Terzan 5 is a massive, dense stellar system in the Milky Way's bulge, reclassified as a "bulge fossil fragment" because it contains multiple generations of stars, unlike typical globular clusters. It's a relic from the early galaxy, containing up to four distinct star populations and a large number of millisecond pulsars, making it a key object for understanding how galaxies form and evolve.
Location: Orbits within the crowded central bulge of the Milky Way, about 22,000 light-years away in the constellation Sagittarius.
Classification: Once thought to be a globular cluster, it's now considered a "bulge fossil fragment"—a remnant of a much larger, primordial clump that contributed to the bulge's formation.
Star populations: Contains multiple generations of stars, with evidence for up to four distinct populations, showing a complex star formation history.
Mass and density: Extremely massive and dense, with about 2 million solar masses in a small area, allowing it to retain gas and dust for new star formation.
Pulsars: Home to a large population of millisecond pulsars, including the fastest known one, due to its high stellar density and collision rate.
X-ray sources: Also contains numerous X-ray sources, including an X-ray burster, likely formed from the high rate of stellar collisions.
Galactic evolution: Its survival as a separate entity provides a unique window into the early, chaotic formation of the Milky Way's bulge.
Self-enrichment: Its ability to form multiple generations of stars shows it was massive enough to retain heavy elements from supernova explosions, a process called self-enrichment. Image Credit NASA/ESA/Hubble/F. Ferraro
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