Astronomical and Imaging Data

Useful Informations

The globular cluster Pyxis is an ancient, sparse star cluster located in the distant outer halo of the Milky Way galaxy. Discovered relatively recently in 1995 in the constellation Pyxis, it is notable for its remote location and kinematic properties, which suggest it may have an extragalactic origin.

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Key Physical Characteristics

• Classification: Pyxis is classified as a globular cluster, meaning it is a spherical collection of old stars tightly bound by gravity. It is considered one of the less massive or “wimpy” globular clusters, with an estimated mass of only about 30,000 solar masses (equivalent to roughly 30,000 stars).
• Age: The cluster is an extremely old object, with an estimated age of approximately 11.5 to 13.3 billion years (Gyr). It is thought to be slightly younger than some of the most ancient globular clusters located closer to the galactic center, suggesting a different formation history.
• Metallicity: Pyxis is metal-poor, a common trait for old halo globular clusters. Its metallicity, expressed as the iron-to-hydrogen ratio relative to the Sun, is typically measured to be around [Fe/H] = −1.2 to −1.4 dex. This means its abundance of elements heavier than helium is about 1/16 to 1/25 that of the Sun.

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Location and Galactic Context

• Distance: Pyxis is located at a large distance from Earth, approximately 130,000 light-years (40 kpc) away.
• Galactic Location: Its distance from the center of the Milky Way is even greater, around 133,000 light-years (41 kpc). This remote location places it squarely in the outer galactic halo, a region where the density of globular clusters drops off significantly.

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Kinematics and Origin Hypothesis

• Radial Velocity: Pyxis has a measured radial velocity (motion along the line of sight) of around +36 km/s relative to the Sun. With respect to a stationary observer at the Sun’s position, this corresponds to a velocity of about −191 km/s.
• Orbital Path: The cluster is currently on a highly eccentric orbit around the Milky Way. Its path is theorized to carry it to distances as close as about 98,000 light-years and as far out as approximately 330,000 light-years from the Galactic center—a distance greater than that of the Large Magellanic Cloud (LMC).
• Extragalactic Origin: Its extreme orbit, location, and presence along the orbital plane of the Large Magellanic Cloud strongly suggest that Pyxis may not be an original Milky Way cluster. The leading hypothesis is that it is a former globular cluster of the LMC or another dwarf galaxy that was subsequently captured by the Milky Way’s gravitational pull. Its study is therefore important for understanding the Milky Way’s history of accretion and merger events.

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