Southern Double-Star Gems
By observing these stars you’ll be walking in the footsteps of some of the Southern Hemisphere’s most important astronomers, in particular James Dunlop and John Herschel. Apart from some sporadic observations made in the 17th century, double-star observing in the Southern Hemisphere was born in the 1820s when Dunlop and Christian Carl Ludwig Rümker, under the patronage of Thomas Brisbane, came to Australia to catalog all stars of 8th magnitude and brighter south of declination –33° (Sky & Telescope: June 2001, page 112). Dunlop discovered many attractive double stars and produced a catalog of 253 pairs based on observations he made between 1825 and 1827 with a speculum-metal, 9-inch f/12 reflecting telescope.
However, the systematic cataloging of southern double stars really began in earnest when John Herschel arrived in Africa at the Cape of Good Hope in 1834. Using his father’s "20-foot reflector" (with several 18¼-inch speculum-metal mirrors) and a 5-inch refractor, Herschel compiled a list of 2,102 pairs during a four-year period. Most of the stars described here were first recorded by these two observers.
Early Summer-Evening Treasures
Delta (d) Tucanae is a beautiful double. The primary is bright white and the companion reddish. They are separated by 7" and are both well seen in a 3-inch (76-millimeter) scope. Similarly, Gamma (g) Volantis is perfect for small apertures. This stunning bright gold and pale-yellow pair is set in a field of scattered faint stars.
Another two-for-one set of doubles is in nearby Carina. The superb pair Upsilon (u) Carinae reminds me of Rigel, with a bright primary and a close fainter companion. This long-period binary has shown little change since John Herschel discovered it in 1836. Located 5' southeast is the fainter, wider, more-equal pair HJ 4252, which shares the field with Upsilon in my 6-inch (15-centimeter) f/8 reflector at 190x. This second set consists of 8.7- and 9.1-magnitude stars separated by 12.2". Both doubles provide an interesting study in contrasts faint and wide versus bright and close.
Doubles Around the Southern Cross
My favorite southern double is Acrux, Alpha (a) Crucis. This magnificent gem is located at the foot of the Southern Cross. Its double nature was first noticed in 1685, but by whom? There is some disagreement in the literature, and the most recent reference I found states that it was first noted by French Jesuit priest Guy Tachard, who was journeying to Siam and en route stopped in Cape Town, South Africa, where a temporary observatory had been set up.
Alpha Crucis is actually a triple. The main pair (stars A and B) are brilliant bluish white suns separated by 4", while 4.8-magnitude C is located 90" away. Through my 6-inch reflector the stars resemble a short-base isosceles triangle. The widely separated A and C components are visible in 10 x 50 binoculars, and a 3-inch scope will even show the A-B pairing in daylight!
At the head of the Southern Cross is Gamma Crucis. It's an optical double, which means that its component suns are not gravitationally linked the stars merely appear together along our same line of sight. I enjoy looking at this object through steadily held 10 x 50 binoculars or a small telescope. The primary is bright orange, and its companion is white. As an added bonus, have a look at the nearby bright wide pair Mu (m) Crucis. Separated by 34.6", these 4th- and 5th-magnitude white stars are an easy target for a 60-mm scope. Since the observations of Dunlop in 1826, the position angle has increased by only 7° and the separation has decreased by 1.3".
Much more of a challenge is my next choice, Beta (b) Muscae. This 1.4" pair is a test for both the observer and the steadiness of the seeing. The pair has certainly opened since 1880, when Henry C. Russell measured the separation as 0.54". On nights of steady seeing, my 6-inch reflector at 190x shows two bluish white globes with a sliver of darkness between them.
Observers who enjoy color-contrasting doubles will appreciate Alpha Circini. The primary is yellow, and its fainter companion is red. This pair is easy in a 3-inch scope. The movement of this system suggests that we are probably seeing its orbit edge on.
These are just a few of the wonderful double stars that populate the southern sky. The article was written with a December observing session in mind, but of course these doubles are visible at other times of the year. If you can tear yourself away from the Magellanic Clouds long enough to explore these pairs, I promise you won’t be disappointed.
|Fine Southern Doubles|
|Gamma Volantis||7 08.7||-70 30||3.9 5.4||14.1"||298°|
|Gamma Velorum AB||8 09.5||-47 20||1.8 4.1||41.4"||221°|
|Gamma Velorum CD||7.3 9.4||22.2"||124°|
|Upsilon Carinae||9 47.1||-65 04||3.0 6.0||4.8"||128°|
|Alpha Crucis AB||12 26.2||-63 06||1.3 1.6||4.0"||114°|
|Alpha Crucis AC||1.3 4.8||90.0"||202°|
|Gamma Crucis||12 31.2||-57 07||1.8 6.5||125.4"||26°|
|Beta Muscae||12 46.3||-68 06||3.5 4.0||1.4"||37°|
|Mu Crucis||12 54.6||-57 11||3.9 5.0||34.6"||17°|
|Alpha Centauri||14 39.6||-60 50||0.0 1.4||13.3"||224°|
|Alpha Circini||14 42.5||-64 59||3.2 8.5||15.6"||227°|
|Theta Indi||21 19.9||-53 27||4.5 6.9||6.7"||273°|
|Delta Tucanae||22 27.3||-64 58||4.5 8.7||7.0"||281°|