Progress that doesn’t go viral

Some recent posts have addressed events in astronomy that have attracted attention from the general public.  Our astronomer would like to explain something that you’ll probably never hear about.

Our astronomer writes:

A paper was recently published in an on-line astronomy archive that you will not have noticed.  In contrast with the strange behaviour of the unaccountably dimming star, which attracted a lot of attention in social media, and the latest supermoon, which was visible to anyone with a clear sky, this is only really of interest to astronomers.  Indeed, many of them will do no more than glance at it and pass on to something in their own area of research.  But it is much more typical of the way science is done.  And it could very well make a much bigger contribution to progress in astronomy than many things that attract a lot of outside interest.

(The authors are a British astronomer working in California and a Turkish astronomer working in his home country.  This is not unusual; astronomy is a very international effort.)

The authors collected data on 60 binary stars and compared them with computer models.  Binary stars are stars that orbit each other, like the Earth and Moon, and are quite common in the sky: the Sun is unusual in being alone.  The important fact about binaries is that, once their orbit is determined, the masses of the stars can be calculated.  It’s an undergraduate exercise, which means that there are few tricks and little complexity involved.  This method has been used to “weigh” stars for nearly two centuries, so the idea is not breaking new ground.

The interest comes in because the authors were looking at highly-evolved stars, those that aren’t quietly shining like the Sun but are changing their structure, sometimes drastically.  While models of the Sun are pretty firm, a lot isn’t understood very well about stars off the Main Sequence.  The theory thus being tested involves difficult things like the effect of magnetic fields on mass-loss over the star’s lifetime, as well as internal convection (turbulent flow is one of the stubbornly hard problems of physics).

The theory comes out well, for the most part.  42 of the 60 binaries fit the computer models within the accuracy of the data.  Another 15 don’t do well, but for easily understood reasons (limitations in the models, mostly).  For two, they can be made to fit if something unusual has happened during their evolution; for one, the authors frankly admit themselves stumped.

This paper is a classic comparison of theory and observation, illustrating a basic operation of science.  And it shows that a mature theory gets most things right (the basis of current stellar structure models was worked out between about 1920 and 1950).  At the same time, it is ambitious enough (looking at evolved stars) to push the theory to the point that some things need adjusting, and to throw up one true puzzle.  This is how astronomy normally works.