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 UMa. Amor 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. angstrom 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. Animation ALT-A 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 hotkey. 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. Apollo 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). arcminute 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 degree. arcsecond 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. Arp 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. artifact 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. ASSA 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. asteroid 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 Earth.) The first four asteroids are sometimes good binocular targets. Most of the others require decent telescopes to find. 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 astrometric 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. Aten 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. AU 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). B-V 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 Flashlight Realistic 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 horizon. 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. Balmer Lyman 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. Barnard 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. Bayer 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 BBO-B 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. BBWo 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. BD CD CP CPD SD 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 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. 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. blend 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.