The first realistic distance of 1.09kpc was found by Trumpler in 1930, during his extensive cluster surveys in both hemispheres (Lick. Obs. Bull., 14, 173). Yet it was only when good photometric observations were obtained between about 1959 and 1965. that the estimates tended to "settle" around 2.3 to 2.4kpc. Important examples of estimations at this time and the modern determinations includes:
| Hernandez; PASP, 22, 416 (1960) | 2.1 kpc. |
| Buscombe; Mt. Stromlo Mim.. N6 (1963) | 2.3 kpc. |
| Feast; MNRAS. 126, 11 (1963) | 2.36±0.32kpc. |
| Feast; MNRAS, 130, 264.267 (1965) | 2.36 kpc. |
| Dluznevskaja; NautchInf (Russia) (1965). N2 | 2.35 kpc. |
| Kennedy; Mt. Stromlo Mim., N9 (1966) | 2.36 kpc. |
| Graham; MNRAS. 135, 377 (1967) | 2.3 kpc. |
| Perry, C.L. et.al ; Astron. J., 81, 8, 632-638 (1976) | 2.3 kpc. |
| Sagar & Cannon; Astro. & Astroph. Suppl. Ser., 111, 75-84 (1995) | <2.1±0.2 kpc. |
| Koenig, Ingo et. al.; Astron. Gesellschaft Meeting (AGM), 14, 35, Jan (1998) | 2.34±0.35kpc |
| Sanner et.al.; Astro. & Astroph. Suppl. Ser., 369, 511-526 (2001) | 1.9±0.3 kpc. |
AOST2 says the distance is 2.4kpc. Sky Catalogue 2000.0
Vol.2 says 2.34kpc. Both of these values are the rounded
results of measures made in the years 1966 and 1967, and
this value has remained fairly constant between 1965 until
the beginning of the 1990's.
Presently the best distance estimates, with the lowest
error, is by Sagar. R. & Cannon, R.D. (1995) of
2.1±0.2kpc (6 850±650ly.) and Sanner et.al.;
2001) of 1.9±0.3kpc. Another of the latest distance
determinations was found using the brighter component stars
by Koenig, Ingo et. al. (AGM), 14, p.35 Jan (1998)in
distance modulus as V–Mv of 11.85±0.30
magnitudes, and this was also confirmed by the red M2 Iab
supergiant as 11.84±0.50. This translated into
2.34±0.35kpc (7 630±114ly) and
2.36±0.57kpc (8 000±186ly, respectively. The
result suggests reconfirms the distances quoted in the
1960's.
There may be some significance to these results, as the
brighter stars are giving a slightly different distance of
0.2 to 0.3pc. to those of the the much deeper magnitude
surveys. Combined with the "double" main sequences suggest
that the Jewel Box might be superimposed cluster - as
hinted at in Sagar and Cannon's paper (1995). The only
other likelyhood is that the observations between the
bright and fainter stars is somehow flawed - either by the
measurements or in the stellar evolution theory. Problems
may also still exist in the determination of precise
distances mainly because of the uncertainty in the amount
of interstellar gas obscuring this part of the Milky Way.
Either way, observations of this cluster will still be
important in the decades to come.
“Southern Astronomical Delights” © Andrew James (2002) Sydney, Australia