Viewing Double Stars

Double stars, also known as binary star systems, include two stars that orbit around a common point, called the center of mass (according to Kepler’s Law), or a common center of gravity. Individually, the stars are referred to as components. The brightest/primary component is component A and the secondary or generally faintest star is component B. Each component is quite potent in that it can affect its partner’s life course. Furthermore, some stars are easier to detect than others. During the night sky, some stars can give off the impression of being a binary star system, yet they are actually just very close to one another. When the concept first came around, astronomers thought that all stars appeared as double stars. However, in 1802, Sir William Herschel found that many of the stars that seemed to be close to each other essentially had changed in position relative to one another. There are many different types of double stars. Here’s some information on four of them.

Visual double stars are systems that consist of the ability to see the individual stars through a telescope. The main reason that such systems have been researched and investigated is for astrometric purposes. The function of astrometrics is to accurately measure the position of a star on the celestial sphere in order to learn about the characteristics of the orbit and obtain visual estimates of the differences in the measure of the stars’ brightness between the two components (magnitude). More recently, high precision measurements of the global magnitude and colors of these systems are taken by observers using photometric techniques. An example of an optical double star is Mizar, the second star from the left in the constellation of The Big Dipper. The star is quite visible to the naked eye and should be easy for any real astronomer to point out. The star also has a component B called Alcor. Because both stars are binaries, the entire Mizar complex has a system of four stars in total. Another type of double or binary star is an astrometric binary star. These appear to be single stars that have binarity due to perturbations of the position in respect to the “reference stars.” These systems are measured and studied photographically. Through studies of astrometric double stars, information regarding things like the luminosity, the semi-major axis of the astrometric orbit, and the position of the center of mass of the system can be obtained. The third type is spectroscopic double stars. These are systems where the stars are so close together that they emerge as a single star, even when viewed from a telescope. The study of their spectra can provide the orbital period, the mass ratio of the system, and other important information. Lastly, eclipsing binaries are (in some cases) systems where the orbital plane of a binary star system is oriented precisely to the plane of the sky in the effect of one star passing right in front of another, causing its light to block out during an eclipse. Such binaries can also be visual or spectroscopic double stars.

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