Southern Doubles, Stars and Variables
10 Mar 2009
RA : 12h
Dec : -30° to -90°
Constellations : Hya, Ant, Cen, Vel, Car, Cru, Mus, Cha, Oct.
Best Observed : Feb - Jul (Text Ordered by RA)
RA : 12h
Dec : -30° to -90°
DS 00h 01h 02h 03h 04h 05h 06h 07h 08h 09h 10h 11h
NEW 12h 13h 14h 15h 16h 17h 18h 19h 20h 21h 22h 23h

Positions given as;
I.e. (19583-6018), are;
19h 58.3m
-60° 15'
This follows the current
WDS Conventions.

" or "arcsec
In arc seconds or
' or 'arcmin
In arc minutes or
mas - milli arc seconds

( ° ) Angle in degrees.
Measured from
North through East

v - visual (naked-eye)
p - photographic
V - Photometric Visual
B - Photometric Blue
MV - Absolute @ 10pc.

pc. - parsecs
ly. - light-years
AU - Astronomical Unit


T: Periastron (yr.)
P: Period (yr.)
a: Semi-Major Axis (arc sec.)
e: Eccentricity
i: Inclination
Ω: Orbital Node (°)
ω: Angle True Orbit (°)


R.A. 12 Hours

DON 1095 (12047-6703) is a very faint pair first discovered in 1929 by H.F. Donner. Visually, the primary is around 10th magnitude, and lies 14.7'N of the PNe, NGC 4071. By magnitude, this reddish pair is 10.7 and 13.8, with the separation of 4.5 arcsec along PA 339°. Little is known about this pair, and no one else has bothered to contribute much about it at all. DON 1095 A is listed in the Tycho catalogue as T 8986:2081:1 / GSC 35994:1, with the B-V of 0.988 confirming its reddish colour. Surprisingly, the parallax on the star is large 141.7±47.4 milliarc seconds (mas), corresponding to a close distance of 7.06±1.56pc. (23.0±5.1 ly.) A 20cm easily resolves the pair, though 25cm or larger would certainly improve this.

Southern Asterism (12052-6358) Cruxs Diamond” or The Crosss Diamond” lies in the SW corner of the constellation of Crux. This involves the stars in south-western Crux and the pairs (ζ1,2) Zeta, (θ) Theta and (η) Eta Crucis, and the star j Centauri. (This is not J Cen, which is a completely different star further to the west.) The diamond shape lies on its side whose centre is some 2.4°SW of Acrux. These four stars produce an asterism that is trapezium or diamond shaped that can be seen with the naked-eye - even in the city-lights. Each axis is 3.2° and 1.4° It is useful in trying to find several pair in the regions. We already have the Jewel Box in this constellation, so should not we name this Cruxs Diamond” or something like the Crosss Diamond”.?

W Crucis (12120-5847) is an EB-type eclipsing binary that lies 3°W of the bright star δ Cru / Delta Crucis and positioned in the middle of three 8th magnitude stars which almost lie in a perfectly straight line. The two either side making good comparison stars. Although this star is relatively faint, the variations in brightness are larger than most eclipsing binaries. The magnitude range is from 8.5v to 9.3v and the UBV light observations show the magnitude range as 9.04B and 10.38B. What is most unusual about W Cru is the long 198.53 days (6.2 months) orbital period. The light-curve parameters determine both stars are separated by 270 million kilometres and that the primary is a supergiant and the secondary a giant. Classed as a giant eclipsing binary, the primary displays the mean diameter of 93 times that of the Sun, while the companion is 116 times. In luminosity, the stars are 10 383 L⊚ and 2 884 L⊚ while the temperature of the two stars is 4 200K and 3 940K. Total mass ΣML⊚ of W UMa is 19.85, with the individual masses calculated to be 11.67 ML⊚ and 8.18 ML⊚, respectively. Its primary, because of its diameter, is likely in a later state of evolution and certainly has exceeded its Roche Lobe, and consequently the gravitational proximity makes this star appears tear-drop in shape. W Crus secondary component is believed to be also highly distorted, filling some 85% of the Roche Lobe size criteria. Due to the separation and long orbital period, the effects of the distortion could be considered minimal especially when compared to the effects produced by the total light curve. The combined spectrum reveals the spectral and luminosity class as G2 Iab - indicating supergiant stars. Bright emission lines have characterised this in the observed spectra.

RMK 14 / HIP 59654 (12140-4543) in Centaurus is a surprisingly bright pair in a field of challenging and other faint objects. Lying 2.9°NE (PA 52°) of the faint planetary nebula of Longmore 6. This orange and white pair has 5.60 and 6.76 magnitudes components. Comparing this to the Delta-m Catalog in the WDS (2001), the four stated values; being 0.95, 1.17, 1.19 and 1.21, respectively, give a mean μ of 1.13, and close to the 1.16 in the WDS 2001 Main Pair Catalog.” Both stars are separated by 2.7 arcsec along PA 243° (2002). The changes since Rümker discovered this pair find the separation reduced from 4.0 to 2.7 arcsec and the PA reducing by 5°. AOST2 describes Rmk 14 as;

Separation and angle of this orange and white pair are slowly lessening; it is an easy object dominating a well-sprinkled star field and common proper motion indicates that the stars are connected.

The spectral class of the primary is K3III (B-V=1.400) while the Tycho gives only the photometric data for both stars as Rmk 14 A ; B-V being 1.501±0.007 and Rmk 14 B; 1.216±0.008. Rmk 14 A is also spectroscopic binary. If the pair is truly connected then Rmk 14s period is certainly long. Evidence from the proper motions suggest attachment but the period might be long of the 5.76±0.83mas parallax from the Hipparcos satellite is to be believed. The distance here being 173pc or 566ly. This lovely pair is great in telescopes larger than 10.5cm.

ε Mus/ Epsilon Muscae / HIP59929 / SAO251830 / HD106849 (12176-6758) lies 2.2°NW from α Muscae, along Muscas western wing. ε Mus mean magnitude is 4.06 and the spectral class of M5 III. Considered as a SRb red variable, the period varies over about forty days, fluctuating between about 3.99 and 4.31 mag. Like all SRb variables, each display unpredictable variations caused by multiple simultaneous periods. Paradoxically, SRbs can also remain at constant brightness for long periods, before again resuming their fluctuations. Little information exists on this variable, with the period and amplitude being imprecise. Parallax πis 0.01080±0.0048 and the high proper motion of pmRA = -231.26 pmDE = -26.37

[Note: Motions are similar to Epsilon (ε) Crucis/ HIP60260/ SAO251862/ HD107442 (12213-6024) some 7.5° due north of ε Mus in both magnitude, π = 0.01430±0.0056; pmRA = -171.06 pmDE=91.82 K3/K4III B-V=1.389.]

ζ1,2 Mus / Zeta (1,2) Muscae / HDO 215 / HIP 60320 / SAO 251866 / HD107566 (12221-6731) is a 5.15 magnitude star 35'NE (PA 46°) of ε Mus or 1.7°E and 14'S of the planetary nebula, NGC 4071. Again, this is a bright star with a faint companion, separated by 32.4 arcsec at PA 130°. In an attractive background of stars, this white/ yellowish pair can easily be seen in 7.5cm. Magnitudes are 5.2 and 10.7.

α Cru / Δ122 & Δ123 / α 1,2 Cru / Alpha Crucis / Acrux / α 2 Cru - TOK 7 Ca, Cb, Cd (12266-6306) is likely in the Top 3 of all southern doubles and multiples and certainly in the Top 5 of the entire sky and is often the first or second targets for newcomers to amateur astronomy. α Cru is one of the brightest star being the 14th bright star in the night time sky.

John Herschel also certainly began his observational program in the right way. It was he first object he observed with his telescopes in South Afica, which was made on the 22nd February 1834. from the Cape of Good Hope.

Acrux is also one of the most prominent of the southern binaries, whose magnificent 1st magnitude sapphire blue pair of 1.1 and 1.5 mag. is presently separated now about 4 arcsec. There is some evidence that the two are slowing making a very slow reduction in separation. This is a must see” double for southern observers shares its prominence with Spica / α Virginis - both spectrally, chemically and approximately by mass.

In my own opinion Acrux is the second best double in the sky and only just narrowly behind Alpha Centauri - though any northern observer might have either Mizar or Alberio in these two places, but Acrux is indeed much brighter. In regards of the constellation of Crux, it is likely the best or second best object, whose place is strongly challenged by NGC 4755 - The Jewel Box - but I should leave that debate to you! The C component is some 90 arcsec away, and is about 4.8 magnitude, and is easily seen in 7×50 binoculars with care. I saw Acrux a moment ago while writing this text. I looking out my backdoor (late-December 2002), however, because of the thick smoke from the Sydney bushfires, both appear orange - more like p Eridani in fact!) An additional 12.5 mag α Cru D??? field star lies about midway and slightly east of the wide pair, making an obvious right- angle triangle. Acruxs field is also sprinkled with many stars.

The Discoverer of Acrux

According to R.H. Allen Star Lore: Their Name and Meaning”, and stated by Robert Burnham in his Celestial Handbook, that Acruxs duplicity was discovered by;

...some Jesuit missionaries sent by King Louis XIV to Siam in 1685.

There till remains significant debate on whether they saw the main AB pair or only the more distant C star some 90 arcsec away - neither of which is often mentioned in many common astronomical sources. The Binary Stars” by Robert Aitken says that it was Father Fontenay discovered the pair from the Cape of Good Hope in 1685, though I have never found out the source of this statement. This incidentally place Acrux as the FIRST far southern double star to be discovered and even before Alpha Centauri in 1689!

However as to the true discovery of the duplicity of Acrux, it is likely that Robert Aitken maybe correct about this. Since Burnham made the statement in the first place, the discoverer has remained as the statement above. (I have seen no statements since the Burnhams Celestial Handbook was published in any text that gives Father Fontenay as the discoverer - the it is uncertain if he saw the AB pair or the AC pair). The discovery of the main α Cru AB was truly identified by James Dunlop in 1826, and the WDS has recognised this as such - a point I will get too in a moment.

Acrux as a Multiple System

Acrux is actually a very complex multiple with at least five (5) components, whose structure and connection being still quite open for some debate. The A component α1 Cru A is actually the spectroscopic binary α Cru Aa whose period is some seventy-nine (79) days long. The first estimations of this period were made by Neubauer, F.J.; Lick. Obs. Bull., 15, 190 (1932) gave both A and B as spectroscopic binaries both with the period was about a day. In 1934, Luyten, W.J ; PASP., 47, 196 (1935) found that the periods could be matched to the data as having 59.31 and 56 days, respectively! Luyten had also produced the orbital elements for both stars. Thackeray, A.D. and Hill, G. ; The System of Alpha Crucis.”; MNRAS., 268, p.55-59 (1974) said that the period was more like 75.769 days. They also found that they could not explain the spectroscopic α Cru B data, and from their additional observations, they concluded;

There is no evidence from the material [Neubauer or Thackeray/ Hill] that star α Cru B is a binary, and in particular no evidence for variations in a period of near twenty-four (24) hours.

Much disinformation appears in the literature, many sources having a combination of data extracted from various papers or common amateur astronomical texts.

WDS Information: The WDS 2001 on the three main stars gives the following information;

WDS Name    1st  LAST   1st  LAST   1st LAST       Mags.         Spec.  pmRA  pmDec.
DUN 252 AB  1826 1994  05.4" 04.0"  114  114  A=1.25 B=1.15 mag. B1V   -0.36 ± -012  
DUN 252 AC  1826 1991  87.2" 90.0"  200  202  A=1.25 C=4.80 mag. B4IV  -042 ±  -012  
DUN 252 BC  1859 1947  89.8" 90.6"  205  205  B=2.3  C=6.0 mag.? B1V   -0.36 ± -012  

Δ252 AB and AC have similar proper motions and are certainly related. However in 1997 Token found a further three nearby stars as possible components were found by speckle interferometry. These are now catalogued as TOK 7 Ca, TOK 7 Cb and TOK 7 Cc, whose designations were added by the United States Naval Observatory (U.S.N.O.) on the 1st November 2001. It is uncertain if these star could eer be seen visually as the main star are overwhelmed these components brightness. There 1997 positions were made as follows;

WDS Name  WDS Desig.   Sep.  P.A.
TOK 7 Ca  12266-6306   2.2   052 
TOK 7 Cb  12266-6306   4.9   214 
TOK 7 Cc  12266-6306   6.7   119 

The WDS 2001 Notes” gives;

DUN 252 /α1 is a spectroscopic binary and shares common proper motion with HR 4729 and 4731.” It is currently thought there are at least five (5) components in the system. Both the coordinates and proper motions are similar being obtained from ACRS, PPM, IRS, FK5 catalogs.

A Classification Major Problem

Acrux is catalogued in the WDS (and elsewhere) as Dunlop 252 (Δ252), and this appears in Megastar and many other sources. However, this is perplexingly wrong. James Dunlop in his double star catalog (Dunlop, J.; XVII. Approximate Places of Double Stars in the Southern Hemisphere, observed at Parramatta in New South Wales.”; Memoirs. Astronomical Society of London, 3, 257; (1829)) is;

Δ122 and Δ123 (12266-6306) are given for Alpha (α) Crucis - with DUN 122 for the AB pair and DUN 123 for the AB-C pair. At first I was fooled by this in Megastar Versions 3.0, and 4.0.31 and 5.0.5, as if you type either Δ252, Δ122 or Δ123 you still get Alpha Crucis. But if you look at the data box you still get Δ252. It turns out the Megastar cannot identify the pair in question, it finds the last star listed in that catalog you are looking up. Hence Δ122 and Δ123 dont actually exist! DUN 252 is also used in Sky Catalog 2000.0. Amazingly, Astronomical Objects for Southern Telescopes (AOST2) fail to mention the catalog number as Δ252 for Alpha Crucis. (Note : This is object No.522 in AOST2 - coincidently this is 252 if you switch the first two numbers!)

Furthermore, Dunlops pairs appear in Right Ascension order. Hence, Δ124 is γ Cru (Gacrux) and Δ125 is Beta Crucis (Mimosa and NEVER Becrux!) Here is an example. According to James Dunlop from the same source given above, DUN 252 is actually the Bode star 29 Tucanae, whose 1827 position is 23h 34m 00s -65° 22' (23h 43m 37s -64° 24'). Dunlop says this is an orange sf 6 and 8 magnitude stars a position angle of 27.4° and the separation of 3.4' - a wide double indeed! 29 Tuc is located 2°NW of 4.5 magnitude Epsilon (epsilon;) Tucanae. This particular pair also doesnt appear in the WDS for obvious reasons. I also checked in Dunlops 629 deep-sky object to look for DUN 255, and find it is an elliptical nebula” no where near Acrux.

The source of this error is likely the Index of Double Stars (IDS) of 1963, and is the first reference I could find with this problem. Worst the usage has now become quite universal and the error appears in many many sources and has stayed so for many decades.

Again this highlights many problems with southern stars that have been perpetuated over the years. This interesting problem, in my opinion should be corrected in the WDS in the near future.

γ Cru / Gamma Crucis / Δ124 / Gacrux (12312-5707) at 1.67 magnitude is the 23rd brightest star in the sky, An orange-red colour, Gamma Crucis in the northern part of the Cross and is left with the unfortunate proper name of Gacrux. This both ugly and inelegant name still makes me wince, and was first given it by Elijah H. Burritt in the 1840s. When viewed with binoculars or a small telescope, the bluish-white 6.39 mag companion (Hartung in AOST1 says white) makes the pair Δ124 AB, and this contrasts nicely with orange-red primary. For some reason, the WSD 2001 gives it a slightly lower 6.45 magnitude, though much of the earlier data present the view that the magnitude was 6.71v. The component is presently a very easy 125.4 arcsec away (1991), and it seems that this distance over time continues to increase by an amazing about 39.2 arcsec per century. From this we can estimate that in 2002, the separation is about 129.7 arcsec, and 130.1 in 2003. Also the PA since Dunlop discovered the pair in PA has also decreased by 17° or about 0.8° per decade. Todays PA is about 26° (2002).

The brightest star in this multiple is Δ124 A is catalogued as HIP 61084 / PPM 341058 / SAO 240019 / HD 108903. This star displays a significant proper motion of -264.33±0.47mas.yr-1 in Dec and 27.94±0.54mas.yr-1, so it is heading almost due south from its position and roughly towards Acrux. It parallax is given as 37.09±0.67mas, which gives the distance to Gacrux as 26.96±0.49pc. or 87.93±1.60ly. Spectrally the primary is M3.5 III, and the B-V is 1.516, meaning it is a cool giant star. γ Cru A itself is a variable star that has been designated 34G Cru or as NSV 5672, which varies irregularly between 1.60V and 1.67V magnitude. The prism image is noticeable revealing several prominent dark lines that have also been discussed by Hartung in ASOT1 (1968). This the first star I ever looked at with a spectroscope, and I could see the main lines Hartung was talking about immediately - two in the red and orange, two in the yellow and green, a strong green band, and (several lines in the violet. This is a great example of the spectra of stars, and worthy to someone who has not seen this before.

Δ124 B is of spectral class A3V, displaying a B-V of 0.167. Listed as T8654:3421:1 and not in the Hipparcos catalogue, the measures however, are still as telling. All three parameters - pm RA, pmDec and Parallax are well outside the primarys data. I.e. pmRA; 10.20±2.80mas, pmDec; -15.90±2.10, while the parallax is 4.20±2.80 - suggesting distance of 238pc. or 776ly. There is also significant agreement in the PPM star catalogue, so both these stars are certainly unrelated.

Another star is classed in the pair, and this is Δ124 C. Dunlop discovered this deep yellow star in 1825, but it was not measured until H.C. Russell did so twice in 1879 - and it hasnt been measured since! At 9.5 magnitude it lies (according to the WDS01 at 155.1 arcsec from the primary along PA 82°. Looking at the Tycho position of some ten years ago (1992), the only catalogue available with this star, the position is now more like ~234° along PA 78°. The Tycho data also gives a significantly different proper motion and parallax; being pmRA; 103.30±13.5, pmDec 23.70±14.5 and the parallax of 19.40±14.00. Although these errors are quite large, they are in different directions, suggesting this star is unrelated to either Δ124 A or Δ124 B. Although the star hasnt had a spectrum obtained as yet, the B-V of 0.66 tells us it must be either a late G-type star or early K-type star.

So although all three stars are not physically connected, the AB pair has a wonderful colour contrast, that is especially attractive with smaller instruments, with the third star adding to the star-spangled field. This is a wide and easy double, easily seen in apertures as low as 5cm, and still colourful in larger apertures.

β Mus / Beta Musca / Bemusca”/ R 207 / Syd 1-207 / HIP 62322 / SAO252019 (12463-6806) rates somewhere in the Top 10 of southern pairs, and is a must see” for all southern amateurs. Culminating on the 12th May at 9pm and 27th March at midnight, Beta Musca is the most northerly star of the main trapezium-shaped body of the Southern Fly, and appears 1.3°N along PA 40°, from α Mus / Alpha Musca (12372-6908). A renown binary system, R 207 was first seen by H.C. Russell at Sydney Observatory mid-evening on the 15th April 1878. In 1882, Russell claimed in New Double Stars found while measuring Herschels Cape Stars in 1882”, that β Musca was, ...this is one of the closest doubles I know.” Measures made during the 1880s on this star were prolific - twice by Russell himself, and eight times by J.A.Pollock. H.C. Russell was well aware of the significance of this pair, and had the fortune to find R 207 having its fastest orbital motion in the orbit during its closest approach (periastron) sometime in the early 1870s - later determined from the observed orbital motion as 1872.290 - 10th April 1872. As Herschel had missed this obvious but difficult pair, it was natural to assume that the star had begun to significantly widened since Herschels observations.

Magnitudes are 3.6 and 3.9, with the combined magnitude being 3.04, but the Sixth Orbit Catalog” gives 3.04 and 3.52. PA is presently about 45° (2002), with the separation is c.1.3 arcsec. Presently both stars can be easily seen in 20cm, but remains difficult in anything less than 15cm.

AOST2 says about Beta Muscae;

...the stars reached a separation of 1.4″ in about 1950 and now seem to be gradually closing again; 12.5cm will separate the stars on a night of good seeing. This is a long-period binary, and a fine object in a starry field.

The Washington Double Star Catalogue 1996 (WDS96) states in the notes that the period is 383.12 yr., a=1.74 arc sec, while the motion is direct. Component A has also been observed to have a variable radial velocity, so it might be also binary itself. The "6th Catalog of Binary Stars" (2002)) gives the orbital elements as;

Elements : P = 383.120 yr T = 1872.29 a = 1.735 e = 0.526 i = 61.30° ω = 98.32° Ω = 161.81°
Beta Muscae : R207

The most reliable data to date was first published by F. Mourao (1963) and appeared in (Bul.Astr.Obs. R.Bel., 5,143 (1964)). All the orbital elements remain uncertain, as only a quarter of orbit has been observed, and this is all just after periastron - the most critical position for determining orbital elements. I have used the latest data to draw the apparent orbit. Closest approach was sometime in the beginning of 1872.150 then widened until its first maxima were reached in the mid 1960s. Presently the pair is slowly decreasing in separation, and this will continue until 2 029AD, where the separation reaches 1.25 arcsec - the second minima in the orbit. Again the orbit increases in separation, reaching the greatest apparent separation of 1.715 arc ec in 2 165AD. About the same time, observations of β Mus will increase in precision, and the true period, etc. of the system will be properly known. Over the next hundred years or so, roughly after about 2 165AD, the separation and position angle will change more rapidly. By 2 258AD, the separation again reaches the minimum separation of 0.38 arcsec, making it difficult pair in most amateur telescopes below 30cm, and even larger if the seeing is poor.

Orientation of the true orbit is inclined 8.3° to the east-west line, with P and A being the date of the respective periastron and apastron of the orbit. (See the Figure Above)

Distance of β Mus

Hipparcos found the distance as 95.41±5.56pc (311.2±18.1 ly) from the Sun, using the measured parallax of 10.48±0.65mas. The results here seem questionable. For example, the parallax measured in the Tycho catalogue gives the distance as 39±4pc (25.60±6.10mas), more the two-and-a-half times larger than Hipparcos. Using the dynamical parallax, the theoretical distance of the system derived from the magnitudes and orbital data, finds the distance of 50.6pc or 164.9 ly. I again calculated this using the full Baize-Romani method (first determined in 1945) and obtained 51pc. Such divergence in the values makes the mass and size of the orbit as doubtful. From the available data, I think that the Hipparcos distance is likely too large, and it is certainly much closer than the quoted error. It is possible that the proximity of the two stars might be causing an incorrect measurement. Hipparcos had significant problems in measuring pairs between about 1.5 and 0.2 arcsec due to the configuration of the optical system. β Musca, because of the stars brightness, lies on the boundary of this region. Overall, I think the 51pc. distance is likely closest to the truth. Compared to the Solar System, Beta Muscaes true orbital dimensions are huge. If the Hipparcos distance of 95.41pc. is correct, then the dimensions of the true, then at the time of periastron passage the two stars are some 42.4 A.U. apart, this increases to the furthest apastron distance of nearly 200 A.U. - five times the orbit of Pluto! If the correct distance is closer at 50.6pc, then the proportion of the orbital size is smaller, dimensioning periastron at 22.3 A.U. and apastron as 104 A.U.

Proper motions suggests the motion of travel is moving further south each century by some -40.40 arcsec in RA and -10.32 arcsec in declination, with Beta Musca is moving way from us at about +42kms-1. Spectral class of both main sequence stars is B2V and B2.5V, implying effective surface temperatures of 24 400K and 20 950K, respectively. The measured B-V for both stars combined light is -0.178, reflecting its visual blue colour. Absolute magnitudes (Mabs) are -1.3 and -1.0, respectively, making the two stars 275 LL⊚ and 208 LL⊚ times the luminosity of the Sun. Using this data, and applying the results from the dynamical parallax, produces the minimum value for the total combined solar mass of 4.6 ML⊚, and individual masses of 2.33 ML⊚ and 2.28 ML⊚, respectively. Furthermore, the minimum radii of each star are about 3.0 RL⊚ and 2.6 RL⊚, roughly four million kilometres across. Limitations on mass, radius and absolute magnitude using evolution theory place possible value about 30% to 40% higher than the ones quoted above.

In all, this is an interesting system and challenging for those with moderate apertures. Due to the brightness of the two stars, cleanly separating the two is highly dependant on the seeing. Using 20cm (C8) at 225X and 333X, I have observed the pair on more than three or four dozen times, seeing the two Airy disks clearly separated perhaps only 40% of the time. (Incidentally, I have tried without success to observe this pair using the 30cm (11.75-inch) Refractor at Sydney Observatory in 1977 and 1978. This was mainly limited by the seeing, however!)

β Cru / Δ125 / I 362AB / Beta (β) Crucis / Mimosa (12477-5941) is a blue supergiant star and rank as the 20th naked-eye star in the sky having the diameter some thirteen times larger than the Sun. Beta Crucis "A" is a Beta Cepheid-type variable star, some of which appear also in the nearby Jewel Box / NGC 4755 which is about 1°SE of Mimosa. Observation of Beta Cepheids are often found to vary in periods anywhere between 0.1 to 0.7 days, displaying only small changes in luminosity by between 0.1 and 0.3 magnitudes. Spectral classes are often found between B0 to B3, and this applies across all the seven main categories of luminosity classes. β Crucis is of B0.5III-IV spectral type. Common bright examples of field Beta Cepheids included nearby to the Jewel Box - δ Crucis λ Crucis and β Centauri (See NSP 16 at Planetary Nebula Homepage),and β Canis Majoris.

Beta Crucis varies between 1.28V and 1.31V magnitude (0.034V in recent estimates) over 0.2365072 days. It also shows seven different harmonic periods combining to producing the main "P0" period. These other periods are;

P1 0.16343 (0.029V mag)
P2 0.121383 (0.01V mag)
P02 0.249373 (0.016V mag)
P13 0.24273 (0.024V mag)
P24 0.23931 (0.028V mag)
P03 0.16342 (0.016V mag)
P14 0.16171 (0.029V mag)

β Crucis has a nearby companion that is also known as Δ125 AC, is about 2.5'SW or 361.7 arcsec (2002 AD) away along PA 23°. It appears as a deep ruby red of about 8.6 magnitude star EsB-365. Easily seen even in 7.5 cm telescopes, and admired by Sir John Herschel, it truly came to prominence within the Espin Birmingham 1900s catalogue because of its remarkable spectra. The displayed colour is a testament to both its grandfather-like age and remarkably cool temperature. EsB-365 is technically known as a carbon star, mainly as the spectral lines of Carbon and Nitrogen are quite prominent, while the other common lines are either faint or are not seen at all. The positions have decreased by 12.4 arcsec in 124 years since last measured, while the PA has remanded fixed at 23°. This 10.0 arcsec per century suggest that in the sky both stars will join together in the year 4 550 AD. About the year 4 050 AD and 5 050AD, this will become one of the best colour contrasting pairs in the entire sky. It is still almost certain that these two stars are not associated.

A second companion is I 362 AB. which is another white star of 11.4 magnitude and extends about 44.3 arcseconds along PA 322° from the primary.The positions of the primary and this star have not moved since Innes discovery in 1901.

PLEASE NOTE: The name for MIMOSA lately has the name BECRUX now used in modern star atlases and software, including Astronomy 1999. This was introduced in 1993 by American observers. In no circumstances should this name be used nor promoted!

Comment: Nearby is the Jewel Box / NGC 4755. This magnificent open star cluster, the was discovered by James Dunlop (Δ301) that John Herschel named the Jewel Box NGC 4755, which is both east, and slightly south of β Crucis. NGC 4755 is a small A” shaped asterism and is different than most open cluster because of its 6′ size and numerous bright stars. This is a nice cluster in a starry field whose distance is some 2.4 kpc. (See NGC 4755 Text on this Homepage.)

μ Cru / Δ126 / Mu Crucis (12546-5711) is located 2.7°N, and a little east, of β Cru. This broad, bright bluish pair has no common name. (Mucrux might be a suitable name.) Discovered by Dunlop in 1826, and listed as Δ126 in his catalogue, this beautiful blue duo is located in a star-studded field. It is certainly easy to resolve in small telescopes. The magnitudes are 4.03 and 5.10, while the separation is a wide 34.7″ at PA 17°. The spectral class are B2IIV-V and B5Vne, respectively. Looking at the Hipparcos data some credence adds that this pair is likely attached, as the proper motions and distances are quite similar. I.e. Distances are μ1 = 116±7pc (or 361±24ly.) and μ2 = 111±7pc (or 381±25ly.), Common proper motions; π1 = RA:-30.45±0.37mas and Dec:-13.55±0.41mas. And μ1 = RA:-32.45±0.37mas and Dec:-10.93±0.40mas. This gives the mean distance to μ Cru as 113.5pc or 370ly. If this is so, then the two stars are separated in space by some 0.0192pc or 0.063ly. suggesting the period maybe as long 30 000 years. The second star, μ2 Crucis, is Gamma Cassiopeia-type (GCAS) variable, changing by 0.19 mag in an unknown period. Little has changed positions since discovery, and the twelve other observations have shown little additional data. This pair is a good test for 7x50s binoculars that can just separate the duo, but likely during poor seeing, this might be a bit more difficult.

Δ127 (12598-5555) is one of the nicest pairs I have seen and I have taken great delight in observing this pair numerous times in the last thirty odd years of double star observing. Puzzlingly missed in both the AOSTs and most other noted deep-sky descriptions, like because it is overshadowed by the brighter gems in nearby Crux and Centaurus. This pair cqan be found in mid-southern Centaurus and the north-eastern corner of Crux. It is easily located some 1.4°ENE (PA 61°) of the magnificent Mu Crucis (Δ125). Dunlop discovered the pair sometime in 1825, although his first measure appears as 1826 in the WDS01. Δ127 appears as two distinct blue stars of 8.27v and 8.98v magnitude and is presently aligned along PA 126°. Easily seen in 7.5cm, the separation has slowly widened from 13.0 arcsec to its present 16.8 arcsec (2002). Dunlop listed the pair as Anonym.” and gave the 1827 position as RA: 12h 49m 00s Dec: -54° 57', (1827) precessed to 12h 59.2m -55° 53' (2000), and identifies the 7th and 8th magnitude star about 5'W of the positions given for Δ127; in the WDS01. Although Dunlops separation is given as 10 arcsec, it is likely that this is an error by him, as the measures since then make it impossible for it to have been this close. The WDS01 seems to be aligning itself to use the Herschel measures that the observation was made in 1835 for the separation and using Dunlops 1826 PA of 51°sf. (129°). Is this really an error in the WDS01?

Looking at the general observational data, Δ127 A is HIP 63422/ PPM 341501/ SAO 240455, which Hipparcos gives as the 8.27 mag. as stated above. Also the parallax is 13.54±3.02, the spectral class is B7IV/V and the B-V is 0.040. Δ127 B is HIP 63424 / PPM 341502 / SAO 240457 and gives the fainter magnitude of 8.98. Parallax is stated as 14.15±7.02mas, suggesting the mean distance of 70.7pc. Dun 127 Bs spectral class of B9 matches near to the B-V of 0.110.

Also from these data, both the parallaxes and proper motions seem very similar, and we can conclude that these two stars have a very high probability of being gravitationally attached. The Tycho data matches the Hipparcos data quite well except for the cpm which are significantly different. Tycho gives the magnitudes as 8.21 and 8.90, while the PPM says 8.5 and 9.1. Overall the Δm;s are 0.71, 0.69 and 0.6, and although not listed in the WDS02s Deltam catalogue, the mean of the Hipparcos-Tycho value of 0.70 seems about right. The Guide Star Catalogue lists the primary twice as GSC 8652:1908 and GSC 8652:192. It is likely the measuring machine has read the individual stars once as the A component and again as the combined AB component.

Overall, Δ127 is contained in a field of many 10th and 11th magnitude stars, which I thought seem to be winding NE to SW across the field. This is a lovely wide pair.

Additional Field Pairs

I have identified four faint pairs in the field using the 20cm where the limiting magnitude was about 13.8. These pairs are not listed as pairs, as such, but they can be seen in the 18'x18' STScI image surrounding Δ127. (See Attached Image)

Δ127 Pair 1 (12599-5548) is a north-south 11th and 12th magnitude pair (PA of 6°) and 18 arcsec apart. Both have the respective magnitudes of 11.3 and 12.6 in the GSC matching fairly well with my estimates. Pair 1s position can be identified easily because it is in the same field about 7.4'N of Δ127.

Δ127 Pair 2 (12594-5550) appears 6.0'NW (PA 325°) of Δ127. I estimated the magnitudes as 11th and 13th and the separation of around 15 arcsec. Using the GSC star data, the magnitude of the primary is GSC 8652:263 at 12.4 magnitude while the secondary star GSC 8652:1538 is 13.3 magnitude. Positional differences is about 17.7 arcsec along PA 80°. It can be identified as the pair is the most eastern of a brood of nine 10th to 12th magnitude stars and also being closest to my Pair 1. I saw both stars displaying just a hint yellowish and slightly dingy-red.

Δ127 Pair 3 (12596-5558) is the second brightest in my listing and lies 4.0'SSW of Δ127. The primary is 11th (GSC says 11.2) magnitude and the companion is 12th (GSC says 12.8, though I estimate it is slightly brighter than this. Pair 3 is 19.6 arcsec apart along PA 246° (WSW). The bright star is GSC 8652:581 and I thought it a tad bluish. I saw no colour in its companion”.

Δ127 Pair 4 (13007-5603) lies some 11'SE (PA 137°) from Δ127 and is the brightest in my listing. Pair 4 A is GSC 8652:116 and is given as a 9.9 magnitude star - the most northerly star of a 2' triangle of c.9th magnitude stars. The Pair 4 B companion is listed as a 12.6 magnitude "Non-Star" even though it appears on the STScI image. I thought it was slightly brighter than this, and certainly brighter than seen in the image. I calculated the distance apart as 15.7 arcsec along PA 314° (NW). This seems about right.

Δ127 Pair 5 (13009-5550) is another pair I first noticed on the STScI image. The GSC gives this as an 11.0 magnitude "Non-Star" though it is clearly visible. I estimate the companion must be at least 13.5 magnitude and the separation must be about 5" to 6"arcsec aligned toward PA 260°. I havent tried to observe this star, but it should be visible in 20cm, and definitely in 25cm or 30cm.

Several other pairs are in this field, though they are of little interest as they are a little too wide. In the 26mm Plössel with 20cm appears at the SW edge of the field (some 18' away from Δ127) is a obvious wide unlisted 9.7 and 11.1 magnitude pair separated by about 36″ around PA 45°. The primary is deep-yellow in colour and its companion seems white.

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Southern Astronomical Delights”
© (2009)
10 Mar 2009