MRSL Reference: Marsalkova, P.: 1975, Astrophysics and Space Science 273. A comparison catalogue of HII-regions. Mu Cep The Mu Cep type of variable star is an older, cooler star that has a fairly regular period of variation of 30 to several thousand days in length, with some irregular changes from time to time. This type of variable usually varies by about one magnitude in brightness. Name The Bright Star catalog sometimes will remark on names commonly used for brighter stars. nanometer A nanometer is one-billionth (.000000001) of a meter. ("Nano" always indicates a billionth. It's the step beyond "micro", or one-millionth.) The wavelength of light is usually measured in either nanometers or Angstroms. For example, visible light has wavelengths between about 400 to 700 nanometers. An Angstrom is one-tenth of a nanometer. Nearby star Many of the nearer stars have special names given by their discoverers, such as Barnard's Star, Proxima Centauri, Kruger 60, Lalande 21185, and so forth. These stars are not likely to appear in standardized catalogs such as the PPM or SAO. The Go to Star menu contains an entry, "Nearby Star", listing a few dozen of these odd designations. Click on one of them, and Guide will recenter on that star. nebula Scattered throughout the sky are vast clouds of gas called nebulae. These clouds come in many forms. Some are gas clouds in the process of condensing into a star. Others are the remains of stars that have exploded. Some are bright, because there is a star inside them that keeps them glowing. Some are dark, and only seen as dark patches against a background of stars. This program makes the distinction between planetary nebulae, symbolized by a circle with four rays, and diffuse nebulae, symbolized by a square. Some are reflection nebulae, where light from a star inside reflects from dust particles, and some are emission nebulae, where light from a nearby star ionizes or excites atoms in the nebula until it glows. You can find a particular nebula using the Go to Nebula menu in the Go To menu. Nebula Databank The Nebula Databank was compiled by Eric-Sven Vesting to evade the problems that came with earlier bright nebula databases. For example, previous versions of Guide gathered nebula data from five separate catalogs. There were few cross-indexes from one catalog to another, and no way to indicate that one designation applied to a part of larger area with a different designation. Also, brightness levels were applied in an inconsistent manner at best. The Nebula Databank contains explicit links between the various nebula catalogs, enabling Guide to show all designations for a given object and to avoid drawing some objects twice (if they appeared in separate catalogs under different names). Also, Eric-Sven Vesting created the nebula isophotes used by Guide to indicate the shapes of most prominent nebulae. Better positional data was generated, usually by comparing catalog positions to actual RealSky images. Neptune Neptune is the eighth planet from the Sun. It is usually visible at about 8th magnitude through small telescopes. Physically, it is very similar to Uranus in its composition. Neptune was originally located mathematically. By 1845, Uranus had been observed for over sixty years, and it was quite apparent from its motion that the gravity of an unseen object was pulling and pushing it in its orbit. Analysis of the motion made it possible to calculate where the unseen object must be, and two people, John Couch Adams, an undergraduate at Cambridge, and Urbain Leverrier, a French mathematician, did this independently. After the math was done, an hour or so of telescopic searching was enough to pin Neptune down. Neptune has several satellites, one of which, Triton, is roughly the size of our Moon. Triton is unique in that its orbit is backwards, or "retrograde", compared to that of all other large objects in the Solar System. If you looked at the Solar System from above, all the planets would seem to go counterclockwise around the Sun, and most of the satellites counterclockwise around their planets. Triton's motion would be clockwise. Much of what we know about Neptune and its satellites comes from the Voyager 2 flyby in 1989, which collected pictures and other data on these objects. neutral hydrogen Neutral hydrogen is, logically enough, hydrogen with no electrical charge. Among other properties, it tends to emit microwaves with a 21-cm wavelength. This happens because a hydrogen atom consists of an electron combined with a proton. Each of these has a quantum mechanical property called "spin"; for purposes of this discussion, think of "spin" as resembling magnetic poles. If the spins of the two particles are the same, then you can add a little energy (perhaps from a collision with another hydrogen atom) and the spins will be opposite. Later, they will tend to fall back to their normal state; the energy is released as a photon of 21-cm microwave energy. A cloud of neutral hydrogen can be detected by this radiation, and its speed and other properties can be determined by examination of the 21-cm emissions. neutron star pulsar A neutron star is a more compressed version of a white dwarf. In these objects, the pressure is high enough to force electrons and protons together into neutrons. An object consisting of nothing but neutrons can collapse to a density of about 100 million tons per cubic centimeter. Neutron stars are usually around ten to twenty kilometers across, yet are more massive than the Sun. They generate tremendous magnetic fields and spin rapidly, generating radio pulses that sweep around line the search beam on a lighthouse. Sometimes these beams intersect the Earth, and we see a pulsar. One example is in the Crab Nebula (M-1). This object was a star that went supernova in 1054, and was visible in daylight for some time. Now there is a nebula and a pulsar in its middle. New moon First quarter Full moon Last quarter The moon takes roughly 29.5 days to go through a full set of phases. It starts out close to the sun; this is a new moon. As it moves away from the sun, we can see it as a thin crescent, and then as a half-lit disk. It is then 90 degrees from the sun. It's half-lit, but only a quarter of the way through its phases, which is why this point is called first quarter. After this, it continues away from the Sun until it is opposite it in the sky. Since it is opposite the sun, it rises at sunset and sets at sunrise (roughly), and is fully lit: hence the name full moon. It continues moving, but now it is catching up with the sun again. It becomes less lit, then only half lit, again 90 degrees from the sun. This is the last quarter phase. It continues to shrink, becoming a crescent again, until it finally catches up to the sun and there is another new moon. Times for all these phases are shown when you "click for more info" on the moon, or ask for Quick Info. NGC The NGC catalog is a list of about 9000 objects such as nebulae, galaxies, and clusters of stars. It stands for "New General Catalog", and you will often hear an object referred to by its NGC number. For example, NGC 104 is a globular cluster several thousands of light-years away, out near the edge of our own galaxy. When you see a symbol such as an oval, circle with a cross inside, or square, followed by a number, all in yellow, that's an NGC object. You can stop these from displaying (or ensure they are always displayed) inside the Data Shown menu. You can find an object by its NGC number inside the Go To menu. NGC2000 The NGC2000 is a modern version of the NGC and IC catalogs compiled by J. L. E. Dreyer in the late nineteenth and early twentieth centuries. Errata compiled by Dreyer and by subsequent workers have been incorporated into the new version and the object types have been updated with information from modern astronomy; the descriptions given are those of Dreyer, and tend to be on the cryptic side at best. This catalog is copyrighted by Sky Publishing Corporation, and is used here by permission. The data should not be used for commercial purposes without the explicit permission of Sky Publishing Corporation. non-star When the Hubble Guide Star Catalog, or GSC, was produced, a computer was used to examine each object in an effort to determine what type of object it was. The vast majority were clearly stars. Most of the remainder were classified as non-stars, meaning that they were too fuzzy or too elongated for the computer to think they were stars. Many of the non-stars are really galaxies; some are asteroids that moved while the image was taken, and some are really scratches on the plate. They can be turned on and off in the star display dialog. nova Every now and then, a star will suddenly climb up in brightness from out of obscurity, stay bright for a week or so, then slowly fade into obscurity over months. Such stars are known as novae and supernovae. Your garden-variety nova is dimmer and more common than a supernova. The causes of novae are a little obscure, but they probably occur in binary stars where one member of the pair is a white dwarf. The stars are so close to one another that the white dwarf's gravity can pull matter from the other star, and when it lands on the dwarf's surface, you can get a nova. Sometimes this happens repeatedly, making a repeating nova such as T Pyx. nova-like Sometimes an object may, based on its spectrum and other data, look a lot like a former nova, in which case it may be labelled as nova-like. Sometimes more detailed examination of the object reveals it to be something else. NSV New Catalog of Suspected Variables Many stars have been observed closely enough to be suspected variable stars, but not closely enough (or for long enough) to determine what kind of variable they are or even if they really vary. Such stars are listed in the New Catalog of Suspected Variables, or NSV. nutation In addition to the slow precession of the earth's axis, which causes it to sweep out a wide circle over a period of 25,800 years, the earth's axis traces out much smaller ovals over an 18.6 year period. This smaller "wobble" is called nutation. It doesn't shift the earth's axis by more than about 10 arcseconds, so it is neglected for most purposes; along with aberration, it is included in the calculation of an object's apparent position at current epoch. obliquity to orbit The obliquity to orbit of an object is the angle between its polar axis and the axis of its orbit. The most important thing it does is to tell you how extreme the seasons would be on that planet. For example: the obliquity of the Earth is about 23.5 degrees. So at the summer solstice, the north pole of the earth tilts toward the sun by 23.5 degrees, and at the winter solstice, it tilts away by the same amount. This leads to fairly extreme temperature changes. Uranus, on the other hand, has an obliquity of about 97.86 degrees, almost a right angle. This means that at its "summer solstice", the north pole points almost directly at the sun, and there is continuous daylight in most of the northern hemisphere of that planet. As the following table shows, most planets have much smaller obliquities, and therefore less extreme seasons: Mercury ~0.1 deg Venus 177.3 deg Earth 23.45 deg Mars 25.19 deg Jupiter 3.12 deg Saturn 26.73 deg Uranus 97.86 deg Neptune 29.56 deg Pluto 118? deg Obs. ID This item tells you the identification number of the survey photographic plate on which this star can be found. occultation double Some stars are close to the path of the moon's orbit, and from time to time, the moon passes in front of, or occults, them. Suppose this happens to a very close binary star, one so close together that even huge telescopes can't see them as binary. As the moon passes in front of such a star, we'll see the brightness drop, but not totally vanish, as one star goes behind the moon, and then see the other star vanish, leaving no light at all. This kind of star is an occultation double. Regrettably, not many stars are close to the Moon's path, so this trick can't be used on every star. open cluster An open cluster of stars doesn't show a great deal of structure. It's a collection of from perhaps a dozen to perhaps several thousand stars that are, appropriately enough, clustered together. The Pleiades are an example of an open cluster, one close enough and bright enough so that most people can see seven closely packed stars with unaided eyesight, and dozens with binoculars. The symbol for an open cluster in this program is a dashed circle, followed by a Messier or NGC number. Open clusters are usually made up of very young stars. Quite a few have nebulae inside them. Unlike globular clusters, many are relatively close by; the Pleiades, for example, are a mere 300 light-years away. You can find an open cluster with the Go To menu. The brighter open clusters have Messier, NGC, or IC numbers and can be found by those numbers. Some of the dimmer clusters have Stock or Collinder or other designations; you can click on the Go to Open Cluster menu item to get a list and select a catalog and number. You can control the display of open clusters in the Data Shown menu. opposition point Alt-U The Opposition point menu item, inside the Go to Coordinate submenu, finds the point on the sky directly opposite the sun. You can also reach this option with the Alt-U hotkey. Orbital arc To determine an object's orbital elements, it is best to make as many measurements of its position as possible, spread out over as long a period as possible. An orbit based on, say, four observations made over a few days will not be as precise as one based on thousands of observations made over a period of years. When you click for "more info" on an asteroid, Guide will usually tell you how many measurements were made of the object and over how long a period ("orbital arc") they were made. That can help you to evaluate the quality of the orbital elements. orbital elements There are several ways to describe the orbit of an object. The most commonly used method is by listing its orbital elements. An object's orbital elements consist of seven numbers: the semimajor axis, eccentricity, inclination of orbit, argument of perihelion, longitude of the ascending node, mean anomaly, and the epoch of elements. Many astronomy programs will let you define the orbit of an object by entering these numbers. Should you wish to add a new asteroid or comet to Guide, you can do so by hitting the Ctrl-K hotkey and entering the orbital elements. Keep in mind that the elements describe the object's position and velocity precisely at one instant, that of the epoch of the elements. For a few months around that instant, they still will usually provide a good match to the object's path. (Since most comets are usually visible for only a few months, this is not a problem for them.) However, as years go by, the gravity of the planets (especially Jupiter) can pull objects into very different orbits. (Jupiter sometimes even throws comets out of the Solar System, or into the Sun, or into closer orbits that take less time to orbit the Sun.) For asteroids, Guide gets around this limitation by using fresh elements every 50 days, so its data is never more than 25 days out of date. Orion type Orion type stars are variable stars with irregular eruptions. They are probably young objects that will evolve into non-varying stars someday. They are found in or near diffuse nebulae. Some show, as well as the irregular outbursts in brightness, more regular changes as they rotate. The irregular changes may reach several magnitudes. Overlay menu Alt-O The Overlay menu contains all the tools required for editing, selecting, and deleting overlays. By default, Guide only provides six overlays, for constellation borders, constellation labels, constellation lines, planet trails, the Palomar Observatory Sky Survey, and the AAVSO Atlas. You can use the Overlay menu to adjust the display of these overlays, and to add and edit your own. You can reach this option from any point in Guide by hitting the Alt-O hotkey. PAL-E This plate in the GSC was taken using the Palomar Schmidt telescope, using a singlet corrector, 103aE emulsion, and a red plexiglass filter. PAL-J This plate in the GSC was taken using the Palomar Schmidt telescope, using a doublet corrector, IIIaJ emulsion, and GG 385 filter. PAL-V1 This plate in the GSC was taken using the Palomar Schmidt telescope, using a singlet corrector, IIaD emulsion, and Wratten 12 filter. Most of the northern plates were taken in this manner. PAL-V4 This plate in the GSC was taken using the Palomar Schmidt telescope, using a singlet corrector, IIaD emulsion, and GG 495 filter. PAL-V5 This plate in the GSC was taken using the Palomar Schmidt telescope, using a doublet corrector, IIIaJ emulsion, and GG 495 filter. parallax trigonometric parallax Parallax is a method used to determine a star's distance. As the Earth orbits the Sun, our motion makes nearby stars appear to wobble back and forth as we see the stars first from one point in our orbit, then from another. This motion is always very small, but with good instruments, it can be measured. The amount of the movement is the star's parallax. Once the parallax is known, the distance easily follows. (This method is also known as trigonometric parallax, to distinguish it from a few non-geometric ways to derive the same information.) Only a small number of stars are close enough for us to measure their parallaxes accurately. For the rest, we need to rely on more complex ways to gauge distance, such as by calculating dynamic parallax. parsec A parsec is a unit of measurement roughly equal to 3.26 light-years. It is the distance from which the angle between the Earth and Sun would appear to be (at most) one arcsecond. It is equal to about 19.2 trillion miles, or 30.8 trillion kilometers. Partial Events The Partial Events option appears in the Extras menu, when in eclipse mode. It provides a way to tell Guide to find not only total/annular events, but partial ones as well. It defaults to being on. Suppose you have a chart of a solar eclipse path on the screen, generated using the Show Eclipse function; and that you've set the Local Events only switch on (telling Guide to only show events visible from the current screen center). If this option is checked (the default), clicking on "Next" or "Previous" will lead to the next or previous event visible from that position, whether it is total, annular, or partial. If the option is not checked, Guide will keep looking until it finds a total or annular event. (Since total or annular events are rarely visible from a given point, Guide may have to search a long time before it finds one! Don't be distressed if the computer seems to lock up for a minute or so while the computation proceeds.) penumbra umbra totality When the moon passes through the earth's shadow at the time of full moon, it's possible that the earth will only partly block the sun's light. If you were on the moon, you would see the sun with a "chunk" removed from it. This part of the shadow is called the penumbra (as opposed to the umbra, the part of the shadow where the sun is completely blocked.) During a lunar eclipse, if the moon only enters the penumbra, it will get slightly dimmer, but usually not by very much, because enough sunlight is hitting it to keep it well-lit. If it partly enters the umbra, then those parts of the moon in the umbra will appear essentially dark (with a tint of orange). Often, the entire moon will enter the umbra; the time during which the moon is fully eclipsed in this manner is called totality. Sun Earth Moon Umbra Penumbra @c 50,310,45 sun @c 300,310,16 earth @c 400,310,4 moon @m 450,367 @l 52,265 @l 450,310 @l 52,355 @l 450,257 periastron If the orbit of a binary star can be determined, then the point where the two stars are closest together is referred to as periastron. In some cases, Guide is able to give a date for when this takes place.