The Amateur Astronomer (eBook)

Publisher’s Note 5
Preface to the First Edition 7
Preface to the Twelfth Edition 9
Contents 11
1 Astronomy as a Hobby 14
2 The Unfolding Universe 16
3 Telescopes and Observatories 30
4 The Solar System 44
5 The Sun 54
6 The Moon 64
7 Occultations and Eclipses 80
8 Aurorae and the Zodiacal Light 90
9 The Nearer Planets 96
10 The Outer Planets 106
11 Comets and Meteors 118
12 The Stellar Sky 132
13 The Nature of a Star 140
14 Double Stars 146
15 Variable Stars 152
16 Star Clusters and Nebulae 162
17 Galaxies 172
18 Beginnings and Endings 180
Appendicies 182
1 Planetary Data 182
2 Planetary Satellites of Magnitude 14.5 or Brighter 183
3 Minor Planet Data 184
4 Elongations and Transits of the Inferior Planets 185
5 Map of Mars 187
6 Jupiter: Transit Work 189
7 Saturn: Intensity Estimates 191
8 Forthcoming Eclipses 192
9 The Limiting Lunar Detail Visible with Different Apertures 193
10 The Lunar Maps 194
11 Some of the More Important Annual Meteor Showers 206
12 The Constellations 207
13 Proper Names of Stars 210
14 Stars of the First Magnitude 217
15 Standard Stars for Each Magnitude 218
16 The Greek Alphabet 220
17 Stellar Spectra 221
18 Limiting Magnitudes and Separations for Various Apertures 222
19 Angular Measure 223
20 Test Double Stars 224
21 Extinction 225
22 Bright Novae 226
23 Messier’s Catalogue 228
24 The Caldwell Catalogue 231
25 The Star Maps 234
Map 235
Map I. Key Map: Ursa Major ( The Great Bear) 235
Map II. Key Map: Orion 237
Map III. Ursa Major, Ursa Minor, Draco, Cepheus, Camelopardus 239
Map IV. Orion, Lepus, Eridanus, Taurus, Cetus, Auriga, Columba, Caelum, Fornax 242
Map V. Gemini, Cancer, Canis Major, Canis Minor, Monoceros, Hydra 246
Map VI. Leo, Virgo, Coma Berenices, Corvus, Crater, Leo Minor 250
Map VII. Cassiopeia, Cepheus, Lacerta, Perseus, Andromeda, Lynx, Triangulum 253
Map VIII. Cygnus, Lyra, Sagitta, Vulpecula, Delphinus, Equuleus, Capricornus, Aquila, Sagittarius, Scutum, Serpens, Aquarius 256
Map IX. Boötes, Corona Borealis, Hercules, Serpens, Ophiuchus, Libra, Scorpio 261
Map X. Pegasus, Andromeda, Pisces, Triangulum, Aries, Cetus, Aquarius, Sculptor, Pisces Australis 265
Map XI. Key Map: Crux Australis ( The Southern Cross) 268
Map XII. Key Map: Orion 271
Map XIII. Crux, Centaurus, Lupus, Circinus, Triangulum Australe, Norma, Ara, Telescopium, Sagittarius, Corona Australis, Scorpio 273
Map XIV. Carina, Vela, Puppis, Pyxis, Volans, Antila 277
Map XV. Octans, Apus, Musca, Chamaeleon, Mensa, Hydrus, Reticulum, Dorado, Pictor, Eridanus, Horologium 280
Map XVI. Pavo, Indus, Tucana, Grus, Phoenix, Microscopium 283
26 The Observation of Variable Stars 286
Binocular Variables 289
Telescopic Variables 293
27 Some Periodical Comets 298
28 Amateur Observatories 299
29 Astronomical Societies 301
30 Bibliography 302
Index 304

Chapter 15
Variable Stars (p.139-140)

Fortunately for us, our Sun is a steady, well-behaved star. It may have periods of unusual activity, when its disk is disturbed by spot-groups and flares, but at least its output of energy does not alter greatly over the lapse of hundreds of centuries. Other suns are not so quiescent. Some of them vary in brightness from day to day, even from hour to hour, either regularly or in an erratic manner.

They swell and shrink, and their temperatures change with their fluctuations, so that any planet circling round them would be subject to most uncomfortable changes of climate. Variable stars are important both to the professional and to the amateur, and the owner of a small instrument can do useful work, particularly as his telescope need not be so perfect as that of the lunar or planetary observer (though, of course, the better the telescope the better the results).

It is true that the regular variables of short period have been closely studied at the great observatories, but there are other stars which seem to delight in springing surprises, so that they need constant watching. Variable stars are of many types, but it is not difficult to give a general classification. First there are the eclipsing binaries, such as Algol in Perseus,which are not true "variables" at all, even though they do seem to alter in brightness.

Perhaps the most important of the true short-period variables are the Cepheids, so named because the star Delta Cephei is the best-known member of the class; the periods range from a few days up to six or seven weeks. Of much shorter period are the RR Lyrae stars, whose periods range between 30 hours and less than 2 hours. Then there are the long-period variables, usually Red Giants of great size and comparatively low temperature,with periods ranging from 70 days to over 2 years. Irregular variables, as their name suggests, behave in an unpredictable manner. Lastly come the violently explosive "temporary stars" or novae. There are several variables which can be followed without any telescope at all.

The most famous of these is Betelgeux, the Red Giant in Orion. It belongs to the irregular class, though there is a very rough period of from 4 to 5 years, and it changes in brightness from magnitude 0 down to 1, so that whereas it may sometimes almost equal the glittering Rigel it may at others be comparable with Aldebaran, the "Eye of the Bull;". The alterations are slow, but they become noticeable over a week or two, and the beginner who estimates the magnitude of Betelgeux every few days will soon be able to detect the fluctuations.

However, most of the interesting variables cannot be followed without a telescope or at least binoculars, since when near minimum they are below naked-eye visibility. Before coming to the proper variables, it will be of interest to say something about the "fake variables", or eclipsing binaries. These might well have been described in the chapter dealing with double stars, but since they do seem to change in brilliancy they come under the scope of the variable star enthusiast. The best-known of these "fakes" is Algol, which lies in the constellation of Perseus and is shown in Map VII. In mythology, Perseus was the hero who slew the fearful Gorgon, Medusa, whose glance turned the hardiest onlooker to stone, and it is fitting that Algol should mark the Gorgon’s severed head.