The Age of the Milky Way

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The Age of the Milky Way
NASA/JPL-Caltech/R. Hurt
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The Milky Way has many parts, each of which may
have a different age.
Globular clusters
Credit 2MASS Project
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To date, completed white dwarf studies target
white dwarfs in the Milky Way disk.
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Ages of the Milky Way Disk from White Dwarfs
Winget et al. (1987) 7-11 billion years Liebert
et al. (1988) 6-11 billion years Hansen
(1999) 6-11 billion years Fontaine et al.
(2001) 8-11 billion years
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Ages of Globular Clusters From White Dwarfs
Messier 4 10-14 billion years (Hansen et al.
2004)
NGC 6397 11-12 billion years (Hansen et al. 2007)
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Ages of Globular Clusters from the Main Sequence
NGC 6752 10-12.5 Gyr Messier 5 8-10
Gyr Messier 13 9.5-12.5 Gyr (Chaboyer et al.
1998) Messier 15 12-15 Gyr (McNamara et al.
2003)
M12
Hargis et al. (2004)
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Radioactive Dating of Stars
Frebel et al. (2007)

• Although stars have differing amounts of metals,
  they tend to have the same ratios.
• In very old stars, radioactive elements like U
  and Th are depleted depending on stars age.
• HE 1523-0901 10-14 Gyr
• CS 31082-001 12-16 Gyr

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The Age of the Universe
Age from combining best data sets 13.58-13.89
Gyr Spergel et al. (2007)
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Conclusions

• The Universe is older than its component parts
• The globular clusters and oldest halo stars were
  formed within a couple billion years of the Big
  Bang and are consistent with each other.
• The ages of globular clusters as derived from
  main sequence stars and white dwarfs are
  consistent.
• The disk of the galaxy is younger than the
  globular clusters and halo stars.

The age of the Milky Way Different parts have
different ages. The oldest parts of the Milky
Way are at 11-13 billion years old, and the disk
of the Milky Way is 6-11 billion years old.