Liller 1

Lore & Background

Liller 1 is located within the galactic bulge and is heavily obscured by dust, being close to the galactic plane. Studies of this object have mostly been conducted in wavelengths other than the optical range. The absorbing clouds of dust are not uniform, and the extinction coefficient RV is 2.5, less than the typically assumed value of 3.1. After Terzan 5, it has the highest rate of stellar collisions of any Milky Way globular cluster. It also has the highest level of emission of gamma rays of any globular cluster, which may be due to a large number of stellar collisions and pulsars, but may also be an unrelated background gamma ray source. Liller 1 and Terzan 5 are remarkably similar. Although globular clusters are typically metal-poor, both have a relatively high metallicity; the abundance of metals for Liller 1 is estimated to be half that of the Sun. In 2021, Liller 1 was found to have two different stellar populations, like Terzan 5. One is about 12 billion years old, but the other is relatively young, at 1 to 3 billion years. This suggests that it may not be a true globular cluster, but a class of star cluster that coalesced to form galactic bulges, known as 'bulge fossil fragments'. The globular cluster contains the rapid burster called MXB 1730-335.

Reader's Guide

Liller 1's significance lies in its extreme properties and its challenge to traditional views of globular clusters. Its high mass (2.3 million solar masses) places it among the most massive globular clusters known, alongside ω Centauri and Terzan 5. Its record-breaking gamma-ray emission and high rate of stellar collisions make it a unique laboratory for studying high-energy astrophysical processes, though the gamma-ray source may be unrelated. The discovery of two distinct stellar populations—one ancient (12 billion years) and one relatively young (1–3 billion years)—suggests that Liller 1 may not be a true globular cluster but rather a 'bulge fossil fragment,' a relic of the formation of galactic bulges. This classification, shared with Terzan 5, has implications for understanding how galaxies like the Milky Way assembled their central regions. Its heavy obscuration by dust has forced astronomers to study it primarily in non-optical wavelengths, highlighting the importance of multi-wavelength astronomy. The presence of the rapid burster MXB 1730-335 adds to its astrophysical interest.

Did You Know?

• Liller 1 was discovered by American astronomer William Liller in 1977.
• Liller 1 contains two stellar populations: one about 12 billion years old and another 1 to 3 billion years old.
• It has a mass of around 2.3 million solar masses, making it one of the most massive globular clusters.