Ages of stars in the Milky Way
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Age Distribution of Stars in the Milky Way
The ages of stars in the Milky Way vary widely depending on their location and population. Studies using large surveys and advanced techniques have provided a clearer picture of how stellar ages are distributed across the Galaxy 1235.
Milky Way Bulge: Predominantly Old Stars
Stars in the Milky Way bulge are generally much older than those in the solar neighborhood. The average age of bulge stars is about 8 billion years (Gyr), roughly twice as old as stars near the Sun, which average around 4 Gyr 1. Most metal-rich bulge stars are older than 8 Gyr, but there is a notable population of younger stars (2–5 Gyr) found mainly in the plane of the bulge 2. This suggests the bulge experienced an intense early starburst, followed by ongoing star formation in certain regions until about 2 Gyr ago 2.
Galactic Disk: Age Gradients and Star Formation History
The disk of the Milky Way shows a wide range of stellar ages. The thick disk began forming stars about 13 Gyr ago, only 0.8 Gyr after the Big Bang, and most of these stars formed around 11 Gyr ago, coinciding with a major merger event (Gaia-Sausage-Enceladus) 367. The thin disk, in contrast, contains younger stars, with ongoing star formation and chemical enrichment over the past several billion years 358.
A vertical age gradient exists in the disk: older, low-mass red giants are found at greater heights above the Galactic plane, while stars closer to the plane have a broader range of ages and metallicities 9. The age distribution in the disk is smooth over the last 10 Gyr, indicating mostly steady evolution 9.
Halo Stars: Ancient and Accreted Populations
The stellar halo is dominated by very old stars, with most being older than 10 Gyr 67. Both the in situ halo (formed within the Milky Way) and the accreted halo (from merged galaxies like Gaia-Enceladus) share similar age distributions, with a sharp cutoff at about 10 Gyr ago, marking the time of major accretion events 6710. Accreted halo stars typically range from 8 to 13 Gyr in age, with the oldest being the most metal-poor 10.
Age-Metallicity and Kinematic Trends
There is no tight correlation between age and metallicity for high-alpha disk stars, which formed over more than 2 Gyr with a wide range of chemical abundances 8. Younger, metal-rich stars in the bulge and disk tend to have kinematics consistent with rotation, while older stars show more dispersion in their velocities 28.
Overall Age of the Milky Way
The oldest stars in the Milky Way formed about 13 Gyr ago, setting an upper limit on the Galaxy’s age 34. The Galaxy has experienced continuous star formation and chemical enrichment since then, with different regions and populations showing distinct age distributions 12356789+1 MORE.
Conclusion
The Milky Way contains stars spanning nearly the entire age of the Universe, from ancient halo and thick disk stars formed over 10 billion years ago to much younger stars in the thin disk and bulge. The age distribution reflects the Galaxy’s complex formation history, including early starbursts, mergers, and ongoing star formation.
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