UT
Universal Time
The time system usually used for timing astronomical
events is Universal Time, also known as UT, or Universal
Time Coordinated, or UTC. This is essentially the civil
time used at Greenwich; corrected for time zones, it is
used throughout the world.
To convert from UT to your local time zone (in the US),
subtract five hours to get EST, six to get CST, seven
for MST, and eight for PST. For Daylight Savings time,
subtract one hour less than this.
For most operations, Guide uses the time zone chosen
in the time dialog.
UV Ceti
The UV Ceti type of variable is a star with occasional
massive flares. When such a flare erupts on one of these
stars, the brightness will climb up, sometimes by six
magnitudes, within several to a dozen seconds. It will
slowly return to normal over several to a dozen minutes.
This type is usually of low luminosity, even lower than
the Sun.
v sin i
If you look at a rapidly rotating star's spectrum, you
will find that its rotational velocity makes things more
"blurry" than you might normally expect. Measuring this
blur gives you a minimum rotational velocity, called
v sin i. The 'v' means the speed of rotation, measured
in kilometers per second. The 'sin i' accounts for the
fact that the star may be tilted toward us, and we can't
measure velocity going "sideways", only that toward us or
away from us. The star could have a lot of sideways spin;
we can't tell. That's why v sin i is a minimum speed;
the real speed isn't known.
V(1,0)
The quantity V(1,0) is the visual magnitude a planet would
have if it were one astronomical unit from the Sun, as seen by
an observer on the Sun. The "1,0" means "one AU away, at zero
phase angle." It is essentially the same quantity as the
absolute magnitude H that is used for asteroids.
V-I
The V-I data for a star indicates the difference between
its V (visual) magnitude and its I (a species of infrared)
magnitude.
van den Bergh
vdB
The van den Bergh catalog of reflection nebulae lists
central stars, their magnitudes, and other information.
Be aware that not all van den Bergh nebulae are shown.
Variability
The Bright Star catalog sometimes will remark on
the variable behavior of stars. These comments usually
involve the kind of variability, its period, and the
maximum and minimum magnitudes (or sometimes just the
amplitude, or difference between max and min).
variable
Many stars change in brightness over time; such a
star is called a variable star.
There are several types of variable stars, such as
Cepheid variables, long period variables, novae
and supernovae, cataclysmic variables, eclipsing
binaries, R CrB stars, and others. They vary over time
frames ranging from hours to 27 years. Some vary on a
regular schedule predictable years in advance; some vary
too irregularly to predict. (Most fall somewhere in
between.) Each variable is given a variable star
designation.
variable star designation
Except for some very bright variable stars such as Beta
Lyrae and Algol, each variable star is assigned a
variable star designation to identify it. The system for
doing this has evolved over time as the number of known
variables has grown. The designation is either a letter
from R to Z, or two letters, or V plus a number, and
a constellation.
At first, variables were designated by a letter and
constellation. The letters ran from R to Z (A to Q were
already in use). Thus, R CrB is the first variable found
in Corona Borealis, and T Pyx is the third found in the
constellation Pyxis.
This worked until a tenth variable was found in a
single constellation. The workaround was to use pairs of
letters, still using only R to Z, in the order RR, RS,
RT, ...RZ, SS, ST, ...SZ, TT, TU, ...TZ, and so on,
up to ZZ. This allowed for 45 more variables per
constellation. When that limit was reached, subsequent
stars were labeled AA, AB, ...AZ, BB, BC, ...BZ, and so
on, up to QQ. Now it was possible to label 334 variables
per constellation.
When even this did not prove to be enough, it was
wisely decided to switch to numbers, so QQ was followed
by V335, V336, and so on. Some constellations are now
past 4000 variables.
VBH
Reference: Van den Bergh, S., Herbst, W.: 1975, Astronomical Journal
80, 208. Catalogue of southern stars embedded in nebulosity.
Venus
Venus is the second planet from the Sun. Because it
is 72% as far from the Sun as we are, it collects twice
as much sunlight per unit area as we do. Its atmosphere
is heavy in carbon dioxide, which does an excellent job
of retaining heat. Between the extra sunlight and CO2,
Venus maintains a surface temperature of about 900 F
(500 Celsius).
Both the US and USSR have sent several probes to Venus.
It's difficult to make a probe that withstands 500 degree
temperatures and an atmosphere 100 times denser than our
own, but it can be done, and images from Venus' surface
show a barren, desertlike landscape. Lightning is almost
continous, providing most of the light on the ground.
The US Magellan spacecraft has produced detailed radar
maps of Venus, showing topography and ground character-
istics.
vernal equinox
Roughly speaking, the vernal equinox is the position
of the Sun as seen from Earth on about March 21. (Vernal
means "spring", and you will sometimes hear of the spring
equinox.) More precisely, the vernal equinox is one of
two places where the sun's path in the sky crosses the
celestial equator. (The other is the autumnal equinox
and is on exactly the opposite side of the sky.)
Right ascension is measured from the vernal equinox;
the RA of the vernal equinox is 0h0m0s. Right now, the
vernal equinox is in the constellation of Pisces. Over
millenia, it slowly drifts around the sky because of
precession.
The time when the sun will reach the vernal equinox is
given in the "click for more info" section for the Sun,
as are times for the autumnal equinox and the solstices.
view downloaded Ax.0
Most people will probably not be able to use the full ten-CD
version of the A1.0 or eleven-CD A2.0 anytime soon. Those
people can, however, download a small regions of Ax.0 data from
this site:
http://www.lowell.edu
If you rename that file to A10.DAT, and put the file in your
Guide directory, then Guide will draw the stars in that data,
just as if they had been extracted from the CD. For this to work,
the data must be downloaded in binary form (the Lowell Web site
allows for binary or ASCII text downloads).
Virgo infall
In theory, the expansion of the universe is supposed to
be a pretty "smooth", "un-lumpy" process; if one galaxy
is twice as far from us as another, it should appear to
recede from us twice as rapidly. In reality, many factors
conspire to make this idea only approximately correct.
One reason such things happen has to do with the nearby
cluster of galaxies in Virgo. Its gravitational pull
modifies our own velocity (and that of some other galaxies).
If you compute that effect and subtract it out, then you
can get galaxy velocities that better match the Big Bang
theory.
At least one galaxy catalog, the PGC, often lists
such velocities "corrected for Virgo infall". When it
provides this data for a galaxy, Guide will list it
when you ask for "more info" on that object.
visible light
Visible light is the small portion of radiation that
the human eye can detect, with wavelengths between
(for humans) about 400 to 700 nanometers.
Humans who by bad luck come to lose an eye lens can sometimes
see a little bit more. If the lens is replaced by a perspex one
they have been known to see the central stars of planetary nebulae
better than most of us. Their vision stops at 300 nm, which is all
right, because the ozone layer cuts out all ultraviolet of shorter
wavelengths.
Insects, fish and birds also are more perceptive to the
wavelength range between 300 and 400 nm.
As the wavelength becomes shorter than 400 nm (violet), the light
is described as ultraviolet (just beyond the visible violet light),
and then X-ray, and gamma rays. As it gets longer than 700 nm,
the light is described as infrared (just beyond the visible red light),
microwave, and radio.
Astronomical objects show many interesting properties outside of
visible light, though of course, you need proper equipment (rather
than just eyes and a telescope) to detect that fact.
visual magnitude
An object's visual magnitude is a measure of its
brightness, or magnitude, as observed by a human eye.
This is usually different from the brightness observed
on photos, or photographic magnitude.
Visual magnitude limit
When you ask for Quick Info, one of the many items shown will
be the visual magnitude limit for the center of the screen, at
your currently set position on the earth and date and time. This
limit is computed using the method described by Brad Schaefer in
the May 1998 issue of _Sky & Telescope_.
Guide will use the temperature, pressure, altitude, and
humidity data supplied in the Location dialog in computing the
visual magnitude limit.
Volumetric mean radius
A planet's volumetric mean radius is the radius it would have
if it were a sphere, with zero flattening (basically, the radius
it would have if it did not rotate).
W Vir
The W Vir type of variable star "looks", based on
its changes in brightness, a lot like a Cepheid variable,
except with a longer period. However, W Vir types have
a different light curve: theirs bumps up briefly on the
decline. Also, W Vir stars have a fixed relationship
between the length of their periods and their luminosity,
just as Cepheids do, but it's a different relationship.
wavelength
You can think of light (and other radiations, such as
infrared, radio, microwave, etc.) as consisting of
waves. Like the sort of waves you see at a beach, there
is a certain distance between consecutive crests; this is
called the wavelength of the radiation. Visible light
consists of radiation with a wavelength of from 400 to 700
nanometers. As one goes from 700 nm to 400 nm, the
light runs from red, to orange, yellow, green, blue,
and violet.
WDS
Washington Double Star
The Washington Double Star catalog is an extensive
list of data on double stars. In many cases, Guide is
able to find and list information from this catalog for
a given star.
white dwarf
A white dwarf is the final stage in the evolution of
a star with less than about eight times the mass of the
Sun. These stars lose much of their mass by blowing away
a strong wind of gas. Eventually, this makes a planetary
nebula, and the star becomes a small object with a huge
density (of the order of tons per cubic centimeter). The
best-known examples are the companion stars to Sirius and
Procyon; these objects have the mass of our Sun
compressed down to the size of the Earth.
A white dwarf starts out with a lot of heat, and is
in fact "white hot". But unlike most stars, it has no
power source. It can stay visible for a long time while
the heat of its creation bleeds off, but it will
eventually die down into obscurity.
A star with 1.4 times the mass of the Sun or less will
probably form a white dwarf after exploding. Heavier
stars form neutron stars and black holes.
Wolf-Rayet
A Wolf-Rayet star is a very luminous star with a high
surface temperature, often with large eruptions and some
irregular small light variations (up to about .1
magnitude).
Writing help information to an ASCII file
You can use the "Save to File" option at the bottom of
any help screen to store the currently displayed help
topic in an ASCII file. You can then use that file in
most word processors.
When you click on this option, Guide will ask for
the name of the file you want to create. Guide will then
open that file and write the help information to it.
X-ray
X-ray radiation is radiation with a wavelength of
roughly 5 to .005 nanometers. This is fairly energetic
radiation, which is why it can penetrate soft tissues and
can be used to find broken bones and such. When detected
from celestial objects, it usually indicates something
tremendously energetic, such as a cataclysmic variable
or an X-ray variable star.
X-ray burster
X-ray burster stars are a form of X-ray variable star
producing bursts of X-rays and visible light. The bursts
last from a few seconds to ten minutes, and amount to .1
magnitude or so. Examples are V801 Ara and V926 Sco.
X-ray irregular
An X-ray irregular star is a close binary system, with
a hot compact object (white dwarf, neutron star or black
hole) with an accretion disk formed by matter from its
companion. This companion is a dwarf star. This type of
variable star changes in brightness irregularly over time
scales from minutes to hours. The changes amount to about
one magnitude or so. Sometimes there is a periodic
variation due to the orbital motion of the stars, as
takes place in V818 Scorpii.
X-ray jet variable
An X-ray jet variable is an X-ray variable star where
the matter falling on the neutron star or white dwarf or
black hole gives rise to jets. In the case of X-ray jet
stars, the jets are moving at a significant fraction of
the speed of light. An example is V1343 Aquarii.
X-ray novalike
An X-ray novalike object is similar to a normal nova,
except that it also releases X-rays. Its outbursts may
last several months. An example is V616 Monoceros.
X-ray reflector
An X-ray reflector object is an X-ray variable with the
reflection effect. The reflection effect comes into play
when the pulsar component irradiates its companion with
X-rays, some of which are scattered toward us. These may
vary by 2 to 3 magnitudes. An example is HZ Herculis.
X-ray variable
X-ray variable stars are close binary stars with one
member a white dwarf, neutron star, or black hole. Mass
flows from the other star to this very compact object,
making an accretion disk. When the matter falls in, the
impact creates X-rays.
Yale
HR
Bright Star
The Yale Catalog of Bright Stars, or HR catalog, is
a list of 9,096 of the brightest stars in the sky. This
isn't a lot of stars -- in fact, it's much smaller than
the 258,857 stars in the SAO catalog or the 18 million
objects in the GSC catalog -- but it does contain a lot
of interesting information on each star, such as its
Bayer and Flamsteed designations, proper motion,
parallax, magnitude, and miscellaneous comments.
You can find a star's HR number by clicking on it with
the right mouse button. (Of course, it may not have
one.) You can go to any HR star by using the Go to Yale
(BSC) option in the Go to Star menu, in the Go To
menu.
YY Ori
Some stars, like YY Ori, show features in their
spectrum that indicate matter is falling onto their
surfaces; in other words, the star has an accretion
disk.
Z Andromedae
The Z Andromedae type of variable stars are close binary
stars, consisting of a hot star, a star of late spectral
type (like an M, R, N, or S), and an extended envelope
of gas around them excited by the hot star's radiation.
The combined brightness of the two stars displays
irregular variations with amplitudes up to 4 magnitudes
(visual).
Z Aqr
The Z Aqr type of variable star has a fairly old
spectral type (like an M, R, N, or S) and shows a regular
period of variability from about 35 to 1200 days in length.
The range of variation is usually less than 2.5 magnitude.
Many of these stars differ from Mira type stars only by
showing smaller light variations.
Z Cam
Z Cam stars are a subclass of U Gem stars. They are
different because sometimes after an outburst they do not
return to normal, but stay between maximum and minimum
for a few cycles. The cycles take from 10 to 40 days,
while the light increase during an outburst is two to five
magnitudes.
zenith
nadir
Go outside and look straight up. That point is the
zenith. Now look straight down. If the earth were a
little more transparent, you could see the nadir, the
point in the sky opposite the zenith.
Because the earth rotates, these points don't stay
fixed among the stars. Because the earth is round, they
also vary from place to place; i.e., the stars you see
overhead are not the same as those someone a thousand
miles away will see.
zodiac
The Zodiac represents the twelve constellations through
which the Sun travels during the year: in order, Aries,
Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius,
Sagittarius, Capricornus, Aquarius, and Pisces. (In
reality, the Sun also passes through the constellation
Ophiuchus, but this was not true when the zodiac was
created.)
You will note that these are the same constellations
used by astrologers. The dates used in newspapers represent
the days during which the Sun is in that constellation.
The days used by astrologers are based on calculations done
4000 years ago, and are now about a month different from
reality.
zoom and recenter
You can "drag" a box with the mouse to define an area
to zoom in on.
First, click with the left mouse button on the point
where you want to recenter. Holding that button down,
move the mouse; as you do so, a box will appear showing
the area you'll zoom in on. Also, a number will appear
showing the new zoom level you will arrive at when you
release the mouse button.
Zoom In
*
When you hit the '*' key at any point in Guide, you
will zoom in by one zoom level. It's similar to a
telescopic zoom: you see less sky but more detail.
Each time you zoom in, the screen size drops by about a
factor of two. When you reach level 20, you can't zoom
in further.
Zoom Out
/
When you hit the '/' key at any point in Guide, you
will zoom out by one zoom level. It's similar to a
telescopic zoom: you see more sky but less detail. Each
time you zoom out, the screen size jumps up by about a
factor of two. When you reach level 1, you can't zoom
out any farther and you are seeing roughly an entire
hemisphere of sky.
Zwicky
The Zwicky catalog of clusters of galaxies is one of
two such catalogs available in this program. (The other
is the Abell catalog.) It contains 9,134 objects, but
only covers the northern half of the sky.
You can find an object by its Zwicky number by using
the Go to Zwicky Cluster option in the Go to Galaxy menu
in the Go To menu.
ZZ Ceti
ZZ Ceti type variable stars are pulsating white dwarfs.
They change in brightness over periods from 30 seconds to
25 minutes, by .001 to .2 magnitudes. There are usually
several cycles running simultaneously. Sometimes flares
of about one magnitude are observed, but these may be
due to UV Ceti type companion stars.
Main Menu
The main menu provides access to the following options:
File menu (set up and go to mark files; print; exit
the program)
Go To menu (find an object, enter coordinates, or find
a point on the horizon)
Settings menu (set your latitude/longitude, time,
"home planet", etc.)
Display menu (control the various markings on the screen)
Data Shown menu (control what celestial objects appear,
when they appear, and how they are shown) (DOS version)
Animation (show solar system objects moving across the
sky; create trails and ephemeris data for them)
Overlay menu (add your own markings to the charts;
create and maintain your own lists of objects)
Tables menu (create lists of events such as lunar phases
for any time span)
Extras menu (advanced options such as viewing A1.0 and
RealSky images, or adding new comets)
The "Abell", "Zwicky", and "Abell + Zwicky"
options let you control display of clusters of galaxies,
and to select the catalogs from which they are drawn.
Guide uses two catalogs for clusters of galaxies.
The Zwicky catalog covers the entire northern half
of the sky, and contains 9,134 objects. The Abell
catalog covers the entire sky, but only contains about
5,000 objects.
Usually, you will probably want to show objects
from both catalogs, which is why "Abell + Zwicky" is
selected by default. Sometimes, you may find this too
cluttered, and will opt to pick one catalog. A check
mark is shown on the selected item.