Age variability: Determining the age of the Sun and other stars is harder than previously thought. Astronomers have discovered that the Sun’s age, determined using astroseismology, fluctuates with the solar cycle, thus deviating by about 300 million years depending on when it is measured. For other, more active stars, the deviations can be much higher—and unlike the Sun, there is no alternative way to determine its age.
How do you know how old a star is? In the case of our Sun, we don’t even have to use astronomical techniques to do this: its age has been determined primarily by dating the oldest minerals in the solar system — those found, for example, in meteorites or in captured interplanetary dust. Because these mineral grains formed in the same primordial cloud and at roughly the same time as our original star.
Stellar vibrations as a means of measurement
However, this is more difficult with other stars: their chemical composition, temperature and luminosity provide only preliminary clues. However, astronomers rely on astroseismology to obtain a more accurate dating. Subtle periodic oscillations in the star’s light are assessed. “You have to imagine a star as a big ball of gas in constant motion. Inside it are currents and waves that make it vibrate,” explains lead author Jérôme Bétrici of the University of Geneva.
These oscillations depend on the mass and structure of the star, but also on its age. “Using astrometry, we can determine the main parameters of individual stars with a precision that is not possible with other standard techniques,” Petreci and colleagues explain. Using special models, the properties of stars can be determined from the frequency and type of stellar oscillations. These models also take into account some of the disturbing effects that affect stellar oscillations.
What role does the solar cycle play?
But there is one factor that has so far remained largely unnoticed: the stellar magnetic field. We know from the Sun that its magnetic field changes over the course of the solar cycle – there are changes in magnetic intensity, but also in polarity. Until now, this has not been taken into account in astroseismic models, as it was considered negligible, Petreci and his colleagues explain.
Astronomers have now verified whether this is indeed the case. To do this, they evaluated two time series of astronomical data going back 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 Sun’s heliospheric age at 95 points in time, three months apart.
Age deviations reach 6.5 percent.
The result was surprising: The Sun’s timing fluctuated more than expected. Depending on when the measurements were made, the determined solar ages varied by up to 6.5 percent. The Sun, which is about 4.6 billion years old, was sometimes 300 million years older, and sometimes younger, depending on when the measurements were made. “In light of the precision that astronomy requires, this is very important,” Petreci and his colleagues say.
As expected, this is due to changes in the solar magnetic field over the solar cycle: as solar activity approaches a maximum, the solar magnetic field becomes stronger and also affects stellar oscillations. As a result, common models estimate that the age is highest during these times. However, near solar minimum, astroseismic dating yields lower values.
Even more serious with foreign stars
This effect can significantly distort the determination of the age of distant stars: “These observations typically cover only a part of the stellar activity cycle, and the time series may be too short to calculate the average effect of the magnetic field,” the astronomers explain. In addition, the Sun is a relatively quiet and inactive star, and the activity of other stars fluctuates more. “For these stars, the effect of magnetic activity can be much more important,” says Petrisi.
This means that astronomy must now adapt common astronomical models to include the influence of stellar magnetic fields. At the same time, methods are also needed to better estimate the star’s current cycle phase, the team explains. “It is urgent to capture robust stellar parameters of magnetic activity,” the astronomers wrote.
Stellar dating also plays an important role in the search and classification of exoplanets. (Astronomy and Astrophysics, 2024; doi: 10.1051/0004-6361/202451365)
Source: University of Geneva
August 21, 2024 – Nadia Podpregar
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