Figure 6. Selected Stars of the Jewel Box
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,
1.0 to 1.2 arcmin each side. Astrometric observation by the
Hipparcos finds the parallax of 1.96±0.63 mas., giving the
distance of 520pc or 1 690ly — some six times smaller than NGC
4755 estimated distance of 2.4kpc. (Note: Quoted distance 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. Surface temperature is estimated as
Is designated as NGC 4755 A / HIP 62894 / SAO 252069 / PPM
359716 / HD 111904 / HR 4887 / GSC 8989:2110 (12h 53.4 -60°
22′), 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.
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°. 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.
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 own variability was confirmed on the 25th March 1977 by Shyam
Although the variability has some of the characteristics
of the Beta Cepheids, 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
variation is less than 0.1 magnitudes.
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 1830s, when he
“°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°g; 21′),
whose magnitudes are given as 7.58v by Feast (1962) and 7.66V, with
most measure values 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. Spectral class is M2 Iab, while Burnham states
the absolute magnitude 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
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 its spectral class. 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 16.8s -60°; 18′ 13″. (Not listed
in Megastar 4.0)
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.
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 derived from their enormous luminosity have
a theoretically stellar radius of about 83.5 million kilometres (~12
R⊙) or about twice the size of the orbit of Mercury.
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
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.
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.
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.
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
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 arcsec 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
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.
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
39s. -60° 21′ 58″, and some 34 arcsec too far east
from the GSC and Tycho star positions 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.
This is also the double star JSP 563, which is 96 arcsec
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 slight increase in stellar brightness when adding
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.
This 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
arcsec 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 visually see the star change in brightness, with the northern
star making a suitable comparison star.
Last Update : 17th July 2012
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