Binary stars and multiple systems

Double stars and multiple star systems are fascinating phenomena in the night sky. Through binoculars, some supposed single stars appear as several stars close together. These can consist of either two or more stars and are called double stars, binary stars, or multiple stars, accordingly. However, at least binoculars are often needed to observe these systems, as many are not visible to the naked eye. But not all double stars are actually double stars.

What is a binary star system?

Double stars consist of two stars that are so close to each other in the sky that they have only a small angular distance to each other seen from the earth. Often it also appears as if they are only one star.

They form a physical unit and are gravitationally coupled to each other, thus orbiting a common center of gravity. The orbital periods are from a few days up to thousands of years. The velocities and the distance to each other change with the rhythm of the orbital period.

Most stars in the universe belong to a binary or multiple system. This is because when new stars form in gas clouds, their proper rotation favors the formation of two or more stars. Single stars like our sun are much rarer.

Optical and physical double stars

In the night sky, we can sometimes observe supposed double stars, which from our point of view, look like binary stars because of their proximity to each other, but in reality, they are very far apart. These double or multiple systems are also called visual or apparent systems. A well-known example is Albireo in the constellation Cygnus (often called Swan constellation), which consists of a bright orange star and a fainter blue star.

Optical binary star Albireo
Optical binary star Albireo

But there are also real binary stars, which were formed together and thus have a similar age. These stars are gravitationally dependent on each other and influence each other. Astronomers can gain valuable information about stellar evolution and galactic dynamics by studying such double stars. One example is the star Mizar, which forms a quadruple system, and Alkor, which is part of a triple system. Both are located in the constellation of the Ursa Major (commonly known as Great Bear or Big Dipper).

Spectroscopic and photometric double stars

As mentioned above, the star Mizar is a true multiple system consisting of four individual stars. Two of these stars, the visual binary stars, can be viewed separately with a pair of binoculars or in a small telescope because they are 14.4 arc seconds apart. The other two stars in the system are so-called spectroscopic binaries. They can only be detected by their individual light spectra, which are altered by rotation.

Photometric double stars are detected by measuring the brightness of a star. When two stars cover each other, there are regular changes in the brightness curve. Algol in the constellation Perseus is an example of such an eclipsing variable star.

The brightest star in the night sky

Many people probably know it - the star Sirius (Latin: α Canis Majoris) in the constellation of Canis Major (also called Greater Dog). It is the brightest star in the night sky and a true double star whose companion was not discovered for a long time.

It was not until 1862 that it was discovered that Sirius is a binary star. The star consists of the very bright main component A and a white dwarf orbiting around A. The white dwarf shines much fainter, with an apparent brightness of roughly 8.5. Although it has a maximum angular separation of only 11.5 arc seconds, its faint light makes it difficult to find.

This also shows that the double or multiple systems can consist of stars that have a high difference in brightness.

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