.

New moon 
First quarter 
Full moon 
Last quarter 
   The moon takes roughly 29.5 days to go through a full
set of phases.  It starts out close to the sun;  this is
a new moon.   As it moves away from the sun,  we can see
it as a thin crescent,  and then as a half-lit disk.  It
is then 90 degrees from the sun.  It's half-lit,  but only
a quarter of the way through its phases,  which is why
this point is called first quarter.
   After this,  it continues away from the Sun until it
is opposite it in the sky.  Since it is opposite the sun,
it rises at sunset and sets at sunrise (roughly),  and is
fully lit:  hence the name full moon.
   It continues moving,  but now it is catching up with
the sun again.  It becomes less lit,  then only half lit,
again 90 degrees from the sun.  This is the last quarter
   It continues to shrink,  becoming a crescent again,
until it finally catches up to the sun and there is
another new moon.  Times for all these phases are
shown when you "click for more info" on the moon, or
ask for Quick Info.

   The NGC catalog is a list of about 9000 objects such as
nebulae, galaxies, and clusters of stars.  It stands
for "New General Catalog",  and you will often hear an
object referred to by its NGC number.  For example, NGC 104
is a globular cluster several thousands of light-years
away,  out near the edge of our own galaxy.
   When you see a symbol such as an oval,  circle with a
cross inside,  or square,  followed by a number,  all in
yellow,  that's an NGC object.  You can stop these from
displaying (or ensure they are always displayed) inside
the Data Shown menu.  You can find an object by its
NGC number inside the Go To menu.

   The NGC2000 is a modern version of the NGC and IC
catalogs compiled by J. L. E. Dreyer in the late
nineteenth and early twentieth centuries.  Errata compiled
by Dreyer and by subsequent workers have been incorporated
into the new version and the object types have been
updated with information from modern astronomy; the
descriptions given are those of Dreyer,  and tend to be on
the cryptic side at best.
   This catalog is copyrighted by Sky Publishing
Corporation,  and is used here by permission.  The data
should not be used for commercial purposes without the
explicit permission of Sky Publishing Corporation.

   When the Hubble Guide Star Catalog,  or GSC,  was
produced,  a computer was used to examine each object in
an effort to determine what type of object it was.  The
vast majority were clearly stars.  Most of the remainder
were classified as non-stars,  meaning that they were
too fuzzy or too elongated for the computer to think
they were stars.  Many of the non-stars are really
galaxies;  some are asteroids that moved while the image
was taken,  and some are really scratches on the plate.
They can be turned on and off in the star display dialog.

   Every now and then,  a star will suddenly climb up in
brightness from out of obscurity,  stay bright for a week
or so,  then slowly fade into obscurity over months.  Such
stars are known as novae and supernovae.
   Your garden-variety nova is dimmer and more common than
a supernova.  The causes of novae are a little obscure,
but they probably occur in binary stars where one member
of the pair is a white dwarf.  The stars are so close to
one another that the white dwarf's gravity can pull matter
from the other star,  and when it lands on the dwarf's
surface,  you can get a nova.  Sometimes this happens
repeatedly,  making a repeating nova such as T Pyx.

   Sometimes an object may,  based on its spectrum and
other data,  look a lot like a former nova,  in which
case it may be labelled as nova-like.  Sometimes more
detailed examination of the object reveals it to be
something else.

New Catalog of Suspected Variables 
   Many stars have been observed closely enough to be
suspected variable stars,  but not closely enough (or for
long enough) to determine what kind of variable they are
or even if they really vary.  Such stars are listed in
the New Catalog of Suspected Variables,  or NSV.

   In addition to the slow precession of the earth's
axis,  which causes it to sweep out a wide circle over a
period of 25,800 years, the earth's axis traces out much
smaller ovals over an 18.6 year period.  This smaller
"wobble" is called nutation.  It doesn't shift the
earth's axis by more than about 10 arcseconds,  so it is
neglected for most purposes;  along with aberration,  it
is included in the calculation of an object's apparent
position at current epoch.

obliquity to orbit 
   The obliquity to orbit of an object is the angle between its
polar axis and the axis of its orbit.  The most important thing it
does is to tell you how extreme the seasons would be on that planet.
   For example:  the obliquity of the Earth is about 23.5 degrees.
So at the summer solstice,  the north pole of the earth tilts toward
the sun by 23.5 degrees,  and at the winter solstice,  it tilts
away by the same amount.  This leads to fairly extreme temperature
   Uranus,  on the other hand,  has an obliquity of about 97.86
degrees,  almost a right angle.  This means that at its "summer
solstice",  the north pole points almost directly at the sun,  and
there is continuous daylight in most of the northern hemisphere of
that planet.  As the following table shows,  most planets have much
smaller obliquities,  and therefore less extreme seasons:

Mercury         ~0.1 deg
Venus          177.3 deg
Earth           23.45 deg
Mars            25.19 deg
Jupiter          3.12 deg
Saturn          26.73 deg
Uranus          97.86 deg
Neptune         29.56 deg
Pluto          118?   deg

Obs. ID 
   This item tells you the identification number of the
survey photographic plate on which this star can be found.

occultation double 
   Some stars are close to the path of the moon's orbit,
and from time to time,  the moon passes in front of,  or
occults, them.  Suppose this happens to a very close
binary star,  one so close together that even huge
telescopes can't see them as binary.  As the moon passes
in front of such a star,  we'll see the brightness drop,
but not totally vanish,  as one star goes behind the moon,
and then see the other star vanish,  leaving no light at
all.  This kind of star is an occultation double.
   Regrettably,  not many stars are close to the Moon's
path,  so this trick can't be used on every star.

open cluster 
   An open cluster of stars doesn't show a great deal of
structure.  It's a collection of from perhaps a dozen to
perhaps several thousand stars that are,  appropriately
enough,  clustered together.  The Pleiades are an example
of an open cluster,  one close enough and bright enough so
that most people can see seven closely packed stars with
unaided eyesight,  and dozens with binoculars.  The symbol
for an open cluster in this program is a dashed circle,
followed by a Messier or NGC number.
   Open clusters are usually made up of very young stars.
Quite a few have nebulae inside them.  Unlike globular
clusters,  many are relatively close by;  the Pleiades,
for example,  are a mere 300 light-years away.
   You can find an open cluster with the Go To menu.  The
brighter open clusters have Messier,  NGC,  or IC numbers
and can be found by those numbers.  Some of the dimmer
clusters have Stock or Collinder or other designations;
you can click on the Go to Open Cluster menu item to
get a list and select a catalog and number.
   You can control the display of open clusters in the
Data Shown menu.

opposition point 
   The Opposition point menu item,  inside the Go to Coordinate
submenu,  finds the point on the sky directly opposite the sun.
   You can also reach this option with the Alt-U hotkey.

Orbital arc 
   To determine an object's orbital elements,  it is best
to make as many measurements of its position as possible,
spread out over as long a period as possible.  An orbit
based on,  say,  four observations made over a few days
will not be as precise as one based on thousands of
observations made over a period of years.
   When you click for "more info" on an asteroid,  Guide
will usually tell you how many measurements were made of
the object and over how long a period ("orbital arc") they
were made.  That can help you to evaluate the quality of
the orbital elements.

orbital elements 
   There are several ways to describe the orbit of an
object.  The most commonly used method is by listing its
orbital elements.
   An object's orbital elements consist of seven numbers:
the semimajor axis,  eccentricity,  inclination of orbit,
argument of perihelion,  longitude of the ascending node,
mean anomaly,  and the epoch of elements.  Many astronomy
programs will let you define the orbit of an object by
entering these numbers.  Should you wish to add a new
asteroid or comet to Guide,  you can do so by hitting
the Ctrl-K hotkey and entering the orbital elements.
   Keep in mind that the elements describe the object's
position and velocity precisely at one instant,  that of
the epoch of the elements.  For a few months around that
instant,  they still will usually provide a good match
to the object's path.  (Since most comets are usually
visible for only a few months,  this is not a problem for
them.)  However,  as years go by,  the gravity of the
planets (especially Jupiter) can pull objects into very
different orbits.  (Jupiter sometimes even throws comets
out of the Solar System,  or into the Sun,  or into
closer orbits that take less time to orbit the Sun.)
For asteroids,  Guide gets around this limitation by
using fresh elements every 50 days,  so its data is never
more than 25 days out of date.

Orion type 
   Orion type stars are variable stars with irregular
eruptions.  They are probably young objects that will
evolve into non-varying stars someday.  They are found in
or near diffuse nebulae.  Some show,  as well as the
irregular outbursts in brightness,  more regular changes
as they rotate.  The irregular changes may reach several

Overlay menu 
   The Overlay menu contains all the tools required for
editing,  selecting,  and deleting overlays.  By default,
Guide only provides six overlays,  for constellation
borders,  constellation labels,  constellation lines,
planet trails,  the Palomar Observatory Sky Survey,  and
the AAVSO Atlas.  You can use the Overlay menu to adjust
the display of these overlays,  and to add and edit your
   You can reach this option from any point in Guide by
hitting the Alt-O hotkey.

   This plate in the GSC was taken using the Palomar
Schmidt telescope,  using a singlet corrector,  103aE
emulsion,  and a red plexiglass filter.

   This plate in the GSC was taken using the Palomar
Schmidt telescope,  using a doublet corrector,  IIIaJ
emulsion,  and GG 385 filter.

   This plate in the GSC was taken using the Palomar
Schmidt telescope,  using a singlet corrector,  IIaD
emulsion,  and Wratten 12 filter.  Most of the northern
plates were taken in this manner.

   This plate in the GSC was taken using the Palomar
Schmidt telescope,  using a singlet corrector,  IIaD
emulsion,  and GG 495 filter.

   This plate in the GSC was taken using the Palomar
Schmidt telescope,  using a doublet corrector,  IIIaJ
emulsion,  and GG 495 filter.

trigonometric parallax 
   Parallax is a method used to determine a star's
distance.  As the Earth orbits the Sun,  our motion makes
nearby stars appear to wobble back and forth as we see
the stars first from one point in our orbit,  then from
another.  This motion is always very small,  but with good
instruments,  it can be measured.  The amount of the
movement is the star's parallax.  Once the parallax is
known,  the distance easily follows.  (This method is also
known as trigonometric parallax,  to distinguish it from
a few non-geometric ways to derive the same information.)
   Only a small number of stars are close enough for us
to measure their parallaxes accurately.  For the rest,  we
need to rely on more complex ways to gauge distance,  such
as by calculating dynamic parallax.

   A parsec is a unit of measurement roughly equal to 3.26
light-years.  It is the distance from which the angle
between the Earth and Sun would appear to be (at most)
one arcsecond.  It is equal to about 19.2 trillion
miles,  or 30.8 trillion kilometers.

Partial Events 
   The Partial Events option appears in the Extras menu,  when
in eclipse mode.  It provides a way to tell Guide to find not
only total/annular events,  but partial ones as well.  It defaults
to being on.
   Suppose you have a chart of a solar eclipse path on the screen,
generated using the Show Eclipse function;  and that you've set
the Local Events only switch on (telling Guide to only show events
visible from the current screen center).  If this option is checked
(the default),  clicking on "Next" or "Previous" will lead to the
next or previous event visible from that position,  whether it is
total,  annular,  or partial.  If the option is not checked,  Guide
will keep looking until it finds a total or annular event.  (Since
total or annular events are rarely visible from a given point,  Guide
may have to search a long time before it finds one!  Don't be
distressed if the computer seems to lock up for a minute or so while
the computation proceeds.)

   When the moon passes through the earth's shadow at the
time of full moon,  it's possible that the earth will
only partly block the sun's light.  If you were on the
moon,  you would see the sun with a "chunk" removed from
it.  This part of the shadow is called the penumbra (as
opposed to the umbra,  the part of the shadow where the
sun is completely blocked.)
   During a lunar eclipse,  if the moon only enters the
penumbra,  it will get slightly dimmer,  but usually not
by very much,  because enough sunlight is hitting it to
keep it well-lit.  If it partly enters the umbra,  then
those parts of the moon in the umbra will appear
essentially dark (with a tint of orange).  Often,  the
entire moon will enter the umbra;  the time during which
the moon is fully eclipsed in this manner is called

  Sun                           Earth           Moon


@c 50,310,45   sun
@c 300,310,16  earth
@c 400,310,4   moon
@m 450,367
@l 52,265
@l 450,310
@l 52,355
@l 450,257

   If the orbit of a binary star can be determined,  then the
point where the two stars are closest together is referred to
as periastron.  In some cases, Guide is able to give a date
for when this takes place.