Two stars circle each other like a pair of figure skaters on ice. Both resemble the Sun, but one shines brighter and has puffed up, while the other remains compact. Astronomers peered into their light using spectrographs, instruments that split light into colors, and found something odd: one star is clearly poorer in heavy elements than the other.
This can happen if one of the stars recently "ate" something from outside — for example, gas or dust that fell onto it and altered its surface composition. Astronomers think the system was born in a region of the Galaxy where heavy elements were scarce, and then the second star became "polluted."
The pair is about 10 billion years old — nearly twice as old as the Sun. One star has already left the main sequence and become a subgiant, while the other is still living an ordinary life. The researchers also noticed that the larger star has a magnetic cycle lasting 8 years, like the Sun's, but shorter.
Such systems are like old clocks that let us test models of stellar evolution. The more we learn about them, the better we understand how stars change over billions of years.
A star's metallicity is the fraction of elements heavier than helium. For the Sun, it's about 1.3%, while for one of the stars in HD 81809, it's 4 times lower.
In Arthur C. Clarke's novel "Rendezvous with Rama," an alien spacecraft uses the Sun's gravity for a maneuver — binary stars often serve as such "gravitational slingshots."