Crux
Crucis
Cru
The Southern Cross

Introduction

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 constellations main asterism of four relatively bright stars. To some the cross shape is more resembling of a kite, but whatever you think it looks like, it is beautiful 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 observers it is circumpolar and therefore never sets. For residents just north of the equator it can also be seen crawling just above the southern horizon around late spring for a few weeks each year. Unfortunately, for the majority of Northern Hemisphere observers this celestial marvel never rises above the southern horizon and can't be seen.

Crux Star Chart

Crux Star Chart - pdf format

Interesting stars

Bright Star, Double Star, Main Cross Star

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

Located 321 light years from Earth, Acrux is a multiple star system of at least 3, 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. From Earth, they are separated by only 4 arc seconds and show no discernable movement over time, suggesting that the orbital period is at least 1500 years.

The brighter star of the two main components, a1, is itself a spectroscopic binary star. It is believed to consist of stars 14 and 10 times the mass of the Sun, orbiting in just 76 days at a separation of only 1 AU.

The 4th star of the system is located about 90 arc seconds from Acrux at mag 4.86. It is also a class B sub-giant and shares the same motion through space as a1 and a2 hence is believed by many that it is gravitationally bound and a true member of the group. However, this is by no means certain and there is much debate regarding the true distance of this star, with some believing it to be several hundred light years further in the background and hence only a chance alignment.

Through a small 80mm (3.1-inch) telescope the two main stars are easily split forming a nice double star with strikingly similar components. Take time also to scan the extra 90 arc seconds to locate the third but mysterious member of the group.

Bright Star, Double Star, Main Cross Star

Mimosa or Becrux (beta Cru) - at mag. 1.30, is the second brightest star in Crux and the 20th brightest star in the night sky. Since beta Cru is so far south as to be unfamiliar to the ancient Greeks and Romans, it does not have a classical name. It is sometimes known as "Becrux", the second star of the cross or as "Mimosa". The latter is a recent name based on its colour and was given by 16th century German astronomer Johann Bayer.

Mimosa is a giant class B blue-white star, 34,000 times more luminous than the Sun, located approximately 350 light years away from Earth. It is a spectroscopic binary with components too close to resolve with a telescope. The pair are believed to orbit every 5 years and 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 for small telescope observers with a 7th mag. star located 373 arc seconds distance, at a PA of 23 degrees.

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

Bright Star, Double Star, Main Cross Star
Gacrux (gamma Cru) - is the northernmost star of the main cross. It is a red giant star at mag. 1.59 and is by far the closest of the four main stars at a mere 88 light years distance. What is probably most striking about Gacrux is the colour contrast with the other three stars of the cross. All three are either blue / blue-white or white, whereas the deep red-orange hue of Gacrux stands out as a notable exception in the crowd.

There is a wide faint unrelated white companion star, mag. 6.4, positioned about 2 arc minutes at a PA angle of 128 degrees and is easily visible in binoculars. As well as being a lot fainter, it is also much more distance than Gacrux - at 400 light years it is nearly 5x further from Earth.

Main Cross Star
δ(Delta Cru) - at mag. 2.78 is the faintest of the four main stars in the Cross. It is a blue-white subgiant star (spectral type: B2IV), located about 360 light-years from Earth. Both the distance and the spectral type of Delta Cru are similar to those of Acrux and Mimosa. Therefore it is reasonable to assume that all three stars are related by birth, although by now they are not close enough to be gravitationally bound. However, despite being by far visually the faintest of the three, Delta is still a magnificent star. It has a very hot 22,550K surface temperature with a luminosity of about 5,600 times that of the Sun. The spectrum of Delta Crucis implies that it has recently stopped hydrogen fusion in its core and is in the process of developing into a red giant. The star is also believed to be just under the limit at which stars go supernova, and hence will end its days as a white dwarf (not unlike Sirius B) rather than go bang in a spectacular explosion. Its age is estimated to be less than 30 million years.

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

Double Star
ι(iota Cru) - 125 light years away, at mag. 4.69 is an orange giant star (class K1). Small / medium telescopes reveal an unrelated nearby mag. 10.2 G8 class star. The separation is 28 arc seconds, PA of 8 degrees.

Double Star
μ(mu Cru) - is the 6th brightest star in Crux. It is 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 a small 70mm (2.7 inch) telescope or even with good 10x50 binoculars. All in all, this is a very nice double star especially suitable for small telescope or binocular observers.

The stars lie at a distance of about 380 light years from Earth.

Deep Sky

Despite its small size Crux contains two big treats for deep sky observers, the most prominent and probably famous dark nebula in the sky and a fantastically brilliant open cluster. Also worth searching for is another half a dozen or so fainter open clusters.

Dark Nebula
Coalsack - Measuring a whopping 7° by 5°, the Coalsack Dark Nebula is the skies most prominent dark nebula and is easily visible to the naked eye under dark skies. It appears majestically as a dark patch silhouetted against the bright southern section of the Milky Way.

Undoubtedly known since prehistoric times, it 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" - the Dark Canopus. It was also called "Macula Magellani" - Magellan's Spot or "Black Magellanic Cloud". The Dark name is not completely true as recently it was proven to be not completely black; it has a very dim glow equal to about 10% of the brightness of the surrounding Milky Way. This faint glimmer comes from reflection of light from obscured stars.

Perhaps surprisingly, the Coalsack is not present in the New General Catalogue, although it is recorded as entry 99 in the Caldwell catalogue; compiled by eminent English amateur astronomer Sir Patrick Moore.

Due to its extremely large size, the Coal Sack is best viewed with the naked eye under dark moonless skies, binoculars or a small wide field telescope.

Coalsack Dark Nebula - Caldwell 99 (ESO Yuri Beletsky)

Open Clusters
Jewel Box or Kappa Crucis Cluster (NGC 4755) - is one of the finest open clusters in the sky, located 6,440 light years away and visible to the naked eye as a 4.2 mag. star. It is a fantastic site in all types of optical instruments. A good pair of 10x50 binoculars reveals a pyramid consisting of the five brightest stars with fainter stars sprinkled in and around it. A small telescope reveals dozen more stars of various subtle colours, mostly red or blue, that appear in two distinct groups, one noticeably brighter than the other. An even larger telescope may show some of the nebulosity associated with the cluster. In total there may be a few hundred stars in this celestial wonder that has an apparent diameter of 10 arc minutes.

French astronomer Nicolas Louis de Lacaille discovered it 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.

The Jewel Box is one of the youngest known clusters, with an estimated age of between 7 and 10 million years.

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

NGC 4609 - after the Jewel Box, at mag. 6.9 is the brightest of a number of fainter open clusters in Crux. What's interesting about this cluster is that it actually lies within the boundaries of the Coalsack. Locating NGC 4609 is easy, first focus on Acrux and then move about 2 degrees directly west until an obvious bow shaped collection of 9th mag. stars appear. The cluster can be seen in a small 80mm (3.1 inch) telescope but is best seen with an aperture of at least 150mm (6-inch). About 15-20 stars are visible, most about 9th or 10th mag. with a maximum diameter of the cluster about 6 arc minutes. In all NGC 4609 may contain up to 40 stars.

There is an unrelated but bright 5.25 mag. star just to the SE of the cluster.

Harvard 5 - sometimes known as Collinder 258 and despite being the 3rd brightest open cluster in Crux at mag. 7.1, is elusive and unimpressive compare with the others. Containing up to 25 members with a diameter of only 5 arc minutes, a minimum 150mm (6 inch) telescope is recommended to view this cluster.

Harvard 5 is relatively easy to find, positioned slightly to the west and about 1/3 of the way along a line connecting Acrux with γ Crux.

NGC 4103 - is a nice small compact (apparent size 6 arc minutes) open cluster of about 50 members. It is located on the western side of Crux and sits in a triangle of 6th mag. stars of which two are noticeably red. NGC 4103 itself is mag. 7.4 and just visible with good 10x50 binoculars, appearing as a faint smudge or glow set against the background Milky Way star field. Observe with larger 11x80 binoculars and the cluster is a much easier target, somewhat irregular in shape with hints of resolution.

A 150mm (6-inch) telescope easily reveals the brightest stars with about 15-20 brighter than 10th mag. They are arranged in multiple chains and the surrounding orange stars provide 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 and only 4 arc minutes in diameter, it is visible in good binoculars on nights of excellent seeing. With a small 100mm (4 inch) telescope, an 8th mag. star is noticeable towards the southeast corner of the cluster. Extend north from this star to the glow of NGC 4349, which subsequently fades off quickly, not unlike the view of a faint comet.

Larger amateur telescope of the order of 300mm (12 inch) aperture resolve many of the cluster members, displaying up to 30 stars lined in chains that cut through space.

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

----------------------------------------------------------------------------------------------------------------------

Crux Star Data Table

Henry Draper Catalogue

(HD)

Hipparcos Catalogue

(HIP)

Bayer
Flamsteed
Name

RA

(J2000)

Dec

(J2000)

Visual Mag. Var. Var. Mag. Range

Period

(days)

Double Sep. PA

Mag.

Primary,

Sec

106490 59747 Delta Crucis --- --- 12h 15m 09s -58d 44m 56s 2.79 --- --- --- --- --- --- ---
108248 / 108249 60718 Alpha Crucis --- Acrux 12h 26m 36s -63d 05m 57s 0.77 --- --- --- Y 4 114 1.25,1.55
108903 61084 Gamma Crucis --- Gacrux 12h 31m 10s -57d 06m 48s 1.59 --- --- --- Y 125 27 1.83,6.45
110829 62268 Iota Crucis --- --- 12h 45m 38s -60d 58m 53s 4.69 --- --- --- Y 28 8 4.7,10.2
111123 62434 Beta Crucis --- Mimosa or Becrux 12h 47m 43s -59d 41m 20s 1.25 --- --- --- Y 373 23 1.28,7.2
112092 / 112091 63003 / 63005 Mu Crucis --- --- 12h 54m 36s -57d 10m 41s 3.97 --- --- --- Y 35 17 4.0,5.1

Crux Deep Sky Data Table

NGC

Caldwell

Harvard

Name(s)

Type

RA

(J2000)

Dec

(J2000)

Visual Magnitude

Apparent Size

Distance

(light years)

Actual Size

(light years)

Number of Stars

--- 99 --- Coalsack Dark Nebula 12h 50m -62d 30m --- 7 x 5 degrees 575 70 x 50 ---
4755 94 --- Jewel Box / Kappa Crucis Cluster Open Cluster 12h 53m 42s -60d 22m 00s 4.2 10' 6440 20 100
4609 98 --- --- Open Cluster 12h 42m 20s -62d 59m 38s 6.9 6' 5000 9 40
--- --- 5 --- Open Cluster 12h 27m 10s -60d 46m 00s 7.1 5' 4000 6 25
4103 --- --- --- Open Cluster 12h 06m 40s -61d 15m 21s 7.4 6' 5300 9 45
4349 --- --- --- Open Cluster 12h 24m 06s -61d 52m 13s 7.4 4' 7000 8 30
4439 --- --- --- Open Cluster 12h 28m 26s -60d 06m 11s 8.4 4' 5800 7 13
4052 --- --- --- Open Cluster 12h 01m 30s -63d 13m 20s 8.8 10' 4000 12 80
4337 --- --- --- Open Cluster 12h 24m 03s -58d 07m 25s 8.9 3.5' 1600 2 40

Sky Highlights - March 2017

Comet
Comet 41P/Tuttle-Giacobini-Kresak now visible with binoculars as it heads towards perihelion

Mercury
Mercury heading towards greatest elongation east

Minor Planet
Vesta now visible with binoculars and small telescopes.

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for March 2017

Northern Hemisphere
Evening
West:- Venus (mag. -4.8 to -4.1 - first half of month), Mars (mag. +1.3 to +1.5), Uranus (mag. +5.9), Mercury (mag. -1.5 to -0.4 - second half of month)
Midnight
Southeast:- Jupiter (mag. -2.3 to -2.5)
Morning
Southwest:- Jupiter
Southeast:- Saturn (mag. +0.5)

Southern Hemisphere
Evening
West:- Venus (first half of month), Mars, Uranus
Midnight
North:- Jupiter
East:- Saturn
Morning
West:- Jupiter
Northeast:- Saturn
East:- Neptune (mag. +8.0 - second half of month)

Deep Sky
Naked eye / binoculars:-
Melotte 111 - Mel 111 - The Coma Star Cluster (Open Cluster)
Messier 44 - M44 - The Praesepe (Open Cluster)

Telescopes:-
Messier 67 - M67 - Open Cluster
Messier 51 - M51 - The Whirlpool Galaxy (Spiral Galaxy)
Messier 97 - M97 - The Owl Nebula (Planetary Nebula)
Messier 101 - M101 - The Pinwheel Galaxy (Spiral Galaxy)
Messier 65 – M65 – Spiral Galaxy
Messier 66 - M66 - Intermediate Spiral Galaxy
Messier 95 - M95 - Barred Spiral Galaxy
Messier 96 - M96 - Intermediate Spiral Galaxy
NGC 4244 - Spiral Galaxy
NGC 4565 - Needle Galaxy - Spiral Galaxy

Shop at Amazon US

Shop at Amazon US

Current Moon Phase

CURRENT MOON

Contributions

If you like the website and want to contribute to the running costs then please do so below. All contributions are most welcome.