Saturday, July 12, 2014

Gyrochronology: Dating the spinning stars

False-color ultraviolet image of our Sun, which currently rotates once
every 24.47 days.
In the ongoing search for extraterrestrial life, scientists have been cataloging planets and stars that resemble our own Earth and Sun, particularly in regards to size, mass, temperature, and distance from each other.  A group of astronomers argue that age should also be taken into consideration when classifying “Sun-like” stars, and they have developed a new method for estimating the age of the star based on its rotation period called “gyrochronology”.

Just like planets, stars rotate around a central axis as they travel through the arms of their galaxy.  Also like planets, their speed of rotation slows over time, though likely due to different physical effects.  The scientists studying these stars proposed that the rotation rate could therefore be used to determine the age, as younger stars should rotate faster than older ones.  Using data from the Kepler spacecraft, the scientists then measured regular pulses in the brightness of distant stars.  They reasoned that these tiny variations were caused by dark spots (like our “sun spots”) on the surface of the stars rotating in and out of view.  Plotting the rotation periods against “known” star ages gave the researchers a scale with which to measure the age of other stars, with particular emphasis on dwarf stars, which are more similar in size to our Sun.  In their study, the astronomers claim to have found 34 “solar analogs” (roughly like our Sun) 22 of which are considered possible “solar twins” (very similar to our Sun).

Although the paper published by the team calls gyrochronology an “independent age derivation,” it still quite clearly relies on other dating methods such as isochrone dating (which uses a star’s brightness and temperature) and astroseismology (which uses a star’s internal structure) to produce the scale with which other stars were measured.  Even then, the methods do not match up particularly well.
Inset from Figure 3 from do Nascimento et al. (2014) plotting asteroseismic ages
against gyrochronologic ages.  The gray area indicates errors less than 20%.
The crossed lines presumably show errors for the individual measurements.
Figure 3 of their paper attempts to show the correlation between gyrochronologic and asteroseismic ages of a particular group of stars, but it is not a very convincing one (r = 0.79).  While some of the stars have similar ages derived from both methods, others do not.  The leftmost star on the graph shows an asteroseismic age of around 1 billion years, yet the calculated gyrochronologic age is almost 3 billion years.  The paper’s appendix reveals more grievous errors as well.  The first star listed in Table 1 has an asteroseismic/isochrone age range of 0.4–2.9 billion years and a gyrochronologic age of 3.91–11.91 billion years.  The second star in the table is reversed, with a 4.2–8.2-billion-year asteroseismic/isochrone age and a 1.37–2.37-billion-year gyrochonologic age.  It seems, then, that even if gyrochronology is a legitimate dating method, it is not a reliable one at all.

Without the numbers given by other dating methods, gyrochronology is at best a relative dating method.  It can determine which stars are older or younger than others, but not their actual ages.  Shouldn’t creationists expect all stars to be the same age, though?  Not necessarily.  While the Bible does indicate that God created stars on Day 4 of Creation Week, it does not preclude more stars from forming since then, just as it does not contradict the birth of more people, animals, and plants.  Interestingly, two of the leading current creationist models of star formation allow for stars with ages of millions to billions of years anyway.  Humphrey’s cosmological model allows for it due to relativistic effects involved in the stretching of space, and the Anisotropic Synchrony Convention model proposes that the Bible defines time differently than modern cosmology.  I cannot say for sure just how long the stars have been around, but I don’t think gyrochronology is a good tool for answering that.

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