Tucanae 47 – A Complex Globular Cluster

OTA:__________CDK17

Camera:_______SBIG STXL11002 with AOX and FW8G (0.63 arsec/pxl)

Observatory:___ Heaven’s Mirror, Chile

EXPOSURES:

Red:______3 x 30, 3 x 60, 6 x 600 sec.

Blue:______3 x 30, 2 x 60, 6 x 600

Green:____4 x 30, 4 x 60, 7 x 600

Total exposure: 3.4 hours

Image Width: 35 arc-minutes

Processed by Alex Woronow (2020) using PixInsight, 3DLuts, Topaz, SWT

The globular cluster Tuc 47 is the second brightest globular cluster in our skies (after Omega Centauri ), and visible to the naked eye, for those who can access a declination of -72 deg. ( Omega Centauri also is a Southern-sky object at a dec. of -47 degrees.) Tuc 47 contains a dense, difficult to resolve, core. This image used 3 different exposures to create an HDR image that reveals stars in, (actually, probably mostly above) the core. (HDR = High Dynamic Range. A Pixinsight process created this HDR image.)

This cluster is a very unusual place! It hosts 21 blue stragglers, hundreds of X-ray sources, two populations of stars with different metallicities, cataclysmic variable stars, neutron stars, a millisecond pulsar, and, likely, a central black hole. To clarify a bit, globular clusters have predominantly old, reddish stars (despite many amateurs rendering showing them as largely blue). This old population of stars conforms to the antiquity of these clusters: the older the cluster, the redder/older its average star population. But some young stars, called “blue stragglers” often occur too. Perhaps, the young blue stars form from the collision of two stars in the dense clusters. But other explanations have been put forward to explain their birth, and none have been proven correct, yet.

Cataclysmic variable stars, you ponder? These arise from a very tight binary pair where one of the stars is a white dwarf. Hydrogen-rich mass transfers from the “normal” member of the binary to the white dwarf, eventually triggering a classical nova, but unlike a nova, this process can repeat many times until the dwarf reaches a critical mass (the Chandrasekhar limit) and disintegrates in a Type 1 Supernova.

High metallicity stars have a relatively high abundance of elements beyond H and He: C, N, O, and Ne. Older stars formed in a young universe largely free of heavier elements, therefore, they have, “low metallicities.” Younger stars, born of the debris of older stars that synthesized heavier elements in their core, and then exploded, have higher metallicities. Tuc 47 has both generations of stars.

Evidence also exists that the more massive stars in Tuc 47 have “settled” to its center, thereby segregating the stars by mass. According to Brogaard, et al. (2017) Tuc 47 is about 11.8 Gyr old while Zoccali, et al. (2001) had estimated the age as 13 Gyr. In either case, that gives TUC 47 beaucoup time to sort its stars any way it wants!

There’s a zoomed flight to Tuc 47 that shows several concentric spheres delineated by rather obvious changes in star densities or mean hue. Some similar features appear in my image. https://www.spacetelescope.org/videos/heic0616a/