Determining the age of the sun and other stars is more difficult than previously thought. The age of the sun determined by asteroseismology fluctuates with the solar cycle and therefore deviates by about 300 million years depending on the time of measurement, as astronomers have discovered. For other, more active stars, the deviations could be even higher, and unlike the Sun, there is often no alternative method for determining their age.
How do we know how old a star is? In the case of our sun, we don’t even need to use astronomical techniques. Its age was primarily determined by dating the oldest minerals in the solar system—found, for example, in meteorites or captured interplanetary dust. These mineral grains originated in the same primordial cloud and almost simultaneously with our home star.
Stellar Vibrations as Measurement Aids
However, this is more difficult for other stars: While their chemical composition, temperature, and luminosity provide initial clues, astronomers rely on asteroseismology for more accurate dating. This involves analyzing subtle, periodic oscillations in the star’s light. “You have to imagine a star as a large ball of gas that is in constant motion. Inside, there are currents and waves that make it vibrate,” explains first author Jérôme Bétrisey from the University of Geneva.
These vibrations depend on the mass and structure of the star, but also on its age. “With asteroseismology, we can determine key parameters of individual stars with a precision that is not achievable with other standard techniques,” explain Bétrisey and his colleagues. Using special models, the star’s properties can be determined from the frequency and type of stellar vibrations. These models also take into account some disruptive effects that influence the stellar oscillations.
What Role Does the Solar Cycle Play?
However, one factor has largely been ignored so far: the stellar magnetic field. From the Sun, we know that its magnetic field changes over the course of the solar cycle—there are changes in magnetic intensity as well as in polarity. Until now, this has not been taken into account in asteroseismological models; it was considered negligible, as Bétrisey and his colleagues explain.
The astronomers have now checked whether this is really the case. To do this, they analyzed two time series of asteroseismological data going back more than 26 years, recorded by two solar observatories—one in space and one on Earth. The data series thus covered two complete solar cycles. Using a common model, the researchers then determined the helioseismic age of the sun at 95 time points, each three months apart.
Age Deviations of Up to 6.5 Percent
The surprising result: The dating of the sun fluctuated more than expected. Depending on the time of measurement, the determined solar ages varied from each other by up to 6.5 percent. The approximately 4.6 billion-year-old Sun is thus 300 million years older or younger depending on the time of measurement. “Given the required precision of asteroseismology, this is quite significant,” state Bétrisey and his colleagues.
The cause was, as suspected, changes in the solar magnetic field over the course of the solar cycle: Whenever solar activity is near its maximum, the intensity of the solar magnetic field increases and also influences the stellar oscillations. As a result, the common models estimate the age to be higher during these times. Near the solar minimum, on the other hand, the asteroseismological datings yield lower values.
Even More Serious for Distant Stars
The age determination of distant stars could be significantly distorted by this effect: “These observations usually cover only part of the stellar activity cycle, and the time series could be too short to average out the magnetic field influence,” the astronomers explain. In addition, the Sun is a relatively quiet, inactive star; the activity of other stars fluctuates considerably more. “For these stars, the influence of magnetic activity could therefore be much more significant,” says Bétrisey.
This means: Astronomy must now adapt the common asteroseismological models to include the influence of stellar magnetic fields. At the same time, methods are needed to better estimate the current cycle phase of a star, as the team explains. “It is urgently necessary that we capture robust stellar parameters for magnetic activity,” the astronomers write.