Multiple stars (Article 100)
At first sight, most stars appear as single, but with the aid of simple optics, many show they are a binary or multiple star system, all bound together by gravity, and one of nature’s best kept secrets. Some binary stars are just visible to the unaided eye, but it is in binoculars and telescopes that we see the fantastic diversity offered by the estimated 60% of the 100 billion stars within our Galaxy. These stars have orbits ranging from hours (where the stars outer atmospheres actually touch) to millions of years. Many also have fabulous contrasting colours, like orange and blue, or red and white. The binary star Albireo, shown in our photo, marks the beak of the Swan in the constellation Cygnus (or the foot of the Northern Cross) and is a wonderful example. In binoculars or a small telescope, this striking pair consists of a golden hued evolved red giant and a hot blue star, orbiting each other every 7300 years. This pair is on show throughout the late summer from Ireland, so have a go on the next clear night. The colours are best seen if you put your instrument slightly out of focus.
Complex multiple stars have complex orbits, with some doubles orbiting other doubles and so on. Where two stars are so close that they cannot be separated visually, we use a technique called spectroscopy, which detects the two stars by their effect on the light received on Earth. Like all stars, those within a binary or multiple system also evolve. When one expands into a red giant, the expansion can bring the evolving star into contact with its companion, where material actually transfers from one star to another! On occasion this gives rise to an explosion called a nova or even a more massive supernova. Stellar evolution can thus convert a stable binary system into a scene of immense violence.
Occasionally, their orbit is aligned with Earth, and as a result we can witness one star eclipse the other, just like in a solar eclipse. These fascinating events can be predicted very accurately for many such eclipsing binaries, as they are known. There are quite a few that can be followed over the course of their eclipse during a few days or even hours, as the normal brightness drops when one star’s light is partially or completely hidden from view.
A number of doubles are not physically attached, but are rather seen along a line of sight in space as viewed from Earth. Such an example is the star Mizar, the second star in the handle of the Plough. The star Alcor is visible to the unaided eye next to Mizar, but are in fact about 3 light years apart.
25-08-2007 Multiple stars (Article 100)
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