Astronomical and Imaging Data

Useful Informations

BH 261 (also known as AL 3 or vdB-Ha 261) is an ancient globular cluster located within the dense and dusty environment of the Milky Way’s Galactic Bulge. Originally discovered as a faint candidate in 1967 by Andrews and Lindsay, its true nature as a globular cluster was only confirmed in 2006 through deep photometric studies that revealed its distinct stellar population.

Discovery and Obscuration

For decades, BH 261 was cataloged as a simple “open cluster” because it is projected against a very rich field of background stars in the constellation Sagittarius.

• The Bulge Filter: Because it sits in the direction of the Galactic Center, the cluster is heavily “contaminated” by foreground disk stars and the dense stellar population of the bulge.
• Reddening: It suffers from significant interstellar reddening (E(B-V) = 0.36), meaning dust grains between us and the cluster scatter its blue light, making the stars appear redder and dimmer than they actually are.

Physical and Chemical Profile

Scientific analysis of BH 261 has identified it as a “metal-poor” system, which is somewhat unusual for clusters located so deep in the predominantly metal-rich bulge.

• Metallicity: Its iron-to-hydrogen ratio ([Fe/H]) is approximately -1.3, making it about 20 times less metal-rich than the Sun.
• Age: It is one of the oldest objects in the Galaxy, with an estimated age of roughly 12 to 13 billion years.
• Stellar Population: The cluster features a prominent Blue Horizontal Branch (BHB). This is a specific group of helium-burning stars that have lost most of their hydrogen envelopes, a classic signature of very old, metal-poor globular clusters.

Location and Dynamics

BH 261 is a relatively “compact” but low-luminosity cluster.

• Distance: It is located approximately 21,000 to 23,000 light-years (about 6.5 to 7.1 kiloparsecs) from Earth.
• Galactocentric Position: It sits roughly 10,000 light-years from the center of the Milky Way.
• Mass: It is a low-mass cluster, estimated at around 24,000 solar masses. Because of this small size, it is highly susceptible to tidal stripping—the process where the Galaxy’s gravity pulls stars away from the cluster’s edges.

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Scientific Importance

BH 261 is a key subject for the Milky Way Bulge Extra-tidal Star Survey. Because it is currently being slowly torn apart by the Galaxy’s gravitational tides, astronomers use it to study how “extra-tidal stars” (stars that have escaped the cluster) contribute to the overall population of the Galactic Bulge. It serves as a “fossil” that helps us reconstruct the chaotic early history of how the center of our galaxy was assembled.

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