Astronomy Report

Southern Cross Constellation

PDF version of this report (prints in 10 or less pages).

 

Authors

Felipe Garrido Goicovic

Matias Marchant Taborga

Juan Soto Rosales

Mario Campos Duque (teacher)

 

School

Colegio Cristóbal Colón

Santiago de Chile

Pasaje Berna 1658, Conchalí

colcrico@ctcreuna.cl

 

Acknowledgements

The authors acknowledge the support provided by the Chilean Astronomy Network (RChA, Red Chilena de Astronomía, http://www.rcha.cl) during the production of this work. The Spanish version of this report has been freed to the public domain under the terms of the License RChA.

 

2003

The Southern Cross

 

One of the constellations more easily seen during the whole year in the South Hemisphere is the Southern Cross (in Spanish “Cruz del Sur”). This is a group of stars readily observable (due to its simple shape) which has been object of veneration in diverse cultures, providing guidance and inspiration for a long time. One of the telescopes at Cerro Paranal [1] has been named after the Southern Cross using the mapudungún name for it, Melipal [2], which means “four tips” (as in the tips of arrows). (N.B.: Mapudungún is the language of the Mapuche, the native people living in central and southern Chile).

 

Our report includes the following aspects:

 

·                     The Southern Cross as lighthouse of literary inspiration.

·                      Knowing the Southern Cross astronomically.

·                     The Southern Cross in comparison to the Northern Cross.

·                     The Southern Cross and the mythology of the original cultures of the Andes.

·                     Classroom activity.

 

I. The Southern Cross as lighthouse of literary inspiration

 

Diverse authors have mentioned the Southern Cross in their works. Dante is one of them. In the Divine Comedy he writes in the first canto of the second part: “I turned right and, directing my attention to the other pole, I distinguished four stars only seen by the first humans. The sky seemed to rejoice in their shine...”. [3]

 

A Chilean author, the Literature Nobel Prize winner Pablo Neruda, wrote in his book “One hundred love sonets” (poem LXXXVI): “Oh Southern Cross, oh threefold of fragrant phosphorum / with four kisses today it penetrated your beauty / and went through the shadow and my hat: / the moon was round because of the chill. / Then with my love, oh diamonds / of blue frost, serenity of the sky, / mirror, you appeared and all was full of night / with your four depots trembling of wine.” [4]

 

These two examples make us think on how this constellation has inspired different poets which, from different latitudes and from different ways of viewing it, have found themselves captivated by this stellar cross; Dante, in the XIII c., observing from far away, and Neruda, in the XX c., from the austral Temuco [5], observing its whole grandeur.

 

II. Knowing the Southern Cross astronomically

 

This constellation is one of the 88 classified by the International Astronomical Union in 1930. Formerly it was part of Centaur after a classification by Ptolemy. Due to the precession of the Earth the Southern Cross became invisible from the North Hemisphere. It was split from Centaur by A. Royer in 1679. [6]

 

The shape of this constellation is that of a cross. It is formed by 4 outstanding stars and a dark nebula. It can be distinguished from other cross found in the same hemisphere (the False Cross) because it has a fifth star near its centre.

 

Its coordinates are A. R. 12.45 h and  Dec. -59.97 degrees, visible between parallels 20 degrees N to 90 degrees S. Its area is 68 square degrees.

Image by astrophotographer Pedro Aguirre V. (http://astrosurf.com/pedro)

 

In Table 1 we show data for the 11 brightest stars of this constellation. We present data for  position, magnitude and spectral class of each star.

 

Table 1 - Source: Catálogo de Brillos de Estrellas [7].

HR	Common Name	Bayer	VarID	RA J2000	DEC J2000	mag	MK Spectral Class	Multiple Count
4853	Becrux (Mimosa)	Beta	Bet Cru	12h 47m 43.2s	-59° 41' 19"	1.25	B0.5III	3
4730	Acrux	Alpha [7]		12h 26m 35.9s	-63° 5' 57"	1.33	B0.5IV	3
4763	Gacrux	Gamma	5672	12h 31m 9.9s	-57° 6' 48"	1.63	M3.5III	3
4731		Alpha [8]		12h 26m 36.5s	-63° 5' 58"	1.73	B1V	3
4656		Delta	Del Cru	12h 15m 8.7s	-58° 44' 56"	2.8	B2IV
4700		Epsilon	5568	12h 21m 21.6s	-60° 24' 4"	3.59	K3-4III
4898		Mu [7]		12h 54m 35.6s	-57° 10' 40"	4.03	B2IV-V
4679		Zeta		12h 18m 26.1s	-64° 0' 11"	4.04	B2.5V
4616		Nu		12h 6m 52.9s	-64° 36' 49"	4.15	F2III
4599		Theta [7]		12h 3m 1.5s	-63° 18' 46"	4.33	Am
4897		Lambda	Lam Cru	12h 54m 39.2s	-59° 8' 48"	4.62	B4Vne

 

Acrux is one of the brightest stars in the sky, being the twelfth in bright. Other interesting Southern Cross data are the distances, temperatures, solar radii and absolute magnitudes, as obtained from the software Starry Night. [7]

 

Table 2 -  Source: Software Starry Night, version 3.1.

 

The following images were obtained from Simbad (astronomical database). [9]

 

 

Comparing the images with the spectral class of the stars (see Table 2), and using as a reference the classification by Morgan and Keenan, we see that:

 

Table 3

HR	Bayer	Absolute Magnitude	Class of star
4853	Beta	-3.92	Giant, white-blue, cepheid
4730	Alpha [7]	-4.22	Subgiant, white-blue
4763	Gamma	-0.6	Red giant
4731	Alpha [8]		Dwarf or from the main sequence
4656	Delta	-2.46	Subgiant
4700	Epsilon	-0.66	Giant
4898	Mu [7]	-1.33	Subgiant
4679	Zeta		Dwarf or from the main sequence
	Nu
	Theta [7]
	Lambda

 

Another interesting information we are able to show about this constellation is the location of the stars in relation to the Milky Way.

 

Table 4 - Source: Catálogo de Brillos de Estrellas [7].

Acrux	Member of the Scorpio Centaur cluster.
Betacrux	Probably a member of the cluster Sco-Cen; member of the association Sco-Cen; member of the Pleiades group.
Delta Cru	Probably a member of the Scorpio Centaur cluster.
Epsilon Cru	Hyades group.

 

Other objects we can distinguish in the Southern Cross are the following:

 

Table 5 - Source: Catálogo de Brillos de Estrellas [7].

Name	HR	Object	Magnitude
NGC	4755	Open cluster	4,2
NGC	4609	Open cluster	6,9
Ru	98	Open cluster	7
Harvard 5		Open cluster	7,1
NGC	4103	Open cluster	7,4
NGC	4349	Open cluster	7,4
NGC	4439	Open cluster	8,4
NGC	4052	Open cluster	8,8
NGC	4337	Open cluster	8,9
Ru	97	Open cluster	9,1
Hogg	14	Open cluster	9,5
Tr	20	Open cluster	10,1
Hogg	15	Open cluster	10,3
PK	298-0,1	Planetary nebula	11
PK	300-0,1	Planetary nebula	11,7
PK	298-1,2	Planetary nebula	12,4
PK	299+2,1	Planetary nebula	12,7
PK	300+0,1	Planetary nebula	12,9
PK	299-0,1	Planetary nebula	13,6
PK	300-1,1	Planetary nebula	13,8

 

The most interesting cluster in the Southern Cross is the Jewel Box (NGC 4755):

 

Image: http://simbad.u-strasbg.fr/sim-fid.pl

 

Some magnitudes in the Jewel Box:

 

Table 6 - Source: http://www.seds.org/messier/xtra/ngc/n4755.html

Right ascension	12h 53.6’
Declination	-60 degrees 20’
Distance	7600 light years
Visual bright	4,2 magnitude
Apparent size	10 arcmin

 

Nicolas Lacaille discovers and then mentions the Jewel Box in Coelum Australe Stelliferum in 1763. This is a young star cluster, approximately 7 million years old. [10]

 

NGC 4755 is made up of diverse stars from which we highlight the following:

 

Table 7 - Source: http://simbad.u-strasbg.fr/Simbad

 

The most remarkable dark nebula of the South, located in the Southern Cross, is the Coal Sack. This nebula is on the galactic plane, about 150 pc from the Sun [6]. It is known since prehistory in this hemisphere. It was observed in 1499 by Vicente Yáñez Pinzón [10].

 

Some data about this nebula:

 

Table8 - Source: http://www.seds.org/messier/xtra/ngc/n4755.html

Right ascension	12h 52’
Declination	-63 degrees18’
Distance	2000 light years
Apparent size	420 by 240 arcmin

 

This nebula is composed of stellar dust that blocks the path of light. Here we see the Coal Sack (in Spanish: Saco de Carbón) in sheer splendour:

Image by astrophotographer Pedro Aguirre V. (http://astrosurf.com/pedro)

 

III. The Southern Cross in comparison to the Northern Cross.

 

Image: http://faculty.rmwc.edu/tmichalik/mlkywayc.htm

 

The image shows the two crosses: Cygnus to the left and Crux to the right. Both extend across the galactic equator. The following data illustrates similarities and differences between the crosses of both hemispheres.

 

	Southern Cross	Cygnus
Icon
Image
Apparent magnitude	Becrux 1.25	Deneb 1.25
Nebula	Coal Sack (dark nebula)	NGC 7000 (bright nebula)
Cluster	Jewel Box (NGC 4755) Open cluster, A-shaped.	M29 Open cluster; shaped as a spoon.
Name	Southern Cross	Northern Cross
Stars of apparent magnitude < 3	4	4
Visible in the range of latitudes...	20 to –90 degrees	90 to –40 degrees
Area	68 square degrees	804 square degrees

 

IV. The Southern Cross and the mythology of the original cultures of the Andes.

 

In a legend about the origin of the Mapuche the Southern Cross is mentioned. In the “Book of Mapuche tales”, compiled by Alicia Morel, we read:

 

“ The children built a ruca (a Mapuche hut) and the Sun entered by the door facing East and left by the one facing West, following the ancient custom of the people of the land, respectful of the cardinal points and regarding the number four as sacred.

 

When at last the Maker of Rain was tired of riding over the clouds and returned to its hiding place behind the hills, water level began to descend and the rivers to follow their normal course. Then the coihue (a native tree) buried itself in the mud as a ship running aground, and when the wind dried the ground the Puma (a native feline) , the Chilla (a native mouse) and the children jumped from the log and looked for a hidden valley to live.

 

The first thing they did, even before building another ruca or looking for a cave to inhabit, was to name their adoptive children. Magical names that would protect them forever. The son was named Manque, the condor that soars in the sky looking after the Earth. The daughter was named Melipal, like the Southern Cross.”