May 24, 2020 – How Old is That Star?

I know how old I am.  Got a birth certificate.

What about stars?  No birth certificate with cute little footprints…do they still do that?  Figuring out a star’s age is a labor of investigative science.  Fortunately, we live in a time where the tools for determining a star’s age are readily available.

That said, tools provide the investigator with results.  As with a table saw, if the user didn’t square up the blade and fence properly, the cut will be off.  That result will impact how the finished project works.  With woodworking small errors can sometimes be hidden within the context of the finished piece.

Not so with astronomy and astrophysics.  Tools must be calibrated at a precision so fine only a minute error might be acceptable.  Why?  Distance.  Distances are so immense small errors get magnified to a point of generating worthless results.  They cannot be hidden.  That’s one reason science is in continuous flux when it comes to findings.  As tools get better, results get better, things become clearer.  Change.

Determining the age of a star has been one of the trickier problems to solve.  You see, stars have an exceedingly long middle age (main sequence) where they appear the same for billions of years.  Early astronomers really had a difficult challenge.  The basic pieces of information needed to begin are the star’s distance, mass, spectral type, and rate of luminosity increase.  Friedrich Bessel made the first distance measurement in 1838.  Friedrich Georg Wilhelm von Struve and S. W. Burnham studied binary stars and their data helped with the determination of stellar mass. Annie J. Cannon developed the modern spectral classification of stars in 1901.  Luminosity was first estimated by Hipparchus and has been refined over the centuries.  However, luminosity increase change is stable only during a star’s middle age and not helpful with young or old stars.

These advances lead to the Hertzsprung-Russell diagram, a visual classification of stellar luminosity, temperature, and spectral class.  Makes a great wall poster.

So, by the early twentieth century astronomers had some tools to estimate a star’s age.  They soon realized however, estimating a star’s age was still challenging and the tools they had needed refinement.  One helpful idea was to examine star clusters, knowing each star in the cluster was a sibling of the others.  Each star was about the same age.  Their tools could be applied to a cluster to get better resolution and it worked.

A new tool was developed after a discovery by Soren Meibom and colleagues that a star’s rotation rate slows down predictably over time. It has proven to be valuable and is called Gyrochronology.

Determining a star’s age is still challenging but we’re getting better all the time.

What’s in the Sky?

May 24; just after sunset; west-northwest:  Venus and Mercury and a crescent Moon line up

If you want to preserve our night sky join us: