AM Her 
   The AM Her type of X-ray variable is a binary star:  a
pulsar with a strong magnetic field orbiting a cool star
(spectral type K or M).  Matter falls from the cool star
to the pulsar's magnetic poles,  which creates light;
this light varies in polarization,  which is why this sort
of object is also known as a "polar".  Usually,  the light
changes are around one magnitude,  but sometimes these
stars are also X-ray reflectors.  That can contribute an
additional 3 magnitudes,  for a total variation of 4 or 5
magnitudes.  Examples of these objects are AM Her and AN

   Objects in the Amor family of asteroids cross the
orbit of Mars and approach that of Earth.  About one in
ten will cross our orbit in the course of a few hundred
to a few thousand years.

   An Angstrom is one-ten-billionth (.0000000001) of a
meter,  or one-tenth of a nanometer.  The wavelength of
light is usually measured in either nanometers or Angstroms. For
example,  for humans visible light has wavelengths between
about 4000 to 7000 Angstroms.

angular diameter 
   All objects in the sky have an angular diameter:  the
amount of sky they appear to cover.  The Sun and Moon,  for
example,  each cover about 1/2 degree of angular diameter.
The stars all cover far less than an arcsecond,  which is
why they appear as pinpoints in a telescope.  The planets
are in between,  and appear as small disks in a telescope.

   The Animation menu lets you move planets, asteroids,
comets,  and artificial satellites in time-lapse animation
across the sky at a desired time rate.  You can also generate
trails showing the path of an object over a given time period,
or generate an ephemeris on your hard disk.
   This item can be reached at any time via the ALT-A
   Help is available on:

   Starting/stopping animation
   Make Ephemeris
   Add a Trail

aperture circle 
   Using the aperture circle can be helpful in matching
what you see in binoculars or a telescope to what's on the
screen.  To use the aperture circle,  you need to first
find the field of view of your binoculars or telescope.
For binoculars,  this is probably in the neighborhood of 3
degrees;  if your binoculars have a manual,  it will most
likely give a more precise figure.
   For a telescope,  the field of view is calculated from
the eyepiece.  Divide its apparent field of view (usually
mentioned in ads for the eyepiece) by the magnification it
provides.  For example,  an eyepiece with an apparent field
of view of 60 degrees and a magnification of 50 would give
a 1.2 degree field of view;  that's the actual size of the
circle you see in the sky.
   Display of the aperture circle is controlled from with
the Measurements dialog.

Aphelion distance 
   This line gives the asteroid's distance from the Sun
at aphelion,  its maximum distance from the Sun.  Since
some asteroids have very highly eccentric orbits,  this
may be much more than the average distance from the Sun.

   Objects in the Apollo family of asteroids cross the
Earth's orbit,  and some come closer than the Moon.

apparent magnitude 
   An object's apparent magnitude is its magnitude as
actually measured from Earth,  with no correction made for
the distance of the object.  For some objects,  such as
Cepheid variables,  we can infer their actual,  inherent
brightness,  also known as their absolute magnitude.

Apparent position at current epoch 
   Positions given as "apparent position at current
epoch" are identical to those for the "mean position at
current epoch",  except that the effects of nutation
and aberration have been included.
   It's important to notice that,  in most cases,  the
only position that will matter to you is the J2000
position. Coordinates in this system will match those read
from star charts,  for example.  In theory,  one would use
the apparent position at current epoch with setting
circles,  or with meridian circles and transits
(specialized devices for measuring the positions of
objects).  In reality,  the precision of most setting
circles won't show the (fairly small) difference between
J2000 coordinates and apparent position coordinates.

AR Lac 
   The AR Lac type of variable star is an eclipsing
binary,  with the stars very close but not in contact,
and both stars being subgiants (meaning about the size
and luminosity of the Sun,  roughly).

   An arcminute is a unit for measuring angles between
objects in the sky.  It's a small unit:  the Moon and Sun,
for example,  are about 30 arcminutes across,  and the
distance from the horizon to the point straight overhead
(the zenith) is 5,400 arcminutes.  A single quote mark,
or ',  is usually used to indicate arcminutes:  i.e.,
"The Moon is 30' across."  There are 60 arcminutes to a

   An arcsecond is a unit for measuring angles between
objects in the sky.  It's the smallest unit used.  If
you looked at a CD-ROM disc from a distance of about
40 km (24 miles),  it would appear to be about one
arcsecond across.  The symbol for arcsecond is a double
quote mark,  or ".  There are 60 arcseconds to an
arcminute,  and 3,600 arcseconds to a degree.

argument of perihelion 
   The argument of perihelion is a measurement used in
defining the shape of an object's orbit.  In combination
with other numbers,  called orbital elements,  it is
part of a complete definition of the orbit of an object.
It is usually represented by a lowercase Omega.

   The Arp catalog contains over 300 peculiar galaxies,
mostly cases where multiple galaxies are interacting with
one another.  You can find an Arp galaxy with the Go to
Arp menu option.

   The Hubble Guide Star Catalog,  or GSC,  classifies
some objects as artifacts.  The computer that assigned
object types sometimes encountered an object that it did
not believe was a real celestial object,  such as a
scratch on the plate or a diffraction spike caused by
a very bright star.
   In reality,  there are several scratches and other
marks that the computer was unable to recognize as
artifacts,  especially on some of the southern plates.
But most are so regular in nature that a human will
immediately recognize them as artificial anyway.

   The Astronomical Society of South Australia (ASSA) maintains certain
variable star charts.

Assumed asteroid diameter 
   Here,  Guide is making a rough guess about the diameter
of the asteroid you clicked on.
   It's not hard to measure how bright an asteroid is (its
magnitude),  and it's also not hard to figure out how far
away it is.  Combine the two,  and you know how much total
sunlight it is reflecting.  But you still don't know if it
is a huge,  but not very reflective,  or if it is small,
but reflects a lot of light.  With suitable equipment,
you can measure the percentage of light reflected (called
its albedo),  and then calculate the asteroid's diameter.
But this has not been done for very many asteroids.
   If an asteroid's albedo is unknown,  Guide assumes it
is .04 (that is,  4% of the sunlight it receives is
reflected),  and calculates a diameter based on this.  If
the asteroid was a perfect reflector (100% of light
reflected),  then it would be five times smaller than the
figure Guide gives.  On the other extreme,  you could
imagine a very non-reflective,  "Stealth" asteroid that
could be as huge as you like.
   In reality,  Guide's guess based on a .04 albedo will
probably be quite good.

   The term asteroid means "starlike".  Asteroids are
small chunks of rock orbiting the Sun;  none is big
enough to be considered a real planet,  though they are
also called "minor planets".
   The first asteroid found,  1 Ceres,  was located by
accident in 1801.  It's the largest,  at about 900 km
(540 miles) across. Three more,  2 Pallas,  3 Juno,  and
4 Vesta,  were found in the following seven years.  These
are the "Big Four" of the asteroids;  after that,  it was
not until 1847 that a fifth was found.  Since then,
thousands have been found.  Most stay between Mars and
Jupiter, but a few come close to and even hit the
Earth.  (Meteor Crater in Arizona is an example of what
can happen when an asteroid a few dozen feet across hits
the Earth.  Some believe really huge asteroids cause mass
extinctions, such as when the dinosaurs vanished from the
   The first four asteroids are sometimes good binocular
targets.  Most of the others require decent telescopes to

Asteroid diameter 
   For some asteroids,  this program can tell you their
diameter,  in kilometers.  These are usually relatively
small:  the largest asteroid,  Ceres,  is about 900 km
(540 miles) across,  and most asteroids are much smaller.
(For comparison,  Earth is 12000 km in diameter.)

astrometric binary 
   In some cases,  a binary star can be watched over a
long time and its orbit measured,  using a telescope and
specialized measuring equipment.  A star that can be
measured in this way is an astrometric binary.
   The quality of the orbital data varies from star to
star,  and depends on how long we've been watching it.
Some stars take millenia to orbit one another,  so our
knowledge of what their orbits look like is based on
seeing a small part,  a bit like guessing the shape of
a highway based on a small segment of road.  Some take
only a few years to orbit one another,  and we have a
good idea of what their orbits are like.

   Astrometry is the science of measuring the positions of
celestial objects.  That sort of measurement has been done in many
ways over the centuries;  right now,  almost all of it is done by
taking an image of the object in question with a CCD,  and comparing
its position to that of known objects in the image.  Usually,  those
"known objects" have positions listed in the GSC,  Ax.0,  or
SAx.0 catalogs.  Soon,  the measurements will probably be made
using positions from the Hipparcos or Tycho catalogs,  which are
much more precise than earlier catalogs.

Asymmetric RR Lyrae 
   Asymmetric RR Lyrae variables are RR Lyrae stars that
take much less time to increase in brightness than they do
to decrease.  Their periods run from 7 to 29 hours,  and
they vary by .5 to 2 magnitudes.

   Objects in the Aten family of asteroids spend most of
their time inside the Earth's orbit.  They may intersect
our orbit when at their farthest from the Sun.

astronomical unit 
   One AU,  or astronomical unit,  is the distance from
the Sun to the Earth,  or about 93 million miles, or about
148 million kilometers.  It makes a convenient unit of
measure within the Solar System.  When we say,  for
example,  that Saturn is 9.5 AU from the Sun,  you can see
right away that it is about ten times farther out than we
are and must be correspondingly colder.  Saying that Saturn
is about 880,000,000 miles out carries no such obvious
information and requires us to absorb too many digits.

autumnal equinox 
   The autumnal equinox is the position in the sky where
the Sun appears to cross the celestial equator on or
close to September 21.  When the Sun reaches this point,
it is considered to mark the beginning of autumn in the
Northern Hemisphere.
   The time when the sun will reach the autumnal equinox
is given in the "click for more info" section for the Sun,
as are times for the vernal equinox and solstices.

B magnitude 
   A B magnitude value indicates the brightness of an object in
a particular photometric band.  For B magnitudes,  that band is
centered on 440 nanometers (4400 Angstroms).

   If you measure the B magnitude of an object and its
V, or visual magnitude,  the difference,  B-V,  is a
good measure of its actual color.  For example,
Betelgeuse has B-V = 1.85,  indicating that it is quite
red (though there are plenty of stars that are far
redder,  and therefore have higher B-V values).

   On the other hand,  Rigel has B-V = -.03,  indicating
that it's a very hot,  blue star.  Most stars fall
between these extremes,  except for a few redder-than-red
stars (mostly carbon stars) and a few bluer-than-blue
stars (mostly young,  high-mass stars).

Background Dialog 
   The Background Dialog can be accessed through the Display Menu.
It provides controls over the color used in chart backgrounds on the
screen, and over the display of the ground (the ground can be filled
in with a solid color,  and you can optionally show objects around the
horizon.)  It shows the following options:

   Normal Colors
   Chart Mode
   Red Mode

   Show Ground
   Horizon Objects

   The default "Normal Colors" shows a black background;  "Chart Mode"
shows a white background.  "Red Mode" uses a black background and shades
of red,  and is intended for use at night (many people find that red light
is less destructive of night vision).  "Flashlight mode" uses a _red_
background;  it turns your monitor into a red-light flashlight,  which
can sometimes provide enough light to find dropped objects at night.
   "Realistic" mode has a light blue background if the Sun is above the
horizon,  black at night,  and shades of dark blue during twilight hours.
One advantage of this is that one can immediately tell if a particular
event will be visible;  and if it occurs during twilight,  one can tell
roughly how bright the sky will be.
   "Show Ground" causes Guide to fill the entire area below the horizon in
a solid color.  "Horizon Objects" helps avoid one common problem in Guide:
maintaining a frame of reference.  This check-box provides a set of objects
(a house,  a barn,  several trees,  two telescopes,  and so on) around the
   Data for horizon objects appears in the text file HORIZON.DAT.  If you
examine it with a text editor,  you will see that it is quite easy to add
or change objects to match the horizon at your own viewing site.  Directions
on how this is done are given at the bottom of HORIZON.DAT.

   When hydrogen is heated,  its spectrum,  like that
of any gas,  shows certain characteristic lines.
Conversely,  if you spot these lines in the spectrum of
an object,  you can assume it contains heated hydrogen,
and even figure out how much and how hot it is.
   In the case of hydrogen,  there are several lines at
particular wavelengths given by the formula:
                  91.1768 nanometers
wavelength = ----------------------------------------
                     2          2
               (1 / a   -  1 / b )
   where a and b are integers.  Two sets of lines are of
particular importance:  if a=1,  you get the Lyman
series,  with wavelengths of 121.6,  102.6,  97.3,  95.0
nanometers,  and so on...  a set of lines getting closer
together,  to a limit of 91.2 nanometers.  All these,  by
the way,  are in the ultraviolet.
   If a=2,  you get the Balmer series,  with wavelengths
of 656.5, 486.3, 434.2, 410.3 nanometers,  another series
that closes in on a limit of 364.7 nanometers.  These lines
are visible.  The 656.5 nm line is called the H alpha
line;  the 486.3 nm line is called the H beta line.

   Many dark nebulae have Barnard numbers.  Objects may be
found by their Barnard numbers using the Go to Barnard option
in the Go to Nebula menu in the Go To menu.

   Most of the brighter stars can be specified by Bayer
Greek-letter and constellation.  Examples are Alpha
Centauri and Tau Ceti.  The letters usually run in order
of brightness,  with the Alpha star brighter than the Beta
which is brighter than the Gamma,  and so on,  but this
is not a hard and fast rule.  In this program,  all the
Greek letters that you see on the screen are Bayer
letters.  You can remove these by turning Bayer letters
off in the Star display dialog.
   You can find a particular Bayer star through the
Go to Bayer/Flamsteed menu,  under the Go to Star menu in
the Go To menu.
   Bayer introduced this method in a noted 1603 star
atlas.  He also introduced several new constellations,
filling in some gaps and creating some in the Southern
Hemisphere,  at the time only recently seen by Europeans.
Twelve are still in use.

Click for Greek alphabet

   This plate in the GSC was taken using the astrograph
(telescope especially designed for imaging large fields)
at the Black Birch Observatory in Blenheim,  New Zealand,
using 103aO emulsion and GG400 filter.

Reference: Brand, J., Blitz, L., Wouterloot, J.G.A.: 1986 Astron.
Astrophys. Suppl. 65, 537. The velocity field of the outer galaxy in
the southern hemisphere. I. Catalogue of nebulous objects.

   The DM or Durchmusterung catalog is divided into
four sections.  The first,  the BD or Bonner Durch-
musterung,  was compiled earliest and extends to -1
degree in declination.  A later effort,  the SD or
Sudentliche Durchmusterung,  extended this to about -23
degrees.  Finally,  the CPD or Cape Photographic Durch-
musterung,  and the CD or Cordoba Durchmusterung,
continue the catalog down to the south pole,  providing
coverage of the entire sky.

Be star 
   A Be star has spectral type B,  meaning a younger,
hotter star than the Sun.  The "e" refers to the fact
that these stars show emission lines due to hydrogen.
   These stars are usually rapidly spinning,  and are
slowly pushing mass into a disk around their equator.
The result is irregular light variation,  making a Gamma
Cas type variable star.  Some vary for other reasons
as well.

Beta Lyrae 
   The Beta Lyrae class of variable star is a combination
of an elliptical variable and an eclipsing binary.  These
objects consist of two stars that eclipse one another as
they orbit.  They are also close enough that their mutual
gravitational pull raises tides on each other,  so they
become elliptical.  This means that at different times,
we view them from different angles and see a different
amount of light,  even when they aren't eclipsing one
another.  This combination makes for a fairly smooth,
continuous change in brightness,  unlike the abrupt
changes of an eclipsing binary.

   A binary is a special (and common) case of a double
star.  In a binary,  two stars that appear close
together are actually orbiting one another.  Alpha
Centauri,  the star closest to the Sun,  consists of two
stars orbiting one another, plus a third star (Proxima
Centauri) orbiting that pair.
   About a third of stars occur in multiple-star systems.
More than two stars in a system is a little unusual;
usually,  such a system is unstable and one star gets
thrown out.
   Sometimes the stars can be separated by telescope;
some,  spectroscopic binaries,  can only be split by
a spectroscope.

BL Her 
   The BL Her type of variable is simply a W Vir type with
a period of less than eight days.

BL Lac 
   BL Lac objects are not really variable stars.  They are
compact quasars with fairly rapid,  irregular changes in
brightness of up to 3 magnitudes.  A few of these objects,
like BL Lacertae itself,  were once thought to be variable
stars until closer examination revealed their true nature.

black hole 
   If a star is more than about 3.2 times as massive as
the Sun when it becomes a supernova,  it may compress
its core even beyond the neutron star stage,  to a point
where no force can resist further compression.  The object
simply continues to shrink until it vanishes from sight,
leaving a black hole.
   To escape from a black hole would require an infinite
amount of energy,  and therefore,  not even light can
leave.  The black hole itself is invisible.  However,
objects falling into it will be heated up as they do so,
and the glow of radiation from this can be detected.

Black stars on white/white on black 
   This option toggles between printouts with black
stars on a white background (the usual choice) and
white stars on a black background.  Not many people
will want the latter,  but for some uses in the field,
such charts can be quite helpful.

blended object 
   The Hubble Guide Star Catalog,  or GSC,  classifies
some objects as blends,  or blended objects.  The term
means that the object appeared to be two nearby objects
(probably stars),  but the computer that assigned the
object types was not really sure what it was looking at.
It is not a very common classification.

Brighten Stars 
   You can hit the + key at any point in Guide to make
the stars a little brighter,  and to raise the limiting
magnitude for all objects.  Each use of this option
adds .5 to the current limiting magnitude.
   You can also add a button for this option to the toolbar,
by using the Toolbar Dialog.