The Southern Cross


Despite having the smallest size of all constellations - a mere 68 square degrees - Crux is the most celebrated of all southern constellations. Its name is Latin for cross, which refers to the main asterism of the four relatively bright stars at its centre. To some the cross shape is more resembling of a kite, but whatever you think it looks like it's superb and unmistakable.

Positioned in the sky at a declination of about -60 degrees, Crux is visible from all parts of the Southern Hemisphere and for many of these observers it's circumpolar. For residents just north of the equator the cross can be spotted during late spring, low down above the southern horizon. Unfortunately, for most Northern Hemisphere observers this superb constellation never manages to climb above the horizon and can never be seen.

Crux Star Chart (credit:- freestarcharts)

Crux Star Chart - pdf format (credit:- freestarcharts)

Interesting Stars

Bright Stars, Double Stars, Main Cross Stars

Acrux (alpha Crucis - α Cru) - at mag. +0.77, is the brightest constellation star and twelfth brightest in the night sky. It also holds the distinction of being the most southerly first magnitude star, positioned just fractionally further south of its nearest rival, Alpha Centauri.

Located 321 light-years from Earth, Acrux is a multiple star system of at least 3 or maybe 4 stars. The two main components, a1 (mag. +1.40) and a2 (mag. +2.09), are the only stars of the system that are visually distinguishable. Both are massive blue-white hot sub-giant B class stars with luminosities of 25,000 and 16,000 times that of the Sun respectively. They are separated by only 4 arc seconds and show no discernable movement over time suggesting that the orbital period is at least 1,500 years.

The brighter star of the two main components, a1, is itself a spectroscopic binary. It's believed to consist of stars 14 and 10 times the mass of the Sun. Physically they have a common orbital period of 76 days and are separated by just 1 AU.

The fourth star of the system is positioned about 90 arc seconds from Acrux and shines at mag +4.86. It's also a class B sub-giant that shares the same motion through space as a1 and a2. Therefore it's believed to be gravitationally bound to the system and a true member. However, this is by no means certain and there is debate regarding the true distance of this star. It could be several hundred light-years further away and hence only a chance alignment.

Through small 80mm (3.1-inch) scopes the two main stars are easily split with strikingly similar components.

Mimosa or Becrux (beta Crucis - β Cru) - at mag. +1.30, is the second brightest constellation star and the 20th brightest in the night sky. Like Acrux, it was unfamiliar to the ancient Greeks and Romans and therefore doesn't have a classical name. It's sometimes known as Becrux, the second star of the cross, or Mimosa. The latter name is based on its colour, which was given to it by 16th century German astronomer Johann Bayer.

Mimosa is a giant class B blue-white star - 34,000 times more luminous than the Sun - that's located approximately 350 light-years distant. It's a spectroscopic binary with components that are believed to orbit every 5 years. They are separated by only 8 AU, just less than the distance from the Sun to Saturn. However, Mimosa does form a line of sight double with a 7th magnitude star, separation 373 arc seconds and at a PA of 23 degrees. The pair of stars are easily separable with small scopes.

Mimosa has a surface temperature of around 28,000 Celsius and is believed to be the hottest of all first magnitude stars. Interestingly it has an iron content of about half that of the Sun, suggesting that it's nearing the end of its hydrogen-fusing stage and following that it's supernova time.

Gacrux (gamma Crucis - γ Cru) - is the northernmost star of the main cross. It's a red giant that shines at mag. +1.59. Gacrux is the closest of the four main stars, at a mere 88 light-years. What's probably most striking is the colour contrast compared to the other three stars of the cross. They are either blue / blue-white or white in colour, whereas the deep red-orange hue of Gacrux stands out as a notable exception.

Gacrux has a wide faint unrelated white mag. +6.4 companion, separation about 2 arc minutes. The PA angle is 128 degrees and both stars are visible in binoculars. At 400 light-years the companion star is nearly 5x further away than Gacrux.

Main Cross Star

Delta Crucis (δ Crucis) - at mag. +2.78, is the faintest of the four stars of the Cross. It's a blue-white subgiant star (spectral type: B2IV), located about 360 light-years from Earth. Both the distance and the spectral type of Delta Crucis are similar to Acrux and Mimosa. Therefore it's reasonable to assume that all three stars are related by birth, although they are now not close enough to be gravitationally bound. Delta Crucis has a very hot 22,550K surface temperature with a luminosity of about 5,600 times that of the Sun. The spectrum of the star implies that it has recently stopped hydrogen fusion in its core and is in the process of developing into a red giant. It has a mass below the Chandrasekhar limit and therefore will end its days as a dim white dwarf rather than go bang in a spectacular explosion. It's estimated to be less than 30 million years old.

Delta Crucis is a Beta Cephei variable with a very small brightness variation. The period is 1.3 hours. The star is also a fast spinner, rotating once ever 1.3 days.

Double Stars

Iota Crucis (ι Cru) - 125 light-years, at mag. +4.69 is an orange giant star (class K1). Small / medium size scopes reveal an unrelated, mag. +10.2, G8 class companion. The separation is 28 arc seconds, PA 8 degrees.

Mu Crucis (μ Cru) - is the 6th brightest star in Crux. It's a wide pair of blue-white (spectral class B) stars of mag. +4.0 and +5.1. With a separation of 35 arc seconds, they are easily spilt in small scopes or even with good 10x50 binoculars. The stars lie at a distance of about 380 light-years. All in all, a nice double star.

Deep Sky

Dark Nebula

Coalsack - Measuring a whopping 7 by 5 degrees, the Coalsack is the most prominent dark nebula in the sky. Under dark skies it's easily visible to the naked eye, appearing as a dark patch silhouetted against the bright star fields of the Milky Way. Undoubtedly known since prehistoric times, the nebula was first recorded in 1499 by legendary Spanish explorer Vincente Yanez Pinzon on a voyage to South America. A few years later, Amerigo Vespucci named it "il Canopo fosco", which means the Dark Canopus. It was also called "Macula Magellani", translation Magellan's Spot, or the "Black Magellanic Cloud". The dark part of the name is not completely true. It was recently proven to exhibit a dim glow equal to about 10% of the brightness of the surrounding Milky Way, which comes from reflection of light from obscured stars.

Surprisingly, the Coalsack was not included in the New General Catalogue, although it is entry 99 in the Caldwell catalogue. Due to its extremely large size, it's best viewed with the naked eye under dark moonless skies or with binoculars.

Coalsack Dark Nebula - Caldwell 99 (credit:- ESO Yuri Beletsky)

Open Clusters

Jewel Box or Kappa Crucis Cluster (NGC 4755) - is one of the finest open clusters in the sky and a fantastic object for all types of optical instruments. It's located 6,440 light-years distant and is visible to the naked eye, appearing like a mag. +4.2 star. A good pair of 10x50 binoculars reveals a pyramid of five bright stars with many fainter members sprinkled in and around it. A small scope shows a dozen or so more stars of various colours that are clustered in two distinct groups. With larger instruments, it's possible to glimpse the associated nebulosity.

French astronomer Nicolas Louis de Lacaille discovered the cluster during his 1751-1752 astronomical expedition to South Africa's Cape of Good Hope. English astronomer Sir John Hershel named it the Jewel Box, describing it as a superb piece of fancy jewellery. It's one of the youngest known open clusters with an estimated age of between 7 and 10 million years. In total, NGC 4755 contains a few hundred stars spread across 10 arc minutes of apparent space.

NGC 4755 Jewel Box Open Cluster (credit:- ESO La Silla Observatory)

NGC 4609 - mag. +6.9, is the brightest of a number of fainter open clusters in Crux. What's interesting about this object is it actually lies within the boundaries of the Coalsack. To locate NGC 4609, first focus on Acrux and then move 2 degrees directly west until you reach an obvious bow shaped collection of 9th magnitude stars. NGC 4609 can be spotted with small scopes but best seen with apertures of at least 150mm (6-inch). A 200mm (8-inch) reflector reveals about 15 to 20 stars, mostly about 9th or 10th magnitude, spread across 6 arc minutes. In total, it contains up to 40 stars. There is an unrelated, but bright mag. +5.25, star to the southeast.

Harvard 5 - mag. +7.1, sometimes known as Collinder 258. Although the constellation's third brightest open cluster, it's elusive and unimpressive compare to the others. Containing up to 25 members across 5 arc minutes, a minimum 150mm (6-inch) telescope is recommended for viewing.

Harvard 5 is relatively easy to find, positioned one-third of the way along a line connecting Acrux with γ Crux.

NGC 4103 - is a nice small compact (apparent size 6 arc minutes) cluster of about 50 members. It's located on the western side of the constellation and sits in a triangle of 6th magnitude stars, two of which are red in colour. NGC 4103 shines at mag. +7.4 and is just about visible with good 10x50 binoculars, appearing as a faint smudge or glow set against the background Milky Way star fields. Observers with larger 11x70 or 20x80 binoculars will find the cluster a much easier target that looks somewhat irregular in shape and hints at resolution.

A 150mm (6-inch) scope easily reveals the brightest stars with about 15 to 20 above 10th magnitude. They are arranged in multiple chains with some orange members providing a pleasant colour contrast.

NGC 4349 - Lying just short of midway between Acrux and ε Crucis, NGC 4349 is a dim but quite interesting open cluster. At mag. +7.4, it's visible in good binoculars on nights of excellent seeing. The cluster spans only 4 arc minutes in diameter. With a small 100mm (4-inch) telescope, an 8th magnitude star is noticeable towards the southeast corner. Extend northwards and the glow of NGC 4349 will be apparent, although it fades off quickly, not unlike the view of a faint comet.

Larger amateur scopes, of the order of 300mm (12-inch) aperture or greater, resolve many of the cluster members. Up to 30 stars are visible, lined up in chains that cut through space.

NGC 4052, NGC 4337 and NGC 4439 - are three fainter (8th and 9th magnitude) open clusters in Crux, difficult in binoculars and best seen with medium to large size instruments.

Crux Star Data Table

Henry Draper Catalogue (HD)Hipparcos Catalogue (HIP)BayerNameRA (J2000)DEC (J2000)Visual Mag.DoubleSep. (arc secs)PA (deg.)Mag. Primary, Sec
10649059747Delta Crucis---12h 15m 09s-58d 44m 56s+2.79------------
108248/10824960718Alpha CrucisAcrux12h 26m 36s-63d 05m 57s+0.77Y41141.25 / 1.55
10890361084Gamma CrucisGacrux12h 31m 10s-57d 06m 48s+1.59Y125271.83 / 6.45
11082962268Iota Crucis---12h 45m 38s-60d 58m 53s+4.69Y2884.7 / 10.2
11112362434Beta CrucisMimosa or Becrux12h 47m 43s-59d 41m 20s+1.25Y373231.28 / 7.2
112092/11209163003/63005Mu Crucis---12h 54m 36s-57d 10m 41s+3.97Y35174.0 / 5.1

Crux Deep Sky Data Table

NGCCaldwellHarvardName(s)TypeRA (J2000)DEC (J2000)Visual MagnitudeApparent Size (arc mins)Distance (light-years)Actual Size (light-years)Number of Stars
---99---Coalsack Dark Nebula12h 50m-62d 30m---420 x 30057570 x 50---
475594---Jewel Box / Kappa Crucis Cluster Open Cluster12h 53m 42s-60h 22m 00s+4.2106,44020100
460998------Open Cluster12h 42m 20s-62h 59m 38s+6.965,000940
------5---Open Cluster12h 27m 10s-60d 46m 00s+7.154,000625
4103---------Open Cluster12h 06m 40s-61h 15m 21s+7.465,300945
4349---------Open Cluster12h 24m 06s-61h 52m 13s+7.447,000830
4439---------Open Cluster12h 28m 26s-60d 06m 11s+8.445,800713
4052---------Open Cluster12h 01m 30s-63d 13m 20s+8.8104,0001280
4337---------Open Cluster12h 24m 03s-58d 07m 25s+8.93.51,600240

Sky Highlights - July 2017

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for July

Meteor Shower
Southern Delta Aquariids (Aquarids) meteor shower peaks on July 29

Northern Hemisphere
West:- Mercury (mag. -0.5 to +0.3) (second half of month)
Southwest:- Jupiter (mag. -2.0)
South:- Saturn (mag. +0.2)
West:- Jupiter
South:- Saturn
East:- Neptune (mag. +7.8)
Southwest:- Saturn
South:- Neptune
Southeast:- Uranus (mag. +5.8)
East:- Venus (mag. -4.1)

Southern Hemisphere
West:- Mercury (second half of month)
Northwest:- Jupiter
East:- Saturn
West:- Jupiter
North:- Saturn
East:- Neptune
West:- Saturn
North:- Neptune
Northeast:- Venus, Uranus

Deep Sky

Small telescopes:-
Messier 13 - M13 - Great Hercules Globular Cluster
Messier 92 - M92 - Globular Cluster
Messier 11 - M11 - The Wild Duck Cluster (Open Cluster)
Messier 7 - M7 - The Ptolemy Cluster (Open Cluster)
Messier 6 - M6 - The Butterfly Cluster (Open Cluster)
Messier 4 - M4 - Globular Cluster
Messier 8 - M8 - Lagoon Nebula (Emission Nebula)
Messier 16 - M16 - Eagle Nebula (Emission Nebula with Open Cluster)
Messier 20 - M20 - Trifid Nebula (Emission and Reflection Nebula)

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