50 delightful double stars: star pairs show us color combinations we rarely see anywhere else.
You may have heard that even the largest backyard telescope isn't powerful enough to resolve stars into radiant spheres. That's true, but there's so much more to the story.
Even the smallest telescope will show that many naked-eye stars are actually close pairs, called double stars. Telescopes can reveal this because their lenses can mirrors magnify the view. Many doubles can be seen distinctly with only modest powers, often less than 100?. The brighter star in a pair is called the primary, the fainter one the secondary or companion.
Telescopes also show us fantastic star colors. This happens because a telescope collects hundreds to thousands of times more starlight than the eye alone. Our retinas' color receptors, called cone cells, don't respond well to dim light; that's why we don't see many colors in the dark. But a telescope boosts star brightnesses above the threshold needed to trigger color vision, and--voila!--stars that appear white to the naked eye blossom in delicate shades of red, blue, and everything in-between.
A double star might be a bright orange beacon with a dim blue spot on one edge. Or a wide pair of identical points that look like the headlights of a car coming at you from space. Or a great white orb paired with a tiny green one, looking like a star with a planet. Or the two stars might be so close together that they resemble a peanut. The permutations of brightness, color, and separation are almost infinite.
Over the Rainbow
Observers rave about the colors of double stars, which are unlike anything in ordinary experience and often quite hard to describe. Ordinary terrestrial objects reflect some colors but not others, whereas starlight is a mixture of all colors. So when you see a "yellow" star in your telescope, it won't look vivid yellow like a road sign; it will shine with a blend of many colors and appear mostly yellowish-white.
I never tire of paging through William Smyth's 1884 astronomy classic, A Cycle of Celestial Objects. Smyth lists 280 extraordinary color combinations for double stars, including bright white and pale purple, slight yellow and pale garnet, orange and small blue, light rose and dusty red.
Besides being pretty, these colors tell us how hot the stars are. The hottest stars shine with more blue light than red, and the coolest ones--which are still plenty hot!--shine with more red light than blue. Our Sun's surface temperature is about 10,000?F (5,500?C), which is middle-of-the-road as stars go, so its light is strongest in the middle, or yellow, part of the spectrum.
Sometimes, though, our eyes play tricks on us, and this is especially true with double stars. When two stars of different temperatures are seen very close together, the contrast between them makes us perceive their colors differently than we would if we saw each star alone. Nobody ever described a single star as mainly green or purple, but many observers see such colors in double stars--and sometimes two astronomers will see different colors in the very same stars!
Around the Track
By now you're probably wondering if double stars are true physical couples or merely chance alignments. The answer is that a pair could be either. Astronomers call the true couples binary stars and the chance alignments optical doubles.
In an optical double, one star lies far beyond the other; their appearance of closeness is simply a line-of-sight coincidence. In a binary star, the two components are bound together by gravity and orbit a common center of mass. The members of a binary star typically lie just a few hundredths of a light-year apart, whereas the separation between the Sun and its nearest stellar neighbors is at least 4 light-years, which is hundreds of times larger.
How can astronomers tell whether a particular star pair is an optical double or a true binary? Often they can't. But if they can determine that the two stars are about the same distance from Earth and share the same motion through space, they can confidently assume that the system is a binary. Conversely, if they find the pair to lie at different distances and/or to be moving through space in different directions, they know they're looking at an optical double.
Some binaries are close enough for us to detect their orbital motion. Usually it's quite slow and likely to be overlooked by any but the most skilled observers. But sometimes it's so fast that any stargazer can see the two stars changing their relative positions from year to year. Once a known binary has been monitored for many years, astronomers can calculate its orbital period, the time it takes to complete one full revolution. Double-star periods are measured in tens, hundreds, or thousands of years.
Stars don't come only in singles and doubles. Many are members of triples, quadruples, and other multiple systems. Sometimes two orbiting pairs will in turn orbit each other, creating a "double double" system. The variety is endless!
Through the Looking Glass
The table on pages 71 and 72 lists 50 of my favorite bright star pairs. I think they offer a perfect introduction to backyard observing, since most of them are visible to the naked eye (and thus easy to find) and all of them are glorious in even the smallest telescope. Indeed, I compiled the list based on observations I made with my little 60-millimeter (2.4-inch) refractor.
The evening star maps in the Sky Diary section (pages 12-59) mark the locations of some of the brighter doubles in my list, but you'll need more detailed maps to find the fainter ones. I recommend the Bright Star Atlas by Wil Tirion (Willmann-Bell, 1990) and Sky & Telescope's Pocket Sky Atlas by Roger W. Sinnott (Sky Publishing, 2006). Both are inexpensive and easy to use.
You don't need a fancy telescope to see these 50 doubles, but you do need a telescope rather than just binoculars in order to get enough magnification to "split" the components distinctly.
If your telescope came from a department store and has poor-quality eyepieces, you may need to replace them with better ones. I suggest getting a good-quality low-power eyepiece (one with a focal length of 25 to 30 mm) and using this to locate the star you're after. If it's a wide pair, you can enjoy the view in this same eyepiece. But if it's a tight double, switch to a higher-power eyepiece, perhaps one with a focal length around 10 to 15 mm.
Only a tiny percentage of the human race has seen these beautiful double stars. If you join the ranks of those who have, you won't be disappointed.
If you have any questions or would just like to share your observations of these star pairs with me, please write--I'd love to hear from you. Happy observing!
Marking the beak of Cygnus, the Swan, Albireo is one of the finest and most colorful double stars in the heavens. Observers variously describe the brighter component as gold or citrus orange or topaz yellow and the fainter one as royal blue or sapphire blue.
The Orion Nebula is one of the sky's finest telescopic sights. This image from the Hubble Space Telescope captures colors and details far beyond those visible in a backyard telescope. But the tight little quartet of stars in the nebula's heart, shown more clearly in the inset image, are visible in even the smallest instrument. This famous multiple star, [Theta.sup.1] ([[theta].sup.1]) Orionis, is more commonly known as the Trapezium. Energy from its ultrahot components is what causes the nebula to glow.
THE DOUBLE DOUBLE
Lyra is home to numerous double stars, but only one is obvious in the image at right. Look carefully to the upper left of the bright star, Vega. That little dumbbell-shaped star is Epsilon ([epsilon]) Lyrae, the famous Double Double, illustrated below.
TELESCOPES FOR DOUBLE-STAR OBSERVING
Many double stars can be seen in small instruments, but challenging pairs can push a telescope's resolving power (its ability to show fine detail) to the limit. Tight pairs test the observer's skill, the steadiness of the atmosphere, and the optics of the scope. And because so many close doubles demand unusually high magnifications, a tracking mount is very helpful. Almost any quality instrument will show hundreds, if not thousands, of interesting double stars. Bigger scopes allow you to see fainter stars and pairs that are closer together than smaller scopes. An 8-inch as an excellent all-around choice. A Schmidt-Cassegrain telescope like the one shown above provides this aperture in a very compact package.
50 Delightful Double (or Triple or Quadruple) Stars
If you're new to backyard observing, don't be intimidated by the columns of data in the table. They're very easy to explain.
Constellation and Star
First I list the constellation the pair is in; then the star's designation (a Greek letter or Arabic number), which you'll find labeled on star atlases; and then its celestial coordinates.
R.A. and Dec.
"R.A." means right ascension, analogous to longitude on Earth, and "Dec." means declination, analogous to latitude. All star atlases have coordinate grids with R.A. and Dec. clearly labeled.
P.A. and Sep.
The remaining columns describe the star pairs themselves. Position angle (P.A.) refers to the compass direction of the fainter star with respect to the brighter one, measured from celestial north (0[degrees]) through east (90[degrees]) and on around. Separation (Sep.) is the angular distance between the two stars, given in arcseconds ("), where 1??= 1/3600[degrees]. A separation of 2" is very close and requires high magnification, 15x is medium close, and 30x is wide enough to be seen well at low powers.
[m.sub.1] and [m.sub.2]
The magnitudes [m.sub.1] and [m.sub.2] indicate the brightness of the primary star and its companion, respectively. The stellar magnitude scale works like a golf score: smaller numbers refer to brighter stars. Each step corresponds to a factor of about 2.5x. Thus, a 2nd-magnitude star shines about 2 1/2 times brighter than a 3rd-magnitude star but only 40% as brightly as a 1st-magnitude star. In the country on a clear, moonless night, you might see stars as faint as 6th magnitude with your unaided eyes. From bright cities or suburbs, though, you'll generally see stars no fainter than about 4th magnitude. A small telescope will show stars as dim as 11th or 12th magnitude. The faintest double-star component in my table is 9th magnitude--an easy catch.
Note, however, that the greater the brightness difference between the primary and the companion, the tougher they are to split. If at first you don't succeed, try a higher magnification or wait for a night when the air is steadier. None of the 50 doubles in the accompanying table are particularly difficult to split; I've avoided listing pairs with tight separations or large brightness differences.
The description in the last column gives my impression of the stars as seen in my 60-mm refractor. You may see different colors than I do, especially if you use a larger telescope.
If the star has a well-known proper name, I've included it in the description. I've also indicated whether the pair is known or suspected to be a true binary or an optical double; if I've listed an orbital period (P), it's a binary!
SISSY HAAS is the author of the new book Double Stars for Small Telescopes: More than 2,100 Stellar Gems for Backyard Observers, which is available from Sky Publishing. You can write to her at 823 Reamer Ave., Greensburg, PA 15601, or send e-mail to firstname.lastname@example.org.
Constellation Star R.A. Cassiopeia [eta] [0.sup.h] [49.1.sup.m] Aries [gamma] [1.sup.h] [53.5.sup.m] Pisces [alpha] [2.sup.h] [02.0.sup.m] Andromeda [gamma] [2.sup.h] [03.9.sup.m] Triangulum [iota] [2.sup.h] [12.4.sup.m] Perseus [epsilon] [3.sup.h] [57.9.sup.m] Perseus 57 [4.sup.h] [33.4.sup.m] Taurus [theta] [4.sup.h] [28.7.sup.m] Taurus 118 [5.sup.h] [29.3.sup.m] Orion 23 [5.sup.h] [22.8.sup.m] Orion [lambda] [5.sup.h] [35.1.sup.m] Orion [[theta].sup.1] [5.sup.h] [35.3.sup.m] Orion [iota] [5.sup.h] [35.4.sup.m] Orion [sigma] [5.sup.h] [38.7.sup.m] Lepus [gamma] [5.sup.h] [44.5.sup.m] Monoceros [epsilon] [6.sup.h] [23.8.sup.m] Monoceros [beta] [6.sup.h] [28.8.sup.m] Lynx 12 [6.sup.h] [46.2.sup.m] Lynx 38 [9.sup.h] [18.8.sup.m] Gemini [alpha] [7.sup.h] [34.6.sup.m] Puppis N [7.sup.h] [34.3.sup.m] Puppis [kappa] [7.sup.h] [38.8.sup.m] Cancer [iota] [8.sup.h] [46.7.sup.m] Leo [gamma] [10.sup.h] [20.0.sup.m] Leo 54 [10.sup.h] [55.6.sup.m] Leo 93 [11.sup.h] [48.0.sup.m] Ursa Major [xi] [11.sup.h] [18.2.sup.m] Ursa Major [zeta] [13.sup.h] [23.9.sup.m] Hydra N [11.sup.h] [32.3.sup.m] Corvus [delta] [12.sup.h] [29.9.sup.m] Canes Venatici [alpha] [12.sup.h] [56.0.sup.m] Libra [alpha] [14.sup.h] [50.9.sup.m] Serpens [delta] [15.sup.h] [34.8.sup.m] Scorpius [iota] [16.sup.h] [04.4.sup.m] Scorpius [beta] [16.sup.h] [05.4.sup.m] Scorpius [sigma] [16.sup.h] [21.2.sup.m] Draco [iota] [17.sup.h] [32.2.sup.m] Draco [psi] [17.sup.h] [41.9.sup.m] Hercules [alpha] [17.sup.h] [14.6.sup.m] Hercules [rho] [17.sup.h] [23.7.sup.m] Ophiuchus 36 [17.sup.h] [15.3.sup.m] Lyra [[epsilon].sup.1] - [18.sup.h] [44.3.sup.m] [[epsilon].sup.2] [[epsilon].sup.1] [[epsilon].sup.2] Lyra [eta] [19.sup.h] [13.8.sup.m] Serpens [theta] [18.sup.h] [56.2.sup.m] Cygnus [beta] [19.sup.h] [30.7.sup.m] Capricornus [alpha] [20.sup.h] [18.1.sup.m] Delphinus [gamma] [20.sup.h] [46.7.sup.m] Cepheus [beta] [21.sup.h] [28.7.sup.m] Cepheus [iota] [22.sup.h] [03.8.sup.m] Cepheus [delta] [22.sup.h] [29.2.sup.m] Constellation Dec. P.A. Sep. Cassiopeia +57[degrees] 49' 319[degrees] 13.0" Aries +19[degrees] 18' 0[degrees] 7.5" Pisces +02[degrees] 46' 270[degrees] 1.9" Andromeda +42[degrees] 20' 63[degrees] 9.7" Triangulum +30[degrees] 18' 69[degrees] 3.9" Perseus +40[degrees] 01' 9[degrees] 9.0" Perseus +43[degrees] 04' 198[degrees] 120" Taurus +15[degrees] 52' 348[degrees] 337" Taurus +25[degrees] 09' 207[degrees] 4.7" Orion +03[degrees] 33' 29[degrees] 31.6" Orion +09[degrees] 56' 44[degrees] 4.3" Orion -05[degrees] 23' 31[degrees] 8.8" 132[degrees] 12.7" 61[degrees] 13.3" Orion -05[degrees] 55' 141[degrees] 11.3" Orion -02[degrees] 36' 84[degrees] 12.7" 62[degrees] 41.5" Lepus -22[degrees] 27' 350[degrees] 96.9" Monoceros +04[degrees] 36' 29[degrees] 12.1" Monoceros -07[degrees] 02' 133[degrees] 7.1" 108[degrees] 2.9" Lynx +59[degrees] 27' 309[degrees] 8.7" Lynx +36[degrees] 48' 226[degrees] 2.6" Gemini +31[degrees] 53' 62[degrees] 4.2" Puppis -23[degrees] 28' 117[degrees] 9.8" Puppis -26[degrees] 48' 318[degrees] 9.8" Cancer +28[degrees] 46' 308[degrees] 30.7" Leo +19[degrees] 50' 127[degrees] 4.6" Leo +24[degrees] 45' 111[degrees] 6.3" Leo +20[degrees] 13' 357[degrees] 74.1" Ursa Major +31[degrees] 32' 245[degrees] 1.7" Ursa Major +54[degrees] 56' 153[degrees] 14.3" Hydra -29[degrees] 16' 210[degrees] 9.4" Corvus -16[degrees] 31' 217[degrees] 24.9" Canes Venatici +38[degrees] 19' 229[degrees] 19.3" Libra -16[degrees] 03' 315[degrees] 231" Serpens +10[degrees] 32' 174[degrees] 4.0" Scorpius -11[degrees] 22' 48[degrees] 7.5" Scorpius -19[degrees] 48' 20[degrees] 13.6" Scorpius -25[degrees] 36' 273[degrees] 20.0" Draco +55[degrees] 11' 311[degrees] 63.4" Draco +72[degrees]09' 15[degrees] 30.0" Hercules +14[degrees] 23' 104[degrees] 4.8" Hercules +37[degrees] 09' 319[degrees] 4.1" Ophiuchus -26[degrees] 36' 146[degrees] 4.7" Lyra +39[degrees] 40' 174[degrees] 210" 352[degrees] 2.1" 82[degrees] 2.4" Lyra +39[degrees] 09' 80[degrees] 28.1" Serpens +04[degrees] 12' 104[degrees] 22.3" Cygnus +27[degrees] 58' 55[degrees] 34.7" Capricornus -12[degrees] 33' 292[degrees] 381" Delphinus +16[degrees] 07' 266[degrees] 9.1" Cepheus +70[degrees] 34' 248[degrees] 13.2" Cepheus +64[degrees] 38' 275[degrees] 7.9" Cepheus +58[degrees] 25' 191[degrees] 40.6" Constellation [m.sub.1] [m.sub.2] Description Cassiopeia 3.5 7.4 Lovely close pair, Sun yellow and almond brown. P = 480 years. Aries 4.5 4.6 Close, bright identical twins; the " Ram's Eyes". Probable binary. Pisces 4.1 5.2 Very tight pair, a sharp little figure 8. P = 933 years. Andromeda 2.3 5.0 Almach. Lovely close pair, tangerine and cobalt blue. Binary. Triangulum 5.3 6.7 Glorious contrast, a little smoke-ball touching a bright yellow star. Binary. Perseus 2.9 8.9 Glorious contrast, a tiny blue dot beside a bright white star. Perseus 6.1 6.8 Wide near-equals, different in color! Ivory and peach. Optical. Taurus 3.4 3.9 Wide twins that dominate the Hyades star cluster. Taurus 5.8 6.7 Dim but attractively close pair, easily found. Probable binary. Orion 5.0 6.8 Wide and easy little pair, white and ashy blue. Orion 3.5 5.5 Close pair, lemon white and smaller blue-violet. Binary. Orion 6.6 7.5 The Trapezium. Quadruple 6.6 5.1 star in the Orion Nebula; 5.1 6.4 fantastic. Binary. Orion 2.9 7.0 Fantastic contrast, a speck beside a bright star. Orion 3.8 6.6 Nice triple, straw 3.8 6.3 yellow and two small blues. Possible binary. Lepus 3.6 6.3 Gorgeous wide pair, Sun yellow and brick red. Probably optical. Monoceros 4.4 6.6 Lovely contrast, a bright star and a little smoke-puff, just apart. Binary. Monoceros 4.6 5.0 Grand sight! A tight 5.0 5.3 triple, all three stars identical. Likely a true threesome. Lynx 5.4 7.0 Pleasing unequal pair, just wide enough to be easy. Possible binary. Lynx 3.9 6.1 Striking contrast, a little smoke-ball touching a bright white star. Binary. Gemini 1.9 3.0 Castor. Stunning pair of brilliant stars, split by a hair. P = 445 years. Puppis 5.8 5.9 Close bright twins in a field sprinkled with stars. Possible binary. Puppis 4.4 4.6 Bright white twins, close but easily split. Probable binary. Cancer 4.1 6.0 Sun yellow and royal blue, gloriously vivid. Optical. Leo 2.4 3.6 Algieba. Brilliant close pair, a grapefruit- colored figure 8. P = 619 years. Leo 4.5 6.3 Lovely close pair, banana yellow and sapphire. Binary. Leo 4.6 9.0 Pretty pair, a bright yellow star and wide little dot. Presumably optical. Ursa Major 4.3 4.8 Very close, a bright lemony-white figure 8. P = 60 years. Ursa Major 2.2 3.9 A super-wide pair; the brighter star (Mizar) is a binary. Hydra 5.6 5.7 Close twins, lovely grapefruit-orange in color. Probable binary. Corvus 3.0 8.5 Great star and a tiny dot, like a star with a planet! Probably optical. Canes Venatici 2.9 5.5 Cor Caroli. Lovely wide pair, deep white and dusty sea green. Optical. Libra 2.7 5.2 Zubenelgenubi. Very wide pair of beautifully bright stars. Probably optical. Serpens 4.2 5.2 Bright pair of touching stars, quite pretty. P = 1,038 years. Scorpius 4.9 7.3 This pair forms a "double double" with another pair right next to it! Scorpius 2.6 4.5 Graffias. Wide, bright, and easy pair, lucid white and cobalt blue. Optical. Scorpius 2.9 8.4 Great star and a tiny dot, like a star with a planet! Optical. Draco 4.9 4.9 Wide, bright identical twins; the "Dragon's Eyes". Probably optical. Draco 4.6 5.6 Wide and easy pair, both stars goldish-white. Optical. Hercules 3.5 5.4 Rasalgethi. A little globe touching a bright red star, breathtaking! P = 3,600 years. Hercules 4.5 5.4 Attractive blue-white couple, split by a hair. Binary. Ophiuchus 5.1 5.1 Citrus orange twins, split by a hair. Very pretty. Possible binary. Lyra 5.0 5.3 The "Double Double," grand. Wide pair, each star a close pair! 5.0 6.1 P = 1,725 years. 5.3 5.4 P = 724 years. Lyra 4.4 8.6 Bright star and a ghostly speck in a field sprinkled with stars. Serpens 4.6 4.9 Alya. Beautiful deep- white twins; bright, wide, and easy. Optical. Cygnus 3.4 4.7 Albireo. Bright citrus orange and vivid royal blue, breathtaking. Optical. Capricornus 3.7 4.3 Algedi. Super-wide pair of bright stars, remarkably alike. Optical. Delphinus 4.4 5.0 Gorgeous close pair, grapefruit orange in color. P = 3,249 years. Cepheus 3.2 8.6 Alfirk. Bright white star touching an olive-green dot, breathtaking. Cepheus 4.4 6.4 Bright white touching azure blue, very pretty. P = 3,800 years. Cepheus 4.2 6.1 Beautiful colors, a [beta] Cygni clone (see above). GREEK LETTERS FOR BRIGHT STARS The brightest stars in each constellation are assigned Greek letters. A constellation's most brilliant star is usually (but not always) called Alpha, the first letter of the Greek alphabet. [alpha] Alpha [beta] Beta [gamma] Gamma [delta] Delta [epsilon] Epsilon [zeta] Zeta [eta] Eta [theta] Theta [iota] Iota [kappa] Kappa [lambda] Lambda [iota] Mu [iota] Nu [iota] Xi [omicron] Omicron [iota] Pi [rho] Rho [sigma] Sigma [tau] Tau [upsilon] Upsilon [phi] Phi [chi] Chi [psi] [Psi] [omega] Omega Equinox 2000.0 coordinates, position angles, separations, and magnitudes are from the online version of the Washington Double Star Catalog by Brian D. Mason, Gary L. Wycoff, and William I. Hartkopf of the U.S. Naval Observatory (http://ad.usno.navy.mil/wds).
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|Title Annotation:||observing double stars|
|Date:||Jan 1, 2007|
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