A celestial do-si-do

A common -- although, as it turns out, completely understandable -- error is to say that the Earth and other planets orbit the Sun.

No, I'm not recommending a return to the geocentric model, where the Earth is at the center of the universe and everything orbits around it in perfect circles, as decreed by the Almighty at the moment of creation (which, of course, was six thousand years ago).  The inaccuracy I'm referring to is much smaller than that -- but is still significant.

Instead of saying "the planets orbit the Sun," the more precise way to state it is that the planets and the Sun all orbit their common center of gravity.  Newton's Third Law describes how every force exerted creates an equal and opposite force -- so just as the Sun is pulling on the Earth, the Earth is pulling on the Sun.  The result is that both are in a dance around the system's center of gravity.  Given the Sun's vastly larger mass, their mutual center of gravity is well inside the Sun, so to say "the Earth orbits the Sun" is a sufficiently close approximation to account for what we observe on a daily basis.

The effect is big enough, though, that this is one of the ways that exoplanets have been discovered -- mostly in nearby systems, where it's easier to see.  A star with an unseen companion gets pulled around as they orbit their common center of gravity, so from our perspective it looks like the star has a slight wobble.  As the wobble is bigger if the planet has a larger mass, this technique has been used mostly to find exoplanets that are gas giants, like Jupiter and Saturn, which are big enough to sling their host star around more effectively.

Sometimes, though, looking for a stellar wobble results in discovering something else -- an invisible object much too massive to be a planet, in a celestial do-si-do with a star.

That was the subject of a paper published this week in The Open Journal of Astrophysics, describing research led by Kareem El-Badry of Caltech.  The team found 21 stars with heavy but invisible companions, which from their size appear to be neutron stars, the collapsed, ultra-dense cores left behind by giant stars after they exhaust their fuel.

The curious thing is that prior to the formation of a neutron star, the giant star went supernova -- so why didn't that colossal explosion completely blow away the Sun-like star it's paired with?  The simple answer is we don't know.  "We still do not have a complete model for how these binaries form," El-Badry said.  "In principle, the progenitor to the neutron star should have become huge and interacted with the solar-type star during its late-stage evolution.  The huge star would have knocked the little star around, likely temporarily engulfing it.  Later, the neutron star progenitor would have exploded in a supernova, which, according to models, should have unbound the binary systems, sending the neutron stars and Sun-like stars careening in opposite directions...  The discovery of these new systems shows that at least some binaries survive these cataclysmic processes even though models cannot yet fully explain how."

If El-Badry et al.'s research bears up, it will be the first time neutron stars have been detected purely by their gravitational effects.

So that's today's cool news from science.  A stellar dance between a Sun-like star and a collapsed, super-dense neutron star.  And I love that El-Badry ends with the words, "... models cannot yet fully explain how."  Focus on the word "yet."  These are the sorts of things that push science forward -- some unexplained observation that makes scientists scratch their heads.  As Isaac Asimov put it, "The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!', but '... that's funny.'"

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