Beta Draconis
Binary star system and third-brightest star in Draco.
Beta Draconis, also designated β Draconis, is a binary star system that ranks as the third-brightest point of light in the northern constellation Draco, which never sets for many northern observers. Its combined apparent visual magnitude of 2.79 makes it readily visible without optical aid. Parallax data from the Hipparcos satellite places the system roughly 380 light-years (120 parsecs) from the Sun, and it is moving closer at a radial velocity of −21 km/s.
The system comprises a bright giant star, designated Beta Draconis A, and a fainter dwarf companion, Beta Draconis B. The companion orbits the primary once every roughly four thousand years, is about 11 magnitudes dimmer, and is separated from it by 4.2 arcseconds. The primary star has a spectral classification of G2Ib-IIa, indicating a mix of bright giant and supergiant characteristics, and is considered a standard for that class. It is about 65 million years old and is undergoing its first convective dredge-up. With about six times the Sun’s mass and roughly 40 times its radius, Beta Draconis A shines with about 950 times the Sun’s luminosity. Its effective temperature of 5,160 K gives it the yellow hue typical of a G-type star. The star exhibits strong chromospheric emission, producing X-ray and far-ultraviolet radiation, and possesses a detectable magnetic field with a longitudinal strength of −1.16 ± 0.25 G.
Beta Draconis lies on or near the cepheid instability strip but varies only slightly, with a range of about one-hundredth of a magnitude. This microvariability was confirmed by Gabriel Cristian Neagu using data from the TESS and Hipparcos missions, and the finding was reported to the American Association of Variable Star Observers (AAVSO) for inclusion in the Variable Star Index.
The system’s Bayer designation is Beta Draconis, Latinised from β Draconis. The components are labeled A and B following the convention of the Washington Multiplicity Catalog, adopted by the International Astronomical Union. The traditional name Rastaban (also spelled Rastaben) comes from the Arabic phrase *ra's ath-thu'ban*, meaning “head of the serpent/dragon”; this name has also been applied to Gamma Draconis. Other historical names include Asuia and Alwaid, the latter from the Arabic *al-ʽawāʼidh* (“the old mother camels”). In 2016, the IAU’s Working Group on Star Names approved Rastaban for Beta Draconis A.
- Bayer designation
- β Draconis
- Apparent visual magnitude
- 2.79 (combined)
- Distance from sun
- about 380 light-years (120 parsecs)
- Radial velocity
- −21 km/s (drifting closer)
- Spectral class of primary
- G2Ib-IIa
- Mass of primary
- 6 times the Sun
- Radius of primary
- roughly 40 times the Sun
- Luminosity of primary
- about 950 times the Sun
- Effective temperature of primary
- 5,160 K
- Magnetic field strength (longitudinal)
- −1.16±0.25 G
Lore & Background
The system bore the traditional name Rastaban, derived from the Arabic phrase ra's ath-thu'ban 'head of the serpent/dragon', a name also used for Gamma Draconis. It was also known as Asuia and Alwaid, the latter from the Arabic al-ʽawāʼidh 'the old mother camels'. In 2016, the IAU Working Group on Star Names approved the name Rastaban for the component Beta Draconis A. Beta Draconis is part of the asterism of the Mother Camels (Arabic al'awa'id), along with Gamma Draconis, Mu Draconis, Nu Draconis, and Xi Draconis, later known as the Quinque Dromedarii. In Chinese, it is part of the asterism Celestial Flail (天棓), and is known as 天棓三 (Tiān Bàng sān, the Third Star of Celestial Flail).
The primary, Beta Draconis A, is a bright giant with a stellar classification of G2Ib-IIa, showing mixed features of a bright giant and a supergiant star, and is listed as a standard star for that class. It is about 65 million years old and currently undergoing its first convective dredge-up. The star has a particularly strong chromospheric emission generating X-ray and far-UV radiation, and a detectable magnetic field with a longitudinal field strength of −1.16±0.25 G. Beta Draconis lies on or near the cepheid instability strip, yet only appears to be a microvariable with a range of about 1/100 of a magnitude. Its variability was confirmed by Gabriel Cristian Neagu using data from the TESS and Hipparcos missions, and reported to the AAVSO Variable Star Index.
Reader's Guide
Beta Draconis is significant as a nearby binary system whose primary serves as a standard star for the G2Ib-IIa spectral class, providing a benchmark for classifying similar stars. Its position near the cepheid instability strip, combined with its confirmed microvariability of about 1/100 of a magnitude, makes it a useful target for studying low-amplitude pulsations in evolved stars. The system's strong chromospheric emission and detectable magnetic field offer insights into magnetic activity in giant stars. Historically, its traditional name Rastaban and inclusion in the Mother Camels asterism reflect its cultural importance in Arabic astronomy, while its Chinese designation as part of the Celestial Flail underscores its role in non-Western star lore. The binary nature, with a companion 11 magnitudes fainter and a separation of 4.2″, provides a long-period (about four millennia) orbital system for study. The system's radial velocity of −21 km/s indicates it is drifting closer to the Sun, which may affect future observations.
Did You Know?
- Beta Draconis is the third-brightest star in the constellation Draco.
- The system's variability of about 1/100 of a magnitude was confirmed using TESS and Hipparcos data.
- The traditional name Rastaban derives from Arabic for 'head of the serpent/dragon'.
More in Binary and Multiple Stars, Part 3 1-24
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