Sigma Leonids Lyrids Mu Virginids Alpha Bootids Phi Bootids Alpha Scorpiids Eta Aquarids Kappa Serpentids Delta Draconids Virginids April Fireballs Showers in April and May Apr. 4 Kappa Serpentids: Radiant--near Corona Borealis. 4 or 5 per hour from Apr. 1 to 7. Apr. 7 Delta Draconids: Radiant--near Cepheus border. From Mar. 28 to Apr. 7. Slow meteors at about 5 per hour. Apr. 10 Virginids: Radiant-- near Gamma in bowl of Virgo. 20 per hour. Apr. 15 April Fireballs: Radiant-- between The Water Jar and Scutum, very erratic. From April 15 to 30 many bright bolides from Southeastern sky. Apr. 17 Sigma Leonids: Radiant-- at Leo-Virgo border, actually has moved into Virgo in recent years. Weak shower of 1 to 2 per hour. Apr. 22 Lyrids: Radiant-- near Vega. 15 per hour, bright and long lasting meteors. From Comet Thatcher. April 25 Mu Virginids: Radiant--near Libra. 7 to 10 per hour of medium speed meteors. Apr. 28 Alpha Bootids: Radiant-- near Arcturus. From Apr. 14 to May 13. Slow meteors with fine trails. May 1 Phi Bootids: Radiant--near Hercules. From Apr. 16 to May 12. 6 per hour. May 3 Alpha Scorpiids: Radiant-- Near Antares. From Apr. 16 to May 9. May 4 Eta Aquarids: Radiant-- near Water Jar. From Apr. 21 to May 12. 21 per hour, yellow with bright trails. Comet Halley debris. Size on red POSS Size on blue POSS The POSS, or Palomar Observatory Sky Survey, shows objects on red and blue sensitive photographic plates. Some galaxies may be measured on both plates. The difference between plates can tell you something about the distribution of red (usually older) stars versus the distribution of blue (mostly younger) stars. slope parameter The slope parameter of an asteroid is a number describing how the magnitude of the asteroid varies as a function of changing illumination, or phase angle. Most asteroids are brightest when the sun illuminates them fully (near opposition), and their brightness drops off when partly illuminated. The extent of that dropoff is specified by the slope parameter. The slope parameter can then be used in reverse to predict the asteroid magnitude for any given illumination. It is usually given the symbol 'G'. For almost all asteroids, the slope parameter is about .15. In fact, if the asteroid has not been closely studied, it's common to simply assume a slope parameter of .15. Slow irregular The slow irregular type of variable star either shows no signs of being periodic, or isn't consistently so. Stars are often considered this simply because they have not been studied long enough to determine their real nature. As a rule, these are giant stars. Solar central meridian Solar observers often need to refer observations to the solar central meridian, which, together with the tilt of the pole, indicates the part of the Sun currently turned toward us. In reality, different parts of the Sun rotate at different rates, much as on Jupiter and Saturn. By convention, the Sun is assumed to rotate once every 25.38 days; because we are orbiting the sun, it appears to rotate only once every 27.2752 days. solar constant planetary solar const planetary solar constant The solar constant is a measure of the amount of radiated energy, at all wavelengths, emitted by the Sun. At our distance from the Sun (one astronomical unit), it is 1367.6 watts per square meter. Each planet has its own planetary solar constant, ranging from Mercury (9936.9 watts per square meter) to Pluto (.87 watts per square meter). This is given in "more info" for that planet. Solar rotation number Solar observers often make use of the solar rotation number in describing their data. This system starts with rotation #1 on 9 Nov 1854, and a new rotation begins when the solar central meridian is equal to zero (that is, the sun has made one apparent rotation). That takes about 27.275 days, on average. This is useful in describing the behavior of solar features. One can say, for example, that "the sunspot I saw on rotation number 1923 moved north by rotation 1924, and had faded out by 1925." When you click for "more info" on the Sun, Guide will report the current rotation number and the date on which it began. solstice summer solstice winter solstice The solstices (meaning "the sun stands still") mark the points where the sun appears at its maximum and minimum declination. The summer solstice marks its greatest declination, and usually occurs within a day or two of June 21. It marks the beginning of summer in the Northern Hemisphere. The winter solstice marks the sun's lowest declination, and usually occurs within a day or two of December 21. The times of the solstices are given in the "click for more info" section for the sun, as are times for the vernal equinox and autumnal equinox. spectral type A star's spectrum is usually first classified by spectral type. The spectral type consists of one of the letters O, B, A, F, G, K, M, R, N, S, followed by a digit. (Generations of astronomy students have been able to remember the letters through "Oh Be A Fine Girl, Kiss Me Right Now, Sweetheart".) The numbers provide gradations between letters; an F9 star, for example, is only slightly hotter than a G0 star. The type gives an idea of the star's color and temperature. Type O stars are young, blue, extremely hot stars, with surface temperatures of about 40,000 degrees Kelvin (about 70,000 F.) As you progress through the letters, one reaches successively cooler stars and the color changes to white (for A stars), yellow (G stars), orange (K stars), and red (M stars). M stars have surfaces as "cold" as 3000 degrees Kelvin (5000 F.) (Notice that these are all surface temperatures. The cores of stars usually run more in tens of millions of degrees.) R, N, and S stars are somewhat off the scale. These are extremely old stars. Your average star, such as the Sun (which is type G2), produces energy by fusing hydrogen nuclei into helium. R, N, and S stars are so old that they have mostly run out of hydrogen to fuse. They produce energy by fusing helium nuclei into carbon nuclei, which is why they are also called carbon stars. Spectroscopic Binaries Some binary stars are so close together that even the largest telescopes see them as one point of light. However, if you examine their spectrum, you may find that each line is doubled. This is caused by the Doppler shift. One star is approaching us, and its lines are a little bluer than usual; the other is receding, and its lines are redder. The difference tells you their relative speeds. As they orbit one another, the lines switch places, telling you their period, their relative masses, and other useful information. Such stars are called spectroscopic binaries. spectrum spectra You have probably seen at some point a picture of white light passing through a triangular prism, and coming out split into colors running from red to blue. This is the visible part of the spectrum. A similar color-splitting phenomenon produces rainbows. If you take such a prism and put it behind a telescope, you can split the light from a star into its component colors. The result isn't equally bright in all colors. Some colors will appear with greater or lesser intensity. These colors correspond to different elements being heated up on the star's surface and atmosphere, so the presence and absence and intensity of certain colors (or "lines", so-called because they show up as lines in the spectrum) tells you what elements are in that star and in what quantities they occur. You can also find out how fast the star is approaching or receding relative to the Earth, or determine if it is a close binary star, or (roughly) how fast it spins on its axis, or how hot it is. Astronomers first classify a star's spectrum by spectral type; for stars from the SAO catalog, this is all the information on their spectrum that this program has. For Bright Star Catalog stars, the type is usually followed by the star's luminosity class, and sometimes a list of chemical elements apparent in the spectrum plus added information. speed of apparent motion In observing asteroids, it's sometimes helpful to have some idea as to how rapidly they appear to be moving across the sky. This is especially true with those passing near the Earth, which can move at apparent speeds of up to several degrees per hour. When you click for "more info" about an asteroid, Guide will calculate the apparent angular speed and the position angle of the motion (the direction in which the asteroid is headed). This is shown as the speed of apparent motion. Spin Left Click on this menu option to rotate the CCD frame five degrees to the left. You can also explicitly set the tilt angle of the camera with the "Angle=" menu option. Spin Right Click on this menu option to rotate the CCD frame five degrees to the right. You can also explicitly set the tilt angle of the camera with the "Angle=" menu option. SS Cyg SS Cyg stars are a subclass of U Gem stars. These variable stars increase in brightness by 2 to 6 magnitudes over one or two days, then go back to normal over a few days. They repeat this semi-periodically. The period between outbursts for a given star can range from ten to a few thousand days. Starting/stopping animation Ctrl-A In Windows, animation is mostly controlled with five buttons on a toolbar in the Animation Dialog option in the Animation menu. (In DOS, the same five buttons appear directly inside the Animation Menu.) The buttons look like this: The "double left arrow" causes animation to run backwards, into the past. The "double right arrow" makes it run into the future. The single arrows animate by one step each time they are clicked. The square in the center stops animation completely. In Windows, there are four additional radio buttons used to select slightly different ways in which animation can work. The default is the "RA/dec" button. In this mode, stars are fixed, and your viewpoint remains centered at the same RA/dec position as time changes. (This is the only mode available in DOS.) The second button, "Moving", can only be selected after you have clicked on a moving object (asteroid, comet, planet, natural or artificial satellite). When this button is selected, your viewpoint will be kept centered on the moving object. As the object moves, stars will drift by in the background. The third button, "Horizon", keeps your viewpoint centered on the same alt/az position as time changes. If you are looking to the southeast and start animation in this mode, you will remain looking to the southeast while stars appear to rise on the screen. If you've turned the horizon on, the horizon will stay fixed. The fourth button, "Proper Motion", resembles the first "RA/dec" in that you will stay centered on the same RA/dec position as time changes. But the stars will be moved according to proper motion. Because this is a very slow movement indeed, you generally need an enormous step size (say, 10 years/step) to see it at all. @m 50,80 dbl left @l 57,70 @l 57,90 @l 50,80 @m 57,80 dbl left @l 64,70 @l 64,90 @l 57,80 @m 75,80 sing left @l 82,70 @l 82,90 @l 75,80 @m 95,72 box @l 105,72 @l 105,88 @l 95,88 @l 95,72 @m 125,80 sing rt @l 118,70 @l 118,90 @l 125,80 @m 143,80 dbl rt @l 136,70 @l 136,90 @l 143,80 @m 150,80 dbl rt @l 143,70 @l 143,90 @l 150,80 Str Reference: O. Struve and W.C. Straka: 1962, Publ. Astron. Soc. Pac. 74.474. Notes on diffuse galactic nebulae. SU UMa SU UMa stars are a subclass of U Gem stars. They are different because along with normal U Gem type outbursts, they have occasional "super-outbursts". A super-outburst is about two magnitudes brighter than a normal one and lasts more than five times as long, but occurs about three times less frequently. These stars complete an orbit in less than 2 or 3 hours. Sun The Sun is far and away the closest star to us. The second closest, Proxima Centauri, is about 250,000 times farther away. The Sun is an enormous ball of (mostly) hydrogen and helium. It has a diameter 109 times that of the Earth, and is 333,000 times more massive. The Sun and other stars produce their energy by nuclear fusion. Deep in the Sun's core, the temperature and density are sufficient to push hydrogen nuclei together to form helium nuclei. Each second, the Sun converts 1,500,000 million pounds of hydrogen into 1,490,000 pounds of helium. The difference is accounted for by changing mass into energy, according to the well-known E=mc squared. This is an immense amount of power. The Sun produces enough energy to boil every drop of water in the ocean, every second. It has done this every second for five billion years, and is expected to do so for another five billion. Examination of the Sun with telescopes reveals the occasional appearance of sunspots, areas that are a little cooler than their surroundings. Their appearance follows a fairly irregular 11 year cycle, although some cycles have been much shorter or longer and sometimes no pattern at all has been apparent. Sunspots are linked with emissions of radio energy and of charged particles, causing the aurora to be seen on Earth. supernova Supernovae are extremely rare events. One occurs in our galaxy, it is estimated, about once every three centuries or so. When it does happen, one star can suddenly produce, at its peak, more power than the entire galaxy is producing. We have not been so fortunate as to see a supernova in this galaxy since the early 1600s, just before the invention of the telescope. However, several supernovae have been seen in other galaxies, including one in the Large Magellanic Cloud (a very nearby galaxy) in 1987. Supernovae are now found in other galaxies at the rate of about one a month. We also can look at the remains of these explosions, such as the nebulae left behind. After the explosion, the star generally becomes either a neutron star, white dwarf or black hole. Supernovae come in two types, Type I supernovae and type II supernovae. Supernovae are not named in the same way as other, more "normal" stars. They are named by year and a letter, as in 1993J or 1885A. You can find them with the Go to Supernova option in the Go to Star menu. supplemental plate The Hubble Guide Star Catalog, or GSC, was originally made with a few "problem areas", mostly places near very bright stars where the light from the bright star washed out dimmer stars nearby. Those problems, seen in the first version (number 1.0), were fixed in version 1.1 by taking some supplemental plates to patch those areas. Regrettably, the usual information about exposure start and so on isn't available for these plates. Switch To/From Red Normal colors Red mode ALT-R It is possible that you may have some desire to use this program at night out in the field. If so, having your eyes adjust to darkness would be tough if you looked at a bright white screen. On the other hand, red light is not very destructive to night vision. Clicking on this option turns all stars, objects and menu items to (or from) red, making the screen easier on night-adapted eyes. This item can be toggled at any time via the ALT-R hotkey; or, you can reach it from the display menu. In Windows Guide, this can only affect the charts drawn by Guide. The colors of the menu and title bars are controlled by Windows; you can reset them in the Control Panel (Win3.1) or Color Manager in the system group (Win95). SX Arietis SX Arietis type variable stars are blue-hot (spectral type B) stars with uneven distributions of helium and strong magnetic fields. As they rotate, the amount of helium seen in the spectrum and the intensity of the magnetic field appear to vary, while the brightness changes by about .1 magnitude. These stars, also called helium variables, are high-temperature versions of the Alpha CVn stars. SX Her The SX Her type of variable star has a fairly regular period of variation of 30 to 1100 days in length. Most semiregular long-period variables are older, cooler stars of spectral type M, R, N, or S, but the SX Her type is apt to be of type F, G, or K, not very unlike the Sun. This type varies from .1 up to 4 magnitudes in brightness. SX Phe The SX Phe type of variable star is much like a Delta Sct type star. They are pulsating subdwarf stars (smaller than the Sun) with spectral type A2 to F5. They are found in globular clusters. Symmetric RR Lyrae Symmetric RR Lyrae variables are RR Lyrae stars that have a light curve that looks much like a sine wave, a smooth variation from dark to light. Synchronous rotation An object that has synchronous rotation has an orbital period that matches its rotational period. This is true of the Moon (and of almost all satellites in the solar system). The result is that, for example, the Moon keeps one face toward the earth, and Io keeps one face toward Jupiter, and so on. T Tauri T Tauri type stars are probably about the youngest visible stars. They form in rich clouds of gas: as a lump in the gas forms, it creates a gravitational field that sucks in more gas, creating more gravity, and so on. As this takes place, the gas falling in starts to form an accretion disk around the star. All of this releases a lot of energy, most of which is soaked up by the surrounding gas and released as infrared (heat) and microwave radiation. Also, this energy will often cause jets to erupt from the poles of the star. Almost always, these stars are found in diffuse nebulae. Tables menu table menu Shift-F9 The tables menu provides several options for generating tables of events such as a "lunar data table" (rise and set times and libration), lunar apogee and perigees, lunar eclipses, Jupiter's satellite events and Great Red Spot appearances, and tables showing what comets or asteroids are visible down to a desired magnitude. There is also a Create Star List option to list all stars in the area currently shown on the screen, down to a desired limit. For each option, you'll be asked to provide a number of days, or of phases, or a magnitude limit. Guide will then show that list, and you can save it to a file or print it if you wish. In DOS, you can also reach the Tables Menu with the Shift-F9 hotkey. Tau Herculids Scorpiids Arietids Ophiuchids June Lyrids June Draconids Delta Aquarids Capricornids Perseids Kappa Cygnids Andromedids Alpha Aurigids Showers in June, July, August, and September June 3 Tau Herculids: Radiant--near Corona Borealis. About a month long, 15 per hour max, most quite faint. June 5 Scorpiids: Radiant--near Ophiuchus. 20 per hour with some fireballs. June 7 Arietids: About 30 per hour. Slow moving with some fireballs. June 13 Ophiuchids: Radiant-- near Scorpius. Only 3 per hour but fast moving bolides are common. Duration--25 days June 16 June Lyrids: Radiant--near Vega. Another part of May Lyrid meteor stream. 15 per hour, faint blue meteors. June 20 Ophiuchids: Radiant-- near Sagittarius. Rate varies from 8 to 20, with occasionally many more. June 30 June Draconids: Radiant--near handle of Big Dipper. Rate varies from 10 to 100 per hour. Pons-Winnecke Comet is parent. July 28 Delta Aquarids: Radiant--near Capricornus. 25 per hour, slow (24 km/sec) with yellow trails. Duration--40 days July 30 Capricornids: Radiant--near Aquarius. Tough to tell these from Delta Aquarids. 10 to 35 per hour with bolides. Aug 10 Perseids: Radiant--near Double cluster. 50 to 100 per hour, yellow with trails and bolides. The best modern dependable shower. Duration--5 days. Aug 20 Kappa Cygnids: Radiant--near Deneb. 12 per hour with many fireballs. Duration--15 days. Aug 31 Andromedids: Radiant-- near Cassiopeia. Occasionally spectacular, usually 20 per hour. Some red fireballs with trails. Biela's Comet parent. Sept 23 Alpha Aurigids: Radiant-- near Capella. 12 per hour, fast with trails.