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The Age of the Milky Way
NASA/JPL-Caltech/R. Hurt
Dedicated to Dr. James Liebert
In many sciences, knowing ages is
crucial to interpreting data.
• In medicine, a person’s age impacts the types and forms of
illness, and therefore interpretation of symptoms.
• In geology, the ages of rock formations can tell us about ice
ages, magnetic pole reversals, volcanic activity, and climate.
• In paleontology, ages of fossils are needed to understand
evolution.
• In archaeology, ages of finds are crucial for dating events and
tracing human activity.
In astronomy, knowing ages of objects is vital
to understanding their formation, evolution,
and underlying physics.
The Milky Way has many parts, each
of which may have a different age.
Globular clusters
Disk
Halo
Bulge
Credit: 2MASS Project
Ages of globular clusters can be obtained
from the main sequence.
M12
NGC 6752:
Messier 5:
10-12.5 Gyr
8-10 Gyr
Messier 13:
9.5-12.5 Gyr
(Chaboyer et al. 1998)
Messier 15:
12-15 Gyr
(McNamara et al. 2003)
Hargis et al. (2004)
Ages of globular clusters can also be obtained
from white dwarfs (thankfully, they agree).
Messier 4: 10-14 billion
years (Hansen et al. 2004)
NGC 6397: 11-12 billion
years (Hansen et al. 2007)
Radioactive dating of stars has constrained the
age of the Milky Way’s halo.
Frebel et al. (2007)
•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
The age of the disk of the Milky Way is hard
to derive, because the disk has been making
new stars constantly for billions of years.
LDN 1622
One idea: look for the oldest white dwarfs
(we’ll learn how and why this week).
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
The age of the Universe is the ultimate
constraint on any theory.
Age from combining best
data sets:
13.58-13.89 Gyr
Spergel et al. (2007)
Conclusions
1. The Universe is older than its component parts
2. The globular clusters and oldest halo stars were formed
within a couple billion years of the Big Bang and are
consistent with each other.
3. The ages of globular clusters as derived from main sequence
stars and white dwarfs are consistent.
4. 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.