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.