NGC 104

Synopsis

Globular cluster; located in Tucana; R.A. 00:24:05.19, dec. -72:04:49.9; culmination early November; magnitude 4.9; distance 4,800 pc (approx 15,650 light years).

Description

Second only to NGC 5139 as the largest, brightest and most spectacular of the galactic globular clusters visible from Earth.

It is among the intrinsically largest and most brilliant, with a diameter somewhere in the vicinity of 64.5 pc (210 light years) and integrated absolute visual magnitude of just under -9.5.

Discussion

Stellar Composition

NGC 104 is atypical of galactic globular clusters in several ways, beginning with a paucity of blue, horizontal-branch stars in general, and RR Lyrae variables in particular.

Among the brightest members are red giant stars with absolute magntudes of about -2. Several irregular red variables have been identified, including several Mira-class objects with periods ranging from ~150 to ~210 days (Burnham 1992, p. 1913).

Several galactic globular clusters have been found, unexpectedly, to host pulsars, and the most prolific source of these is NGC 104. It has been conjectured that the marked increase in stellar concentration towards the centre of this cluster greatly increases the possibility of the kind of interaction between stars that can produce a pulsar (Malin 1993, p. 180).

The integrated spectrum of NGC 104 shows a higher abundance of metals (elements heavier than helium) than is usual. "[C]arbon depletion is weaker in metal-rich clusters, implying that deep mixing is more efficient in low-metallicity stars. The meridional mixing model predicts such a phenomenon (Sweigart and Mengel 1979). ... [E]ssentially the model predicts that the regions in which CN and ON processing occurs extend further from the hydrogen-burning shell in low-metallicity stars. This leads to more CN-processed material getting mixed into the surface layers of such stars. One problem with this model is that it fails to reproduce C and N variations low on the giant branch. It is worth noting in this context that CN variations are present in main-sequence stars in 47 Tuc [NGC 104] and NGC 6752 (Bell et al. 1983; Suntzeff 1989; Briley et al. 1991), and no theoretical models predict that mixing can dredge up CN-processed material when stars are still on the main sequence" (Ashman and Zepf 1998, p. 19).

Observation

An easy target for 7 x 50 binoculars and a great sight in any aperture telescope. 100 mm will begin to resolve the cluster into its component stars. Scattered outliers reach to about 25' in diameter, with the density steadily increasing towards the very bright centre.

References

Ashman, Keith M.; Zepf, Stephen E. 1998: Globular Cluster Systems. Cambridge University Press, 171 pp.

Bell, R.A.; Hesser, J.E.; Cannon, R.D. 1983: Astrophys Jl. 269: 580.

Briley, M.M.; Hesser, J.E.; Bell, R.A. 1991: Astrophys Jl. 373: 482.

Burnham, Robert 1992: Burnham’s Celestial Handbook. Volume 3 – Pavo to Vulpecula. Dover.

Djorgovski, S. 1993: Physical Parameters of Galactic Globular Clusters. In Djorgovski, S.G.; Meylan, G.: Structure and Dymanics of Globular Clusters. ASPC Series, Vol. 50.

Malin, David 1993: A View of the Universe. Cambridge.

Malin, David; Frew, David 1995: Hartung’s Astronomical Objects for Southern Telescopes. 2nd ed.

Pryor, Carlton; Meylan, Georges 1993: Velocity Dispersions for Galactic Globular Clusters.  In Djorgovski, S.G.; Meylan, G.: Structure and Dymanics of Globular Clusters. ASPC Series, Vol. 50.

Suntzeff, N. 1989: In Cayrel, G. et al. (eds.) 1989: The Abundance Spread Within Globular Clusters: Spectroscopy of Individual Stars. Observatoire de Paris, p. 71.

Sweigart, A.V.; Mengel, J.G. 1979: Astrophys Jl. 229: 624.

Tirion, Wil 1991:  Cambridge Star Atlas 2000.0. Cambridge University Press.