Brightness level extremely approximate For most nebulae in the Nebula Databank, the brightness level could be gathered from the Lynd's Bright Nebula catalog, IC, NGC, Sharpless, or van den Bergh catalog data, or from comparison to nearby nebulae. But in some cases in the Southern sky, no brightness level data was available at all; in such cases, an extremely approximate value was assigned based on membership in other catalogs (that is, a nebula appearing in many catalogs was assumed to be brighter than a nebula appearing in only one catalog.) BT magnitude VT magnitude The BT magnitude and VT magnitude values are blue and visual magnitudes as measured by Tycho. They correspond pretty closely to "standard" Johnson visual magnitudes and B magnitudes. The BT system has a peak at 435 nanometers; the VT, at 505 nanometers. BY Draconis BY Draconis-type variables are dwarf stars of spectral type K or M, with quasiperiodic light changes ranging from a few hundredths to .5 magnitude. The period will run from a few hours up to 120 days. These objects vary because some parts of their surface are brighter than others (like sunspots, but on a larger scale). As they rotate, we see more or fewer spots and therefore more or less light. Some of these stars also show flares, similar to UV Ceti stars, in which case they belong to both types of variable star. Callisto Callisto is the outermost of the four large satellites of Jupiter found by Galileo. It has a heavily cratered surface, indicating minimal recent geological activity. (Recent volcanoes and such would eliminate the craters, as they do on earth.) Caption on/off Clicking on this menu item toggles the display of the caption in the legend area of the chart. (To change the caption, you should use the Enter Caption menu item.) carbon star Most stars get their energy through nuclear fusion: they use a process in which four hydrogen nuclei are fused into one helium nucleus. This is, for example, how the Sun produces its energy. A carbon star is a star, usually of spectral type R, N, or S, that has run low on hydrogen. It has switched over to fusing helium nuclei into carbon, and also to a cycle in which carbon nuclei fuse with four hydrogen nuclei, then emit a helium nucleus (same end effect as the usual fusion method). This method usually results in a cooler surface temperature, and therefore, a very red star. cataclysmic variable U Gem The U Gem type of variable star is also often called a dwarf nova, or a cataclysmic variable. These are binary stars, one a white dwarf, the other a subgiant star of spectral type K or M (cooler than the Sun, and larger in diameter). They are close enough that tides from the white dwarf rip gas from the bigger star; this matter spirals in on the white dwarf, forming an accretion disk. Usually the stars orbit one another every 1 to 12 hours. Most of the time, these stars have small, sometimes rapid variations in light. From time to time, though, the system will rapidly get brighter by a few magnitudes and, over a period of days to months, go back to normal. How often this happens varies from star to star, but any given U Gem star has something resembling a period; that is, the star will blaze up at semiregular intervals. The greater the interval, the greater the increase in brightness. These stars are also often eclipsing binaries. They are divided into SS Cyg, SU UMa, and Z Cam types. Cause(s) of variability Stars vary in brightness for a number of reasons, and sometimes the same star will vary for more than one reason. This program will list those reasons (if known) for any variable star you pick on, in the "Click for more info" section. CCD frame The CCD frame provides a way to "frame out" a rectangular region of the screen, to show the area of coverage of a photo or CCD image. The CCD frame dialog provides the controls needed to manipulate the frame. CCD Frame on/off Ctrl-F This menu item toggles the display of the "CCD Frame", a rectangle useful in setting up images with CCD or film cameras. When you toggle this option ON, the CCD Frame Dialog box will appear, giving you control over where the frame is shown, its size and orientation, and whether it stays in the center of the chart. You can toggle this option using the Ctrl-F hotkey. Ced Reference: Cederblad, Sven: 1946, Meddelande fran Lunds Astronomiska Observatorium Series II, No. 119. Studies of Bright Diffuse Galactic Nebulae - With special regard to their spital distribution. Published by the Observatory Lund, Sweden. celestial equator Imagine that the Earth was a transparent sphere with a big light in the center. Imagine further that someone ran around the Equator and put a line of paint there. The shadow cast by that paint out into infinity would cut the sky in half; half the sky to the north, half to the south. This dividing plane is called the celestial equator. It is the plane from which declination is measured; a point exactly on this plane has a declination of 0 degrees. Centaur The Centaur class of asteroids consists of the (very few) asteroids with orbits between Saturn and Neptune. The first such object discovered was 2060 Chiron in 1977; this object is now known to also behave a little like a comet, with some signs of a coma. It orbits between Saturn and Uranus, with a period of 50 years. central meridian The central meridian of a planet tells you what part of that planet is currently facing you. For example, the feature Syrtis Major on Mars is located near longitude 300 degrees; it is therefore best observed when the central meridian is near this value. When the Earth is observed from other planets, its central meridian is also shown. If you (for example) set the "home planet" to be the Moon, using the home planet dialog, and then click for "more info" on the Earth, it might show a central meridian of 150 degrees, indicating that the Pacific Ocean is facing the moon at that time. Cepheid variable Cepheid One of the earliest known variable stars was Delta Cephei. This star regularly grows brighter and dimmer by about one magnitude over a period of about five days. In this case, the star is physically growing brighter and dimmer (and its surface gets hotter and colder and its spectrum varies as well.) Stars that exhibit this sort of behavior are called Cepheid variables. Cepheid type variables usually have periods ranging from a few days to a few weeks. The changes in some, such as Delta Cephei, can be easily followed from night to night by estimating its magnitude. This is usually done by comparing the variable's brightness to nearby comparison stars. One unusual property of Cepheids is that the length of a Cepheid's period can give a close idea as to its total output of energy, or luminosity. Comparing that output to the brightness we actually see gives an idea as to how far away the star is. For more normal stars, determining luminosity and distance is a much more difficult and uncertain task. The fact that Cepheid variables have this property was used earlier in this century to determine the distance to nearby galaxies. CGCG Many galaxies will have listed, under their "alternate designations", a CGCG, or Catalog of Galaxies and Clusters of Galaxies, designation. You can find an object by its CGCG number by using the Go to CGCG option in the Go to Galaxy menu under the Go To menu. chart area You can accomplish two tasks by clicking in the chart area. If you click in this area with the LEFT mouse button, the chart clears and recenters at the point you clicked on. You can pan around the sky in this manner. You can also click on an object in this area with the RIGHT mouse button. In this case, the program will find out information on that object and show a summary in the center of the screen, listing such information as the object's name(s), rise/set times, magnitude, and so forth. Underneath will be two buttons, "OK" and "more info"; clicking on the second button will cause Guide to show more detailed information about the object. Chart mode Ctrl-F3 Normally, Guide shows white stars on a black background. The Chart Mode option toggles to black stars on a white background. This option is especially useful in doing color printouts in Windows; you set this mode and reset the colors as you would like them to appear on the printout. (When you return to white stars on black, your other colors will be restored as well; a different set of colors is stored for each mode.) You can also get this option with the Ctrl-F3 hotkey, or through the background dialog. Chinese calendar The Chinese calendar is both lunisolar (meaning the months line up with lunar phases and the years line up with seasons, both at the same time), and observational (meaning that it's based on the real motions of the moon and sun, rather than on a mathematical approximation.) The result is a calendar of truly immense complexity. The fact that it is lunisolar means that (as with the Hebrew calendar) some years have twelve months and 353, 354, or 355 days; while other "leap" years have thirteen months, and 383, 384, or 385 days. And also like the Hebrew calendar, months have 29 or 30 days. However, the Hebrew calendar inserts leap years in a regular cycle, with seven years out of 19 having the extra intercalary month; and this added month is always at the middle of the year. The Chinese calendar adds such months at almost exactly the same frequency, but in an odd order and at any point in the year. For example, one can have months "one", "two", "three", "three (intercalary)", "four", and so on up to "twelve". The Chinese calendar runs either on a sixty-year cycle, or by numbering years since an emperor ascended the throne, or by numbering from the sixty-year cycle that started in 2637 BC. Guide follows this last convention; for example, it will tell you that 28 January 1998 Gregorian is "Chinese New Years Day" for year 4635. The years of the sixty-year cycle are named, and Guide will also show these names. (Year 4635, which is the fifteenth year of the current cycle, is named wu-yin, for example.) Those interested in full details, including how the calendar is computed and C/C++ source code, should look at: http://www.projectpluto.com/calendar.htm circumpolar cpolar text not vis. text Depending on where you are on the earth, some objects will be either always visible (or circumpolar) or eternally not visible. Since such objects neither rise or set, times of rising and setting aren't listed for them when you click on them; you are merely informed that they are circumpolar or never visible. (If you choose a different latitude in the Settings menu, you can find a place on the Earth where the object will be visible by moving closer to the Earth's equator.) These objects do move in the sky; they trace circles around the poles, hence the term circumpolar. Civil twilight Nautical twilight Astronomical twilight Twilight Twilight is defined in three ways. Civil twilight occurs when the sun is six degrees below the horizon. The next "darker" form of twilight is nautical twilight, with the sun 12 degrees below the horizon. Astronomical twilight occurs when the sun is 18 degrees below the horizon. In polar latitudes, nights may pass where one or more of these may not occur. Twilight times are given (if they occur for your current position, as defined by your latitude and longitude in the Settings menu) when you "click for more info" concerning the sun, or ask for Quick Info. At the end of civil twilight, one is legally required (in some US states) to turn on automobile headlights. During nautical twilight, one can take sightings of stars and planets (it is dark enough for these objects to be visible, but bright enough for the horizon to be seen). At the end of astronomical twilight, the sky has reached its maximum darkness. Clear caption If you have entered some caption text, clicking on this option will clear it all out so you can start over. It is grayed out if there is no caption text currently entered. Clear RealSky images Alt-F11 If you have generated a series of RealSky images, you will eventually want to clear them from your screen and hard drive. The Clear RealSky images option in the Extras menu provides a way to do this. Click on this option, and you'll be asked to confirm that you really do wish to erase the data. If you do confirm it, Guide will delete the RealSky images in the Guide directory and will redraw the screen, free of images. You can also access this option with the Alt-F11 hotkey. Click for a list of constellations !Andromeda Crater Norma Vela Antlia Crux Octans Virgo Apus Cygnus Ophiuchus Volans Aquarius Delphinus Orion Vulpecula Aquila Dorado Pavo Ara Draco Pegasus Aries Equuleus Perseus Auriga Eridanus Phoenix Bootes Fornax Pictor Caelum Gemini Pisces Camelopardalis Grus Piscis Austrinus Cancer Hercules Puppis Canes Venatici Horologium Pyxis Canis Major Hydra Reticulum Canis Minor Hydrus Sagitta Capricornus Indus Sagittarius Carina Lacerta Scorpius Cassiopeia Leo Sculptor Centaurus Leo Minor Scutum Cepheus Lepus Serpens Cetus Libra Sextans Chamaeleon Lupus Taurus Circinus Lynx Telescopium Columba Lyra Triangulum Coma Berenices Mensa Triangulum Australe Corona Australis Microscopium Tucana Corona Borealis Monoceros Ursa Major Corvus Musca Ursa Minor Click for a list of meteor showers Alpha Aurigids Delta Leonids Ophiuchids Alpha Bootids Draconids Orionids Alpha Scorpiids Eta Aquarids Pegasids Andromedids Geminids Perseids April Fireballs June Draconids Phi Bootids Arietids June Lyrids Piscids Camelopardalids Kappa Cygnids Quadrantids Capricornids Kappa Serpentids Scorpiids Coma Berenicids Leo Minorids Sigma Hydrids Corona-Australids Leonids Sigma Leonids Delta Aquarids Lyrids Tau Herculids Delta Arietids March Geminids Taurids Delta Cancrids Monocerids Ursids Delta Draconids Mu Virginids Virginids Showers in April and May Showers in January, February, and March Showers in June, July, August, and September Showers in October, November, and December Click for Greek alphabet Greek letter Greek-alphabetical Greek letters are included here because they are used for Bayer designations for stars. ! @g (means "use greek letters") à Alpha æ Eta ì Nu ò Tau á Beta ç Theta í Xi ó Upsilon â Gamma è Iota î Omicron ô Phi ã Delta é Kappa ï Pi õ Chi ä Epsilon ê Lambda ð Rho ö Psi å Zeta ë Mu ñ Sigma ÷ Omega @g (return to European) ! In general, the Alpha star in a constellation will be brighter than the Beta, which in turn will be brighter than the Gamma... but there are numerous exceptions to this rule. cluster It's not uncommon for stars to form in groups, known as clusters. There are two types of clusters, open clusters and globular clusters. The Pleiades are an example of an open cluster; M-13 (Messier 13) is an example of a globular cluster. clusters of galaxies cluster of galaxies Guide supports the display of clusters of galaxies from the Abell and Zwicky catalogs. Data for these objects include their sizes, positions, and magnitude and redshift (in some cases). Cohen Reference: Cohen, M.: 1980, Astronomical Journal 85, 29. Red and nebulous objects in dark clouds. A survey. colongitude The lunar colongitude is among the data listed when you click for "more info" about the Moon, and when you create a lunar data table. It provides a simple way to describe the location of the lunar terminator, and is therefore useful in figuring out which lunar features are best positioned for observing. The colongitude describes the lunar longitude on the lunar equator where the sun appears to be rising. Near New Moon, the sun is rising on the moon's western edge (near Mare Crisium), at lunar longitude -90. (It's common to always show a positive colongitude, however, so it would be reported as 270.) Near first quarter, the sun is rising on the center of the near side, at lunar longitude 0, and the colongitude is therefore also zero. Near full moon, the sun is rising near the eastern edge of the moon, at lunar longitude +90; and at last quarter, it is rising as seen from the center of the far side of the moon, at lunar longitude 180. Color The Bright Star catalog sometimes will remark on colors observed in stars. Usually, it's difficult to see a star's color without some help (such as binoculars), but some of the brightest stars have obvious blue or red tints. The colors are usually fairly subtle. It doesn't help matters that human vision at night is mostly in shades of gray; the parts of the eye that detect color differences require more light than is usually available. The usual way of measuring color quantitatively is to look at a star through colored filters and measure its brightness. The colors and densities of these filters are standardized. The difference in brightness is a good measure of the color. The most commonly used filters are the B(lue) and V(isual) filters, leading to a "B-V" value. comet "Comet" is a word derived from the Greek for "hairy star", and comets are usually distinguished by a tail formed by gas and dust boiled from their surfaces when they approach the Sun. A few come close enough to be spectacular, even with the unaided eye; many more are either too faint or appear at the wrong time and place to be seen without binoculars or a telescope. Most comets come in from interstellar space, take a pass by the Sun, and head out into outer darkness, never to be seen again. A few have their orbits adjusted by other planets, so that instead of heading back out, they wind up in short-period orbits. Halley's Comet, for example, comes back every 76 years; Swift-Tuttle, every 130 or so; Encke's, every 3.3 years or so. You can find comets through the Go To menu; it has an option that brings up a list of currently visible comets. Click on one, and Guide will recenter on it. common name Many very bright stars were named thousands of years ago, such as Sirius and Betelgeuse and Polaris. The common names used in this program are almost always Arabic, with a few Greek and miscellaneous names added in. This program recognizes about 200 or so of the most commonly used names. You can find them in the Go to Common Star Name menu under the Go To menu. constellation Ancient civilizations divided the stars into groups of constellations. They usually gave them names that, to some minor degree, resembled the shape of the group of stars. Thus, Orion looks a little like a human holding an animal skin and wearing a belt, while Scorpius looks a little like a scorpion's claws and stinger. Naturally, different cultures came up with different sets of constellations. In the interests of standardizing things, there are now 88 officially recognized constell- ations. Each has precisely defined borders so that one can determine with which constellation a star belongs, and each has a name and three letter abbreviation. For example, Orion is abbreviated Ori; Cassiopeia, Cas; Taurus, Tau. These constellations were officially defined in 1930. In this program, you can show constellation lines, constellation borders, and constellation labels. You can also find a desired constellation by clicking on the Go to Constellation menu item in the Go To menu. Click for a list of constellations