The Big Dipper is not actually seven stars? One of them is secretly "paired"?

Author | Feng Ziyang

Audit | Zhao Jingyuan

Editor | Zhao Jingyuan

Mizar is a binary star system consisting of Mizar (ζ Ursa Major in Bayer nomenclature) and Alcor (80 Ursa Major in Flamsteed nomenclature) in the direction of Ursa Major. It is one of the easiest binary stars to observe. Mizar and Alcor can be distinguished by the naked eye without the use of an astronomical telescope. The ancient Arabs even used it as a standard for testing eyesight, making it the oldest "eyesight test chart."

The location of the Kaiyang binary star. Image source: Stellarium

The binary star Kaiyang is located between the two stars Yaoguang at the end of the handle of the Big Dipper and Yuheng on the handle. Kaiyang’s right ascension on the celestial sphere is 13h23m55.92s, and its declination is 54°55′31.5″. It is about 82.9 light-years away from us, with an apparent magnitude of about 2.23. It is the fourth brightest star in the Ursa Major; Fu’s right ascension is 13h25m13.91s, and its declination is 54°59′17.0″. It is about 81.7 light-years away from us, with an apparent magnitude of 3.99, slightly darker than Tianquan (visual magnitude 3.31), the dimmest of the Big Dipper. The angular distance between Kaiyang and Fu is about 12 arc minutes, which means that their actual distance is about 1.2 light-years.

Fuhe Kaiyang. Image source: DSS

Mizar, the English name of Mizar, comes from the ancient Arabic language, meaning "apron, lid, packaging". Around 1617, Italian mathematician Benedetto Castelli first realized that Mizar itself was a binary star system through a telescope, consisting of Mizar A (later named Ursa Majoris ζ1 by Bayer) and Mizar B (later named Ursa Majoris ζ2 by Bayer). The two stars are 14 arc seconds apart, making it the first visual binary star discovered by humans through a telescope. Later, Italian astronomer Galileo Galilei also observed it, trying to find evidence to support the heliocentric theory, but this attempt failed because he made a wrong assumption. In 1890, American astronomer Antonia Maury conducted a series of spectral observations on Mizar A, from which he distinguished the spectral line components of the two stars, making Mizar A the first spectroscopic binary star discovered by humans. In 1908, astronomers discovered that Kaiyang B was also a spectroscopic binary, so Kaiyang was actually a quadruple star system.

Left: Castelli's letter to Galileo, in which he mentioned that he discovered that the Sun itself is a binary star system. Right: Galileo's observation records and calculations. Image source: Reference 7

The English name of Alcor also comes from ancient Arabic, meaning "neglected, faint", and it is named because it is easily obscured by the bright light of the sun. It was not until 2010 that two teams led by American astronomers Eric Mamajek and Neil Zimmerman independently discovered that Alcor is also a binary star system. The fainter companion star is called Alcor B, a faint red dwarf star, and is only 1 arc second away from the brighter primary star Alcor A.

Left: Auxiliary B (circled) imaged by Zimmerman's team using the Hale Telescope equipped with a coronagraph, image source: Neil Zimmerman et al 2010 ApJ 709 733. Right: Auxiliary B (bright spot at lower right) imaged by Mamajek's team using the Multi-Mirror Telescope, image source: Eric E. Mamajek et al 2010 AJ 139 919

After centuries of observation, astronomers finally discovered that the so-called Kaiyang binary star system is actually a "nesting doll" consisting of 6 stars. Kaiyang is a quadruple star system composed of two binary star systems, and Fu is also a binary star system. However, astronomers have not yet determined whether Kaiyang and Fu are physical binaries or optical binaries. The so-called physical binary refers to a physical connection between two stars that rotate around a common center of mass. Physical binaries can be divided into visual binaries that can be distinguished only by visual observation, and spectroscopic binaries that can only be distinguished by spectra. Although the two stars in an optical binary appear to be very close in their projections on the celestial sphere, there is actually no physical connection between the two.

The Kaiyang binary actually contains 6 stars. Image source: Reference 5

Although astronomers are not sure whether Kaiyang and Fu are physically connected now, their proper motions show that they move in the same direction as the stars in the Big Dipper except Tianshu and Yaoguang, which is called the Ursa Major Moving Group. The stars in this group may have been an open cluster with a common origin. This means that in the distant past, Kaiyang and Fu were likely physically connected.

As the easiest binary star to observe, the Kaiyang binary star is worth trying for astronomy enthusiasts. For most people, it is easy to find them. At around 8 pm in the spring of the northern hemisphere, the Ursa Major is hanging high in the northeastern sky. Find the spoon-shaped Big Dipper, and count from the first star (Tianshu) at the mouth of the dipper to the handle of the dipper. The sixth star is Kaiyang. If you have good eyesight, you can see a darker star very close to Kaiyang with your naked eyes. That is Fu. If conditions permit, you can also observe them through a telescope.

Schematic diagram of the position of Kaiyang Binary Star in the Big Dipper. Image source: Reference 5

The Kaiyang binary star is like a mysterious and brilliant treasure in the night sky, lighting up the unique starlight for human exploration of the universe. From naked eye observation to telescopes, to spectral analysis and coronagraphs, astronomers gradually gain a deeper understanding of its complex composition. Although the Kaiyang binary star is only a small chapter in the endless galaxy, it exudes a unique charm and carries the endless curiosity and exploration spirit of human beings about the vast starry sky.

References

1. Mizar, Wikipedia, https://en.wikipedia.org/wiki/Mizar

2. Alcor, Wikipedia, https://en.wikipedia.org/wiki/Alcor_(star)

3. Mizar and Alcor, Wikipedia, https://en.wikipedia.org/wiki/Mizar_and_Alcor

4. Bruce McClure, Mizar and Alcor in the bend of the big Dipper, Earth Sky, https://earthsky.org/brightest -stars/mizar-and-alcor-the-horse-and-rider/

5. Giles Sparrow, Mizar and Alcor: the Plough's famous double star, BBC Sky at night, https://www.skyatnightmagazine.com/space-science/mizar-alcor-plough-double-star

6. Daniel Johnson, Meet Mizar and Alocr: the Horse and Rider, Sky and Telescope, https://skyandtelescope.org/observing/celestial-objects-to-watch/meet-mizar-and-alcor-the-horse-and-rider/

7. Galileo e le stelle doppie, http://www.unsaltonelcielo.it/galileo-e-le-stelle-doppie/

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