Stellar Ages
Age, like mass, and composition, is one of the important characteristics
we'd like to know about a star. From examining the properties of stars
of different ages, we can infer the evolutionary history of a star from
its birth to its eventual demise, whether that will be a white dwarf (for
stars like the Sun), neutron star, or black hole.
This requires recognizing other traits that might indicate the relative
age of a star and then comparing those traits to stars of known age. Fortunately,
there are two traits that the HK Project can observe which can be used
to infer a star's age: rotation and activity. Both of these change noticeably
as a star ages. The mean level of activity decreases with age, and rotation
slows. For example, when the Sun was only 1 billion years old, its rotation
period was close to 8 days instead of 25. However, precisely determining
a star's age requires that accruate relationships between observations
of rotation and activity and age be developed.
Using activity to measure age
The HK Project typically observes the activity of 50 stars every clear night.
One measurement takes about 3 to 5 minutes. So, in principle, every time we
make these measurements, we're possibly learning about the age of a star.
However, there are many processes that change activity: rotation,
activity cycles, and even
extra-cyclic activity. Therefore,
while one measurement of a star's activity provides an indication of a star's
age, to accurately pinpoint it, we'd need to observe over a very long time
to determine the mean activity level to use.
Using rotation to measure age
Since stellar rotation slows with age, we could use this instead. While
activity varies about a mean, rotation is relatively constant, although
period modulations should occur in the presence of surface differential
rotation. However, to measure rotation, we need to be able to detect rotational
modulation, and for some stars, particularly older ones, an active region
which is large enough to be picked up by the HK Project instrument might
not occur for a very long time. In addition, many observations need to
be scheduled to follow rotation enough to calculate the rotation period,
and this means that fewer stars can be observed with the telescope time
available.
Making an age calibration
Models of stellar evolution show the progression of a star's
luminosity and surface temperature with time, dependent on its mass. While
measuring the luminosity and temperature of a single star without knowing
the mass won't necessarily give an accurate measurement of its age, there
are several ways we can improve our accruacy. Star clusters have dozens
of stars which all formed at nearly the same time. Therefore, plotting
their luminosity and temperatures will show a distribution that can be
tied to a single age. We can then measure these stars' activity and rotation,
thereby providing the link we need to measure the age of field stars.
However, only the closest star clusters are close enough to have stars
which are bright enough to be observable with the HK Project's instrument.
Nonetheless, it has provided a good place to start. Another source of
information is binary stars. Even though they have only two stars instead
of the dozens in star clusters, each star should have the same age.
Ages in the solar neighborhood
The figure below shows the distribution of age of dwarfs stars near the
Sun's mass in the solar neighborhood. Younger stars are at the top. Lines
connect stars in binary systems. If the age calibration were perfect for
all stars, then all of these lines would be horizonal indicating that
the determined ages of each component are equal (as they should be). When
the lines have a large slope, it indicates that one (or both). of the
stars is at an extreme phase of its normal activity variation. Since all
the stars in the plot could also be varying, the distribution of observed
age differences in these binary systems gives us an indication of the
precision of our age calibration. For the most part, the age difference
is less than 1 billion years, and is typically below 500 million years.
While this isn't incredibly accurate, it suggests that even one measurement
of a star's activity should be useful in determining if the star is old
or young.
The filled circles in the figure show the Sun at different phases of
its activity variations. (From top: top of the activity cycle, mean of
Cycles 21 and 22, mean of Cycle 20, activity minimum, and estimated activity
during the Maunder Minimum.) If we were to make only one observation of
the Sun as a star, it would fall somewhere in this range. From geologic
records, we estimate the age of the Earth (and therefore the Sun) to be
just over 4.5 billion years. However, the range of ages determined from
the range of solar activity and our age calibration would give ages between
3 and 8 billion years! This shows the importance of long-term measurements
to properly determing the mean level of activity needed to accurately
measure the star's age.
Derived ages of dwarf stars in the solar neighborhood. Lines connect binaries. The series of filled circles show the Sun at different phases of its activity.
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News from Mount Wilson: 1/6/2012 - U.S. Forest Service undertakes a fire fuels reduction program on Mount Wilson - More here. 12/1/2011 - The Observatory's Cosmic Cafe is closed for the winter. We will reopen the Cafe and resume our 1 pm guided tours on April 1, 2012.
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