Description of Some Bright Individual Cluster Stars

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NGC 4755 Individual Star MAP

Figure 4a. Selected Stars of the Jewel Box

Star 1 | Star 2 | Star 3 | Star 4 | Star 5 | Star 6 | Star 6a | Star 7 | Star 8 | Star 9 |
| Star 10 | Star 11 | Star 12 | Star 13 | Star 14 | Star 15 | Star 16 | Star 17


Star 1

This is the brightest, or second brightest if “Star 7” is counted as a true cluster member. Given as Star NGC 4755 B, and is the only naked-eye star of the main asterism. Frequently labelled as the “namesake” - Kappa Crucis, the star is listed as HIP62931 / SAO 252077 / HD 1119732 / GSC 8989:3110 (12h 53.8m -60° 22'), this blue or bluish star is 5.89V magnitude and spectral class B4 Ia. Kappa Crucis is the most southerly of main stars, and some ten stars make the bulk of the cluster, which to me appear as roughly like an A-shaped triangle, 1' to 1.2' arc min each side. Astrometric observation by the Hipparcos finds the parallax of 1.96 ± 0.63mas, giving the distance of 520pc or 1 690ly - some six times smaller than NGC 4755 estimated distance of 2.4kpc. (Note: The distance quoted in the literature is made by producing a colour-magnitude diagram, then applying theoretical analysis from known about stars, to find the distance.) Using the 2.1kpc distance, the absolute magnitude (Mv) of the star is about -6 to -7.5, making it some 80 000 to 100 000 times more luminous than the Sun. Its surface temperature is estimated as 15 450K.

Star 2

Is designated as NGC 4755 A / HIP 62894 / SAO 252069 / PPM 359716 / HD 111904 / HR 4887 / GSC 8989:2110 (12h 53.4 -60° 20'), and this bluish-white 5.75 magnitude B9 [Ib] star marks the apex of the 'A'-shaped asterism. Although often listed as the second brightest star in the main asterism, it is really 0.14 magnitudes brighter than Kappa Crucis itself. All Star 2 is thought to be suspected variable NSV 6008, whose tentative magnitude range varies between 5.7 and 6.8. Interestingly, the Guide Star Catalogue gives 6.6 ± 0.4 magnitude and if this is true then perhaps this was when the star was at minimum. (Tycho and Hipparcos gives 5.8 magnitude.) Observers interested in variables might like to, from time to time, check this star with Kappa Crucis and 6.8 magnitude "Star 3" -the eastern most star.

Star 3

Listed as NGC 4755 C / HIP 62953 / SAO 252080 / PPM 359731 / HD 111990 at 6.78 magnitude, this star appear as the most eastern of the main A-shaped asterism and is actually the brightest double star in the Jewel Box. Discovered by van den Bos in 1927, and listed as B805, this pair is not easy for amateur telescopes. The primary is 6.8 bag while the secondary is 13.2 magnitude, as the separated is 4.3"arcsec along PA 264 degrees. At least 35cm on good seeing and transparent nights are required to see the pair clearly, though I read one report that it was glimpsed using 25cm. I cannot claim to have seen is companion, as I lack the suitable aperture. Little is known about this pair, with the only measure being made since discovery. Spectral Class is given as B3 Ib.

Star 4

Lies between Kappa Crucis "B" (Star 1) and Kappa Crucis "A" (Star 2), and is the westward star of the three stars is a slightly bent line, which marks the crossbar of the 'A'. Surrounding this bluish star is an arc or arrow are seven or eight 13th to 14th magnitude stars pointing to the south-west. About 3' across, “this mini-asterism” is visible in 20cm, but I suspect 15cm could possibly glimpse them on dark nights. "Star 4" is designated NGC 4755 1-05 / HIP 62913 / SAO 252070 / PPM 359721 / HD 111934, but does not have a letter designation as given to the oher stars by Arp in 1958. A 6.9 magnitude star of spectral class B1.5 Ib, it was first classified as a suspect variable NSV 6012 by B. Hill in 1967 and finally designation BU Cru when its variability was confirmed on the 25th March 1977 by Shyam Jakate.
Although the variability has some of the characteristics of a Beta Cepheid, it is likely an eclipsing binary of the “E” or “E-II” type, whose variations are thought to be due to partial eclipses of the component stars. Like most of these variables of this type, the period of variation is less than 0.1 magnitudes.

Star 5

This is the main red star NGC 4755 D that lies in the near center 'A'-shaped asterism, and forms a startling contrast with all the other stars. I suspect if it were not for this star, then the Jewel Box would “lose its lustre ”, and be considered like any other of the bright cluster. Certainly Herschel would have never added the written superlatives that he did in the 1830's, when he said;
...centred is this glittering nest of stars, like a ruby set in a set of diamonds and sapphires, is a red giant comparable to Betelgeuse.
In the catalogues it is listed as HIP 62918 / SAO 252073 / PPM 359723 12h 53.7m -60° 21'), whose magnitudes are given as 7.58v (Feast(1962)) and 7.66V, with most measre alues varing only by about ±0.2 magnitude. Earlier B-V values were given as 1.45, but later values are more likely a redder 2.16. Until recently, it was considered of fixed brightness, but it is now deemed as the variable star DU Cru.
Early spectral class observations, like Arp and van Sant (1958) made this a K5 I(ab), but this surely is an M2 Iab supergiant star, and if it is really associated, is likely the most massive of the Jewel Box stars. The spectral class is M2 Iab. Burnham states the absolute magnitude of this star is -5.7, but it is surely 0.5 magnitudes brighter than this. Burnham compares NGC 4755 D in size and brightness to the 1st magnitude star α Orionis or Betelgeuse.

Star 6

This is opposite the line of three stars in the “crossbar”. This 7.97 magnitude star is labelled SAO 252075 / PPM 359726 / T8989:2233:1. For some reason it does not appear in the Hipparcos, and is listed as the 7.8 magnitude "artifact " in the Guide Star Catalogue, but it does feature in the PPM and Tycho ones. I could not find this star ''s spectral class. Its B-V is 0.161 suggest possibly a white "A-type" star. Through 20cm, I thought this star yellowish. According to the GCVS4 (Kholopov, P.N. (1998)) two other stars are considered as variable. Star 6 is suspected to be an elliptical variable (ELL: Type) caused by the close binary that does not undergo eclipses. Observed light variations are caused by the orientation of the "tear-dropped shaped" tidal distortions of the two stars. Highly detailed photometric observations will be required to determine the true period and magnitude variations. The GVSC position for the 2000 Epoch is 12h 54m 16.8" -60° 18' 13". (Not listed in Megastar 4.0)

Star 6a

Lies merely 45"NW of "Star 6". This star is certainly yellow to yellowish in colour, and makes a contrast to the other stars. Amazingly none of the familiar star catalogues bothers to list this star, even though the estimated magnitude is around 10th. I checked all the double star catalogues as well, and "6 and 6a" are not listed as any pair.

Star 7

Identifies as the star HIP 62732 / SAO 252054 / HD 111613 (12h 51.3m -60° 20'), and is one of a few possible outliers of NGC 4755 in field. The Hipparcos data confirms similar parallax of 1.09±0.04 mas. This is stated was in "The Constellations: An Enthusiast 's Guide to the Night Sky." by Motz and Nathanson and in "Burnham 's Celestial Handbook." Vol.2, p.733, but I did find several comments of this in the professional literature. This 5.74 magnitude star lies well away from the principal stars, and is some 17'W from the cluster. Listed as suspected variable NSV 5991, it has changes possibly as much as 0.06 magnitude in an unknown period. We know HIP 62732 is an A-type supergiant of spectral class A1.5-A2 lab, so the temperature must be about 9 060K. Absolute magnitude (MV)is -6.3, making it 90 000 times brighter than the Sun. Such stars have theoretically a stellar radius of about 83.5 million kilometres (~120R) or about twice the size of the orbit of Mercury.

Star 8

Appears as the middle star between Stars 4 and 5 in Figure 2. At 8.6 magnitude, SAO 252071/PPM 359722 is the most obvious of the fainter stars, breaking the fine symmetry of the cluster. Surprisingly in the catalogues, it is mainly ignored, and the Tycho, Hipparcos and Guide Star Catalogues miss it altogether. More unusual, is the drawing by Dunlop in 1826, and the placement of two equal magnitudes star between "Stars 4 and 5". Together both point almost due north, and for such an obvious position within the cluster, it is unlikely that Dunlop could make such an obvious mistake. I thought this star appeared white and that it contrasts well with the other blue or bluish stars. The spectral class is B0.

Star 9

Lies at the bottom of the 'A' midway and slightly inwards, between Stars 1 and 3. I mention this star because it seems to me very blue for such a faint star. The Tycho catalogue (T 8989:2022: 1) states 8.75v magnitude, while the GSC (GSC 8989:2022) says 9.8±0.4. To me this latter value is closest to the truth, but I estimate that even this is one to one-and-a-half magnitudes too bright. Other than the star having B-V of 0.226, little is known about this component of the Jewel Box.
This star is drawn in Dunlop's Southern catalogue as "Figure 13", but the position is decidedly closer to "Star 3" than "Star 1". It is almost as if Dunlop has mirrored the true position. I checked this against the image of NGC 4755 produced by H.C. Russell sometime in May 1891 (Figure 3), and the star is found slightly further from "Star 1", and not enough to explain the discrepancy. It maybe possible that proper motion has changed the placement, but no data exists to support this premise.

Star 10

The brightest star placed outside the proximity of the 'A' asterism, and is listed as SAO 252074/ PPM 359725/T 8989:2130:1. Near Kappa Crucis ("Star 1") and 1.7'S, is 8.3 magnitude star with the spectral class of B8. This deep blue star marks the southern edge of the principal majority of faint stars, not really noticed at first by eye because of its perceived isolation.

Star 11

Is the other very red star is the field lies 3.9' S of Kappa Crucis ("Star 1"), and is partially obscured in Figure 2. This 10.1 mag star is HD 312081 / PPM 779201 / T8989:1657:1, and is very similar in colour and spectral type. HD 312081 has the B-V of l.487, which is quite red among the blue stars. However, it is unlikely this is a cluster member, as the parallax is ten times higher than many other Jewel Box stars.

Star 12

The Beta Cepheid variable star BW Cru. This 9.07 mag star varies between 9.03 and 9.09 in a period of 0.203 days. The spectral type is B2 III, and appears bluish in the telescope. They have listed it as NGC 4755 F / HIP 62949 / SAO 252078 / PPM 359730, and has a proper motion that agrees well with the Hipparchos measured parallax of 0.134±1.37mas. The GSC wrong stated this is a "Nonstar" of 9.0 magnitude

Star 13

The second Beta Cepheid variable is BT Cru, which has magnitude variations between 9.80B and 9.832B magnitude in the period of 0.133 days. This star is catalogued as NGC 4755 IV-18 . Spectrally classed as a B2 main sequence star, this 9.65v magnitude star is also the double star JSP 561. Discovered in 1929 by the South African astronomer R. Jonckheere, this bluish and white pair is separated by 2.1"arc sec along PA 93°. This is the only measure to date. At 9.9 and 10.0 magnitude, the pair is easily separated in 20cm, and I suspect even 15cm could also resolve it as well. I though the magnitude difference was less than this, perhaps about 0.6 magnitudes. For the record, the "A" component is the actual variable.

Star 14

>This is the Beta Cepheid variable designated BS Cru. This star is in many ways the same as more easterly BT Cru mentioned above as Star 13. Magnitudes vary between 9.75V and 9.79 in a period of 0.275 days. The spectral class is B0.5 V. BS Cru is placed at 12h 53m 19.0s -60° 24' 22" in the GVSC 4th Edition, and this is 1.1'S from its true position. At the correct place is the star NGC 4755 G / GSC 8989:2321 / T 8989:2321:1, and has a stated 9.7±0.4 magnitude Tycho gives 10.48 magnitude, but this seems too low.

Star 15

Also the variable BV Cru / NGC 4755 I-06 which varies from 8.77B to about 8.82B in the period of 0.16 days. This B0.5 III sub-giant star shows numerous nebula lines in its spectra. It is listed in GSC as GSC 8989:1916, at 10.2 mag, and in Tycho as T 8989:191 6:1 and 9.43 magnitude
BV Cru is listed in the "General Variable Star Catalogue" (GVSC4) at position 12h 53m 39. Is -60° 21' 58", and some 34" too far east from the star's GSC and Tycho places mentioned above. No other star of this magnitude appears close by. Furthermore, it is listed as the double star JSP 562 (Position: 12h 53m 36.0s -60° 21' 00") which was discovered R. Jonckheere in 1930. Micrometric positions have not changed since discovery, and the near equal 10.9 and 11.0 stars (combined magnitude is 10.2) remain separated by 3.4"arcsec along PA 206°. Easily separated in 15cm, but I suspect even 10.5cm could see the pair. I see both the stars as bluish.

All the positions and magnitudes are quite varied and are well outside the errors in each catalogue. Finding satisfactory explanations for this discrepancy is difficult.

Star 16

This is also the double star JSP 563, which is 96"arc sec from Kappa Crucis. Clean optics is essential to see the pair, as Kappa Crucis tends to obliterate the faint 10.4 and 12.9 mag star. Although separated by 3.8"arcsec at PA 270°, I found the secondary still quite difficult to see, even though I was using 20cm. Seeing both stars in 15cm should be possible, or perhaps just with 10.5cm. I saw the primary as blue, but the colour of the companion was too difficult to see. The position of JSP 563 is 38"SE from the position in the Washington Double Star Catalogue (WDS96). Marked is 10.8 mag GSC 8989:2346. Tycho (star T8989:2345: 1) gives 9.67 mag, which seems a little too bright, even accounting for the star's slight increase in brightness when adding the magnitudes.
Since Jonckheere discovered this star in 1930, the separation has increased to nearly 2.4"arcsec, although the position angle has changed little. It is now certain this is an optical pair, just from the companion from its rapid motion. Considering the distance of the cluster, the secondary must lie in the space between.

Star 17

Is the variable star CN Cru / GSC 8989:3111 (12538-6023), which is 26" E of Kappa Crucis. Two stars of similar brightness are separated by 20" and aligned north to south. The most northern one in the blue coloured Hipparcos Catalogue as HIP 62937 at 9.01 mag and in the Tycho Catalogue as T 8989:3114:1 at 8.4 mag, but it is the southern star that is the variable.
The GSCV4 gives little details on this system, including the period, but the GSCV position is placed at 12h 53m 53s -60° 22' 36". Variations is ranged in brightness from 8.61B and 9.01B dropping in brightness by 0.24. If the period was even slightly deeper than this, it could be possible to see the star change in brightness, with the northern star making a suitable comparison star.

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Last Update : 06 April 2002

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