Deepsky observing database

NGC 2257

Updated 2007 October 20

Description: This cluster lies quite a ways, in a north-easterly direction, from the naked-eye limit of the LMC – far enough to make me double-check the telescope's pointing! The cluster lies about a degree south-east of a prominent triangle (15' sides) of 7th magnitude stars; the southern side of this triangle points directly to the cluster.

NGC 2257 is immediately apparent in this wide field as a soft, round, starless glow, 1.3-arcmin in diameter, set amongst a meandering row of five stars (running NW-SE, north of the cluster; J-B-D-A-C in the sketch). The nebula (well, globular cluster) is only very slightly brighter (ie more dense) to the middle. It's clearly larger than the separation between stars A and D on the sketch.

Star	V	V	Tycho magnitudes	GSC ID	GSC mag
(1)	(2)	(3)	(4)	(5)	(6)
A	11.11	10.89	BT=12.24, VT=11.01	8902 1763	11.58+-0.28
B	11.24	11.13	BT=12.80, VT=11.28	8902 1341	12.02+-0.27
C	11.96	11.95	BT=12.54, VT=12.01	8902 1116	12.10+-0.29
D	12.00	12.04	BT=13.14, VT=12.14	8902 650	12.40+-0.28
E	13.73	–	–	8902 732	14.24+-0.29
F	13.85	–	–	8902 998	14.58+-0.28
G	14.50	–	–	–	–
H	14.79	–	–	–	–
I	15.29	–	–	–	–
J	–	10.57	BT=12.85, VT=10.78	8902 250	11.79+-0.27
K	–	–	–	8902 328	14.71+-0.29

(1) Label on my sketch; same numbering sequence as in Alcaino & Alvarado (1988). I did not record their star "I".
(2) V mags from Alcaino & Alvarado (1988)
(3) V mag deduced from Tycho BT and BV
(3)-(6) from Guide 8.

Move your mouse over the sketch to see the stars labelled as in the table above.

Literature review

Dirsch et al. (2000) give RA 06h30m24s, Dec -64°17' (2000.0) for the cluster, which has an integrated V=11.5 (Elson & Freeman, 1985). It lies 9° NE of the LMC bar (Testa et al. 1995).

Alcaino & Alvarado (1988) carried out photometry of the stars near the cluster and include a finder chart (figure 4, plate 105, p 1922). From this photo, the cluster is clearly < 2' in diameter (see Errata).

Walker (1972) gives the magnitudes of the two brightest stars within NGC 2257 as V=15.32 and V=16.50. The brightest RR Lyrae variables in the cluster are around V=19 (Nemec et al. 1985, Table 14).

Dirsch et al. (2000) used isochrone fitting to the CMD to determine the age of this cluster as 15.8 (+/- 3.7) Gyr; the surrounding field stars are 3.5 Gyr old. They noted that the CMD of NGC 2257 is very similar to that of Galactic globular clusters. NGC 2257 is probably the oldest Magellanic Cloud cluster (Testa et al. 1995).

The cluster's total luminosity is 66,000 Suns, and its mass 37,000 Suns (Elson & Freeman 1985). It lies 163,000 (+/- 33,000) lightyears from the Sun (Testa et al. 1995).

Errata

Oddly, NGC 2257 is shown as 55 arcminutes in diameter in Guide 8 - you can imagine my surprise. The SAC database (version 7.6.4) also lists its diameter as 55'. Both sources cross-reference it to the ESO/Uppsala Survey (Lauberts 1982) where it is listed as ESO 87-SC24. The electronic version of this catalogue gives its diameter as "55:", the ":" indicating uncertainty of the measurement. Sheesh, that's certainly uncertain ;-)

Perhaps the ESO entry was meant to be "55 arcseconds:" ?

The NGC/IC Project website lists it as an "open cluster in the LMC"; the ESO catalogue classifies it as a "Clobular". Something got clobbered, that's for sure.

There's also a bit of variation in the listed positions, which is particulary noticeable when plotted on a star chart because of the proximity of bright stars.

Source	RA	Dec
SIMBAD	06 30 12	-64 19.6
Cartes du Ciel (SAC)	06 30 12	-64 19 00
Guide 8	06 30 13	-64 19 29
ESO/Upp (precessed)	06 30 13	-64 19 30
NED	06 30 13.8	-64 19 32
Cartes du Ciel (NGC2000)	06 30 24	-64 19 00
Dirsch et al. (2000)	06 30 24	-64 17

References

Alcaino, G. & Alvarado, F. (1988) Photoelectric UBVRI sequences in the Magellanic Cloud Clusters Kron 3, NGC 330, NGC 1841, and NGC 2257. AJ, 95(6), 1724-1726. [1988AJ.....95.1724A]

Dirsch, B., Richtler, T., Gieren, W.P. & Hilker, M. (2000) Age and metallicity for six LMC clusters and their surrounding field population. A&A, 360, 133-160. [2000A&A...360..133D]

Elson, R.A.W. & Freeman, K.C. (1985) Structural parameters and masses for three old LMC clusters. Ap.J., 288, 521-530. [1985ApJ...288..521E]

Lauberts, A. (1982) The ESO/Uppsala Survey of the ESO(B) Atlas. [1982ESO...C......0L]

Nemec, J. M., Hesser, J. E. & Ugarte, P. P. (1985) The RR Lyrae stars in and around the LMC globular cluster NGC 2257. Astroph.J.Supp.Ser., 57, 287-328. [1985ApJS...57..287N]

Testa, V., Ferraro, F. R., Brocato, E. & Castellani, V. (1995) The age of old Magellanic Cloud clusters - I. NGC 2257. MNRAS, 275(2), 454-462. [1995MNRAS.275..454T]

Walker, M.F. (1972) Electronographic photometry of star clusters in the Magellanic Clouds - III. The colour-magnitude diagram of NGC 2257. MNRAS, 156, 459 - 469. [1972MNRAS.156..459W]

Keywords: Astronomy, deepsky observing

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