Southern Double-Star Gems

The Southern Cross with Alpha and Beta Centauri
Two things that all Northern Hemisphere observers seem to know about the Southern Hemisphere sky are that it is home to the Southern Cross, and it is loaded with deep-sky treasures. What is less often appreciated is that the southern heavens also harbor some impressive double stars. North is to the upper left in this Akira Fujii photograph.

While most Northerners on a first-time visit to the Southern Hemisphere like to seek out well-known deep-sky treasures such as the Magellanic Clouds, 47 Tucanae, and Eta Carinae, I encourage visitors to add a sampling of the southern sky’s beautiful double stars to their observing lists. I have chosen some of my favorites to get you started.

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

The Indus/Pavo region
This finder chart is approximately oriented for early evening; north is to the right.
Image courtesy Akira Fujii.

So what has the southern sky got to offer? Well, let’s warm up with some colorful doubles. Ernst Johannes Hartung, author of the classic Astronomical Objects for Southern Telescopes, described Theta (q) Indi as having a pale-yellow primary and a reddish companion. Their current separation of about 7" is greater than when John Herschel measured them in 1834. There is also another pair located 4' south of the main pair.

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.

The Carina/Vela region
This finder chart is approximately oriented for early morning in the Southern Hemisphere's summer sky; north is to the upper right.
Image courtesy Akira Fujii.

Our next stop is Gamma Velorum. This is a magnificent multiple-star system located 520 light-years away. The primary is the brightest Wolf-Rayet star known, and the main pair is clearly visible in binoculars. It consists of a white luminous orb with a bluish white 4th-magnitude companion 41.4" to its south-southwest. In addition, about 1' east-southeast, almost at right angle to the main pair, is a second fine double. The brighter of its stars is magnitude 7.3, and both it and its 9.4-magnitude companion appear bluish. Gamma Velorum is set in a background of fainter stars, adding to the appeal of the view.

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

The Southern Cross
The Southern Cross.
Courtesy Akira Fujii.
Now it’s time to take a break. Since you have a few hours to pass before the rest of my selections climb high enough to be seen at their best, you might as well enjoy some of those deep-sky objects that have doubtless also found their way onto your observing list.

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.

Night's End

Richard Jaworski
The author is seen here with the 6-inch f/8 Dobsonian reflector he uses for much of his double-star observing.
Courtesy Helen Jaworski.

Weary eyes will appreciate that I have saved the brightest for last. Alpha Centauri is the best-known binary in the southern sky. (See the photo at the top of the page for an image of the starfield.) The double nature of this star was discovered in December 1689 by Father Richaud at Pondicherry, India, while he was following a remarkably bright sungrazing comet. Observing from the Cape of Good Hope in 1752, the industrious French astronomer Nicolas-Louis de Lacaille provided the first accurate measurements of the pair. In 1839 Thomas Henderson, also at the Cape of Good Hope, measured the distance to the system using stellar parallax. The orbit is inclined 11° to our line of sight and thus forms an eccentric and elongated ellipse. The distance between the stars varies from 11 to 35 astronomical units (1 a.u. is equal to the mean distance between the Sun and Earth). From our perspective, the separation between the stars will be at a minimum in 2038 when they will appear 1.7" apart. The great thing about this system is that the orbital period is only 80 years, and an observer can detect changes in separation and position angle in just a few years.

Separation and position angle diagram
A double star's position angle (p.a.) and separation (sep.) will indicate how the pair will appear in the eyepiece. The directions shown here are for an inverting scope, such as a Newtonian reflector. Other telescopes may have different field orientations.
Sky & Telescope illustration.
Alpha Centauri is thought to be a triple-star system. The C component, Proxima Centauri, is an 11th-magnitude red dwarf located 2° southwest of Alpha. It also happens to be the nearest star to our Sun, lying only 4.22 light-years away. While working at the Union Observatory in Johannesburg, South Africa, the amateur-turned-professional astronomer Robert Innes discovered Proxima in 1915. It is estimated that it takes a million years for Proxima to complete a single orbit around the main pair.

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
Star
 
R.A. (2000)
h   m
Dec.
°   '
Magnitudes
 
Sep.
 
P.A.
 
Gamma Volantis  7 08.7-70 303.9    5.4  14.1"298°
Gamma Velorum AB  8 09.5-47 201.8    4.1  41.4"221°
Gamma Velorum CD7.3    9.4  22.2"124°
Upsilon Carinae  9 47.1-65 043.0    6.0    4.8"128°
Alpha Crucis AB12 26.2-63 061.3    1.6    4.0"114°
Alpha Crucis AC1.3    4.8  90.0"202°
Gamma Crucis12 31.2-57 071.8    6.5125.4"  26°
Beta Muscae12 46.3-68 063.5    4.0    1.4"  37°
Mu Crucis12 54.6-57 113.9    5.0  34.6"  17°
Alpha Centauri14 39.6-60 500.0    1.4  13.3"224°
Alpha Circini14 42.5-64 593.2    8.5  15.6"227°
Theta Indi21 19.9-53 274.5    6.9    6.7"273°
Delta Tucanae22 27.3-64 584.5    8.7    7.0"281°