Some stars play hide-and-seek

Oct 26, 2009 11:37 GMT  ·  By

In astronomy, the term binary system refers to a group of two stars, which are locked to each other through their mutual gravitational forces and orbit a common center of gravity. Instances in which a small star orbits a massive one are not unheard of as well. The difference between such a system and an eclipsing binary is strictly a matter of perspective, as in how we see them from the Earth. Their orbits around their center of gravity must be organized in such a manner that, from time to time, they either fully or partially cover one another, Space Fellowship reports.

In most instances, the two stars are not of the same caliber, size and luminosity. Therefore, when the smaller star moves in front of the larger one, the covered area emits light that no longer reaches observatories on Earth. The telescopes then produce a brightness curve for the system that drops significantly when this juxtaposition happens. Among astronomers, this phase in the star system is known as a primary minimum.

The brightness curve can record another type of minimum – a secondary minimum – when the smaller star moves behind the larger one. The overall luminosity drops again, and is significantly lower (as viewed from our planet) than in the phases where the two stars are next to each other. The difference in brightness between the two stars is the factor that single-handedly determines whether the primary minimum or the secondary minimum is the lowest one.

When the small star has a surface brightness that is lower than that of its companion, the primary minimum is lower. Conversely, if the larger star is the darker of the two, the secondary minimum is more obvious on the brightness curve. Though it may seem counter-intuitive, there are cases in which larger suns are less bright than smaller ones. In fact, this is the defining trait of an entire class of such astronomical objects, known as cool, red giants.

The brightness curve is not only important in determining which of the minimums is lower. It can also provide astronomers with valuable information about the system itself, such as the relationship between the radii of the two stars, the period of their individual orbits, and even the mass of each of the components, if the speed at which they are moving around each other is also determined. The constellation Perseus holds the most famous eclipsing binary star system, the star Algol. It was first studied in 1783 by astronomer John Goodricke, who determined that it must be accompanied by another celestial body. Modern measurements revealed that his hypothesis was correct.