Astronomical and Imaging Data

Useful Informations

UKS 1 (also known as UK Schmidt 1 or BH 240) is a notable globular cluster located in the constellation Sagittarius. Situated deep within the Milky Way’s Galactic bulge, it held the record for several decades as the “faintest” or “dimmest” known globular cluster in our galaxy before more recent infrared surveys discovered even more obscured objects.

Galactic Context and Location

UKS 1 is located at a distance of approximately 25,000 to 26,000 light-years (about 7.8 kiloparsecs) from Earth. It sits extremely close to the Galactic Center, with a galactocentric distance of only about 2,300 light-years (0.7 kpc).

Because of its position in the “inner city” of the Milky Way, it is heavily obscured by interstellar dust. In visible light, the cluster is almost entirely hidden; however, it is a bright and prominent source in the near-infrared spectrum. This high level of “extinction” means that its light is both dimmed and shifted toward redder wavelengths before reaching our telescopes.

Chemical Composition and Age

UKS 1 is considered an intermediate metallicity cluster, bridge-building between the very metal-poor halo clusters and the highly metal-rich bulge population.

• Metallicity ([Fe/H]): Modern spectroscopic measurements place its iron abundance at roughly -0.64 to -0.78 dex. This level of enrichment suggests it formed from gas that had been pre-processed by early generations of stars.
• Alpha-Elements: The cluster shows significant enrichment in alpha-elements ([alpha/Fe] = +0.31). This chemical signature is a classic indicator of rapid star formation in the very early history of the Galaxy, likely over 12 billion years ago.
• Multiple Populations: Recent studies using the Hubble Space Telescope and APOGEE-2 survey have found evidence of “nitrogen-enhanced” stars within UKS 1. This proves that, like many massive globular clusters, it contains multiple generations of stars rather than a single group born all at once.

Structural Parameters and Dynamics

The cluster is classified as highly concentrated, with a concentration parameter (c) of approximately 2.1. It is often discussed as a candidate for a post-core-collapse cluster. This means the gravitational interactions between its stars have caused the core to contract to an extremely high density, resulting in a sharp “cusp” of light at its center rather than a flat, spread-out core.

• Radial Velocity: It moves toward the Sun at approximately +57 km/s.
• Radius: It has a core radius of only about 0.15 arcminutes, illustrating how tightly packed its central stars are.

Scientific Significance

UKS 1 is scientifically important as a “tracer” of the bulge’s formation. Furthermore, it is part of a potential binary globular cluster system. It is positioned very close to another cluster, VVV CL001, leading astronomers to investigate whether the two are gravitationally bound to one another as they orbit the Galactic Center—a rare phenomenon in our galaxy.

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