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Popular Binary Star Systems

There are many binary stars on the night sky, targeted by both amateur and professional astronomers. Some are even visible to the naked eye, while other require the use of spectroscopy to be distinguished. Here I'll describe a few of the most known binary stars.
You'll find that some of these systems are actually trinary stars, that is three stars orbiting a common gravitational center.

Algol (Beta Persei) - The Demon Star

Algol, the famous eclipsing binary star is located 93 light years away, in the constellation of Perseus. Ancient Arabians noticed its clear variation in brightness. They thought there was something spooky about it and named it "al Ghul", the Demon star. In fact, the starsystem has been given evil names in many cultures. The jews called it Satan's Head. In the Greek mythology Algol marked the head of Medusa, the Gorgon.
As a matter of fact, Algol is a "friendly" star: it is a system which greatly helps us in supplying us with data about stellar masses and sizes.
Algol is a spectroscopic binary where the primary star is a blue B8-class star 3.5 times more massive than our sun, with a surface temperature of 12 500 K and a diameter 3 times that of the sun. The second star is a K2-class star with a diameter of 3.5 more than our sun, a surface temperature of 4 500 Kelvin and a mass of about 0.8 MSun. A third star has been found to orbit the first two discovered. It orbits the two inner stars from an average distance of 3 AU, which make an orbit 1.86 years. Algol C, the third star is of A5-class with 1.67 solarmasses, 1.7 solardiameters and a luminosity 4.1 times more than our sun.
Surprisingly this system shows a paradox: the more mass a star has, the faster it evolves. However, in the Algol-system the much less massive (0.8 MSun) giant star is more evolved than the more massive star! The only explanation is that the dimmer star has been significantly more massive in the past.
The system is known to complete a cycle in 68 hours and 49 minutes (the stars are located 0.05 AU apart, roughly 7.5 million km). During this cycle, in which two eclipses occur, one eclipse makes the system drop in brightness from magnitude 2.1 to 3.4 (a 30% drop). This is when the bright blue star is eclipsed by the dim giant star. During the second eclipse, where the blue star passes infront of the other, the drop in intensity is less noticable.

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Alpha Centauri

Alpha Centauri is (A Cen for short), as the name suggests the brightest star in the constellation of Centaurus. The proper name of this star is Rigil Centaurus, which means "the foot of the Centaur" in arabic. It is located too south to be visible from most of the northern hemisphere. It has the same spectral class as our sun: G2 V, but is slightly larger and more massive (about 10% more massive, and a 20% larger diameter). The surface temperature is 5 800 K and the luminosity may be as much as 60% higher. The star is much older than our sun (around 7 billion years).
A Cen is located only 4.2-4.4 light years away and is actually a system of three stars orbiting a common gravitational centre. Alpha Centauri has a close companion: Alpha Centauri B (of spectral class K1 V). These two stars form a binary, and are located about 4.36 light years away. The binary pair completes an orbit (which is very eccentric) in about 80 years. A third red dwarf, Proxima Centauri orbits this binary from a great distance to the system (13 000 AU). It takes about 500 000 years to complete an orbit around these two stars, but it is closer to us: 4.22 light years. This is so far away that the gravitational bond is very weak and astronomers have doubted that the system is a trinary system (system of three stars). Perhaps the stars are only a part of group of six stars orbiting the center of the milky way galaxy together, without any significant gravitational bond.

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Antares - Alpha Scorpii

Antares is located at a distance of 520 light years away, in the constellation of Scorpius. It has derived its name from the similar appearance to the planet Mars. Antares means "Mars-like" (Ant-Ares) in ancient Greece language. The star shines brightly with red tints on the night sky.
Antares is a huge star: it has a mass of 15-18 MSun, but what's really remarkable is its size: the radius is about 4 AU (in total a diameter of 9.24 108 km)! Had it been in the Sun's place it would have swallowed, Mercury, Venus, Earth and even Mars!
The average density of the star is really low. Even the surface temperature is low (not surprising for a size this size): about 3 400 K. At visible wavelengths of the spectrum the star shines about 10 000 times brighter than the sun, but with the mentioned low surface temperature its main output is in the infrared wavelengths, where it is 65 000 times brighter.
Antares is at its final stages of life where it has swollen to a remarkable red supergiant of class M. Antares will likely explode as a supernova in the near future (near in astronomical terms, which could be a few million years from now). It varies slightly in brightness of a period of 6 years and it also throws off some gas, which creates a nebula. This nebula is lit up by a secondary B2-class blue companion star: Antares B. Antares B orbits the red supergiant star at a distance of about 550 AU, which makes an orbit take 2 500 years. The mass of this second companion is 7-8 MSun.

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Beta Lyrae

Beta Lyrae, also known as Sheilak is the second brightest star in the constellation of Lyra, the "Harp". Studies have shown that it is an eclipsing binary starsystem, with a period 12.89 days. It has also been shown to be a spectroscopic binary, where the main star has a closer companion and a third star (classified as B7) is orbiting from a greater distance. The system is located about 880 light years away and is a fine target for amateur astronomers, not only because it is a beautiful binary, but also because the variation in brightness is clearly visible to the naked eye.
Judging from its spectrum, the primary star is of class B0, which might have a mass twice as much as the sun and a surface temperature of 8 000 degrees Kelvin. The star which orbits it closely, and can only be detected through the use of spectroscopy is much more massive: probably 12 MSun and a surface temperature of 13 000 K. The reason why it cannot be seen is that is covered by a very thick layer of gas (an accretion disc) originating from the primary star. The intense gravity from has deformed both these two stars into an ellipsoidal shape.
Beta Lyrae is a remarkable star in many ways and is set as a prototype to classify other binary starsystems that resemble this one (both ecliptic and ellipsoidal in shape), they are called Beta Lyrae stars.

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Rigel - Beta Orionis

Rigel is the bright blue star that marks the foot of Orion - The Hunter. The name was derived from Arabic ("Rijl Jauza al-Yusra" means left foot of the central one). Rigel is located about 770 light years away from the sun.
Shining brightly, it overpowers the luminosity of the sun by a factor of 40 000, with a surface temperature of 11 000 Kelvin.
Astronomers have discovered two companions for this main star. Both these stars, Rigel B and Rigel C orbit around a common gravitational center, separated by a distance of 28 AU. This binary in turn orbits about 2 000 AU away from the massive primary which we see from Earth. Rigel B should be visible through a small telescope, but Rigel A's luminosity overpowers it greatly. As a matter of fact, Rigel's brightness is so large that a nebula 40 light years away from it ("The Witchhead nebula") is reflecting Rigel's blue colour.

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Sirius - The Dog Star

In 1862 it was confirmed that one of our nearest neighbours in space (8.7 light years away), Sirius had a companion star. Sirius B, or the Pup as some call the companion, is a white dwarf. It is separated from the main star by a distance of 20 AU and it takes about 50 years to complete an orbit around the primary star. The fact that Sirius B is a white dwarf strongly suggests that it must have been more massive than the current primary star we see today - the more mass a star has, the faster it evolves, and a white dwarf is a stellar corpse! Even though it has a surface temperature of 25 000 degrees Kelvin (more than four times hotter than the surface of the sun), it is much more faint than our sun (1/400th), because it is a small and compact object.
Sirius A, the star we see with the naked eye, is the brightest star shining on the sky (visual magnitude of - 1.46), from Earth, apart from the Sun. It has a luminosity 23 times more than our sun. Sirius A is an A1-class star with a mass of 2.4 MSun and it can be seen from the all the populated regions of Earth.

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Spica is the most prominent star in the constellation Virgo. It is located about 260 light years away and consists of two B-class stars (B1 and B4) that orbit each other in mere 4 days. The stars are located about 20 million kilometres away from each other, which is enough to make them elliptic in shape. The main star weighs some 11 MSun and is about 2 100 times more luminous than the sun in the visible wavelengths. Though the star is so hot (surface temperature of 22 400 K.) that most of the radiation comes from ultraviolet rays. The combination between the high surface temperature and its radius (7.8 RSun) gives it a real luminosity of more than 13 400 suns.
Spica B has a mass of 7 MSun, a radius of 4 RSun, and a surface temperature of 18 500 degrees. These two stars emit x-rays that originate from the collision of the stellar winds, creating a bowshock.
Studies have shown that Spica has three other, much fainter companions.

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Quick Links

Algol - Space Art
Alpha Centauri
Antares - Space Art
Beta Lyrae - Space Art
Rigel - Space Art

Space Art

Space art 1: The Algol system is a trinary star. When the main star is observed to eclipse the secondary star (which is bigger than the primary), then astronomers don't detect a too large drop in overall brightness. However, when the secondary star eclipses the blue star, the overall brightness seems to drop.

Space Art

Space art 2: The Antares system. Antares is also known as Alfa Scorpii. It is the brightest star in the constellation of Scorpius. The system consists of the red supergiant Antares, and a blue giant star (which is dwarfed by the red supergiant).

Space Art

Space art 3: The famous binary star Beta Lyrae; It is a system of three stars, where two stars orbit each other closely and a third companion (not seen here) has joined the celestial dance from a distance. The primary star has derived so much gas from the secondary (which is visible in the background), that it is occluded from Earth.

Space Art

Space art 4: Beta Orionis - Rigel. The Rigel system consists of a binary starsystem of blue stars, which orbits a large primary star: Rigel A.


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