The Age of the Milky Way

1. The Age of the Milky Way NASA/JPL-Caltech/R. Hurt

2. Disk Bulge Halo The Milky Way has many parts, each of which may have a different age. Globular clusters Credit: 2MASS Project

3. 10-3 Space Density (Number per cubic parsec) 10-4 10-5 Harris et al. (2006) To date, completed white dwarf studies target white dwarfs in the Milky Way disk.

4. 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

5. 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)

6. 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)

7. 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 star’s age. • HE 1523-0901: 10-14 Gyr • CS 31082-001: 12-16 Gyr

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

9. 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.