Astronomical and Imaging Data

Useful Informations

The globular cluster NGC 6256 is a very ancient and highly obscured stellar system located in the southern constellation Scorpius. It is scientifically significant due to its extreme properties and complex stellar population.

Location and Extinction

NGC 6256 is an example of a bulge globular cluster, meaning its orbit and current position keep it confined within the central region of the Milky Way galaxy.

• Distance and Reddening: The cluster is approximately 22 thousand light years from the Sun. Its location toward the Galactic center means its light is severely diminished and reddened by large amounts of intervening interstellar gas and dust. This interstellar extinction makes it appear much fainter than it intrinsically is, giving it an apparent visual magnitude of 11.3.
• Orbital Properties: Its orbital characteristics are consistent with a low-eccentricity orbit entirely contained within the Galactic bulge, suggesting it is a native component of the Galaxy’s old stellar population.

Physical Characteristics

NGC 6256 is an extremely old system, providing a fossil record of the Galaxy’s early assembly.

• Age: It has an estimated age of about 13.0 billion years, placing it among the oldest structures in the Milky Way.
• Metallicity: It is classified as a metal-poor cluster, with an iron abundance ratio, [Fe/H], of approximately minus 1.02 or possibly lower, though early estimates varied significantly due to the extinction.
• Concentration: NGC 6256 is a highly concentrated cluster. It is classified with a concentration class of I or II on the Shapley–Sawyer scale (where I is the most concentrated).
• Core Collapse: Its structure is that of a post core-collapse (PCC) cluster. This means its central region has undergone a runaway dynamical process where the stellar core has contracted severely, leading to an extremely high central density. This collapse is a key factor in its complex stellar population.
• Unique Stellar Populations: The cluster’s color magnitude diagram reveals a complex population of stars. Most notably, it is one of the clusters found to harbor multiple sequences of blue straggler stars, which are anomalously hot and bright stars believed to be formed through stellar collisions or binary star mergers. These sequences provide clues about the cluster’s dynamical evolution, particularly the intense environment created by its core collapse.

Energetic Contents

NGC 6256 is a host to exotic stellar remnants. It is known to contain a millisecond X-ray pulsar, designated IGR J16597-3704, a type of highly magnetized, rapidly spinning neutron star that is accreting matter from a companion star. These objects are often found in the high-density environments of core collapsed globular clusters.

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