Beta Scorpii
A multiple star system in Scorpius with six orbiting components.
Beta Scorpii is a multiple star system located in the southern zodiac constellation Scorpius. It was historically known by the traditional name Acrab, but the International Astronomical Union now restricts that name to the β Scorpii Aa component alone.
When viewed through a small telescope, the system appears as a binary star, with its two main components separated by 13.5 arcseconds and a combined apparent magnitude of 2.50. These two stars are designated β¹ Scorpii and β² Scorpii, and they form the top level of a hierarchy that includes six orbiting components in total.
β¹ Scorpii, the brighter of the pair, consists of two sub-components, β Scorpii A and β Scorpii B, which orbit each other at an angular separation of 0.3 arcseconds with a period of 610 years. β Scorpii A itself is a spectroscopic binary, made up of β Scorpii Aa (also called Acrab) and β Scorpii Ab, separated by 1.42 milliarcseconds and orbiting every 6.82 days.
β² Scorpii also splits into two sub-components, β Scorpii C and β Scorpii E, with an angular separation of 0.1328 arcseconds and an orbital period of 39 years. β Scorpii E is further a spectroscopic binary, containing β Scorpii Ea and β Scorpii Eb, which orbit each other every 10.7 days.
An unrelated seventh-magnitude star, HD 144273, lies 520 arcseconds away and is designated β Scorpii D. Some sources have also referred to component Ab as D. A companion to component B, called β Scorpii G, has been proposed to account for missing mass in the system, but no evidence for its existence has been found. Similarly, β Scorpii F is a theorized companion to component E.
The Bayer designation Beta Scorpii comes from the Latinized form of β Scorpii, abbreviated as Beta Sco or β Sco. The labels β¹ and β² Scorpii refer to the two main components. The sub-component designations—β Scorpii A, Aa, Ab, B, C, E, Ea, and Eb—follow the convention of the Washington Multiplicity Catalog, adopted by the International Astronomical Union.
Historically, Beta Scorpii bore the traditional names Acrab, Akrab, or Elacrab, all derived from the Arabic word al-'Aqrab, meaning "the scorpion," which referred to the entire constellation. It was also called Graffias, from the Greek word for "crab," a name shared with Xi Scorpii.
Quick Facts
- Apparent magnitude
- 2.50 (combined)
- Distance
- 470 light years (145 parsecs)
- Spectral type (combined)
- B1 V
- Age
- 6 to 12 million years
- Number of components
- 6 orbiting components
- Traditional names
- Acrab
- Akrab
- Elacrab
- Graffias
Facts from the source article.
Lore & Background
Observed through a small telescope, Beta Scorpii appears as a binary star with a separation of 13.5 arcseconds and a combined apparent magnitude of 2.50. This pair, designated β¹ Scorpii and β² Scorpii, form the top branches of a hierarchy of six orbiting components. β¹ Scorpii consists of two sub-components, β Scorpii A and β Scorpii B, orbiting at 0.3 arcseconds with a period of 610 years. β Scorpii A is itself a spectroscopic binary, with components Aa (named Acrab) and Ab separated by 1.42 milliarcseconds and an orbital period of 6.82 days. β² Scorpii has sub-components C and E, orbiting at 0.1328 arcseconds with a period of 39 years; component E is a spectroscopic binary with a period of 10.7 days. Component D is the unrelated seventh magnitude star HD 144273, 520 arcseconds away. A companion to component B, β Scorpii G, has been proposed to account for missing mass, but no further evidence of its existence has been found. β Scorpii F refers to a theorised companion to component E.
Reader's Guide
Beta Scorpii is notable as a complex multiple star system that has been studied for its hierarchical orbital architecture. The system is a kinematic member of the Upper Scorpius subgroup of the Scorpius–Centaurus association, a group of thousands of young stars with mean age 11 million years. Analysis of β¹ Scorpii as a single star derived an evolutionary age between 9 and 12 million years, but analysis of the system as a whole suggests an age closer to 6 million years. The two components of β Scorpii A are the most massive members, with combined spectral type B1 V. Component Aa is evolving slightly away from the zero age main sequence, while component Ab has a main sequence luminosity class, a temperature of 26,400 K, and a luminosity of 8,200 L☉. Component C has a stellar classification of B2 V and a mass of 8.5 M☉. Component E is chemically peculiar, possibly a mercury-manganese star, with high abundances of manganese and strontium but unexpectedly low abundances of other metals. Beta Scorpii is 1.01 degree from the ecliptic and can be occulted by the Moon and, very rarely, by planets; it was occulted by Venus on December 9, 1906, and by Jupiter on May 13, 1971. The star appears on the flag of Brazil, symbolising the state of Maranhão.
The Architecture of a Six-Body System
Beta Scorpii is not a single star but a nested hierarchy of orbiting bodies. Through a modest telescope, the system resolves into a binary pair—β¹ and β² Scorpii—separated by 13.5 arcseconds with a combined apparent magnitude of 2.50. Each of these two primary components is itself a double. β¹ Scorpii splits into components A and B, which trace a 610-year orbit at an angular distance of just 0.3 arcseconds. Component A is further a spectroscopic binary (Aa and Ab) locked in a tight 6.82-day orbit at 1.42 milliarcseconds. On the other side, β² Scorpii divides into C and E, completing a 39-year orbit at 0.1328 arcseconds, while E itself harbors a spectroscopic pair (Ea and Eb) with a 10.7-day period. A seventh-magnitude star, HD 144273, sits 520 arcseconds away as an unrelated line-of-sight object. Proposed companions G and F remain unconfirmed, leaving the confirmed count at six gravitationally bound members.
Naming and Identity Across Cultures
The star has carried several names across cultures and centuries. Its Bayer designation, Beta Scorpii, is the formal catalog label, while the traditional proper name Acrab (also rendered Akrab or Elacrab) traces back to the Arabic al-'Aqrab, meaning 'the Scorpion,' referring to the entire constellation. A Greek-derived name, Graffias, meaning 'crab,' was once shared with Xi Scorpii. In 2016, the IAU's Working Group on Star Names made a deliberate choice to assign proper names to individual stars rather than to whole multiple systems. On August 21 of that year, the name Acrab was officially approved for the single component β Scorpii Aa, and it now appears in the IAU's official list. In Chinese astronomy, both β¹ and β² Scorpii belong to the asterism Fáng Xiù, meaning 'Room,' and each is called Fáng Xiù sì, 'the Fourth Star of Room.' The name Graffias also inspired the USS Graffias (AF-29), a United States Navy vessel.
A Young and Massive Stellar Family
Beta Scorpii belongs to the Upper Scorpius subgroup within the Scorpius–Centaurus association, a vast collection of young stars at roughly 470 light-years. The system's age is estimated at around six to twelve million years, making it extraordinarily young by stellar standards. The two components of β Scorpii A dominate in mass at 15 and 10 solar masses, with a combined spectral type of B1 V. Component Aa is drifting slightly off the zero-age main sequence, while Ab sits firmly on it at 26,400 K and 7,900 solar luminosities. Component B, over twenty times fainter than the A pair, is estimated at about 8 solar masses. Component C is a B2 V star of 8 solar masses, 24,000 K, and 3,200 L☉. Component E stands out as chemically peculiar, showing elevated manganese and strontium abundances, possibly classifying it as a mercury-manganese star, though other metal abundances are unexpectedly low.
Encounters, Occultations, and Cultural Echoes
Because Beta Scorpii lies just 1.01 degree from the ecliptic, it is periodically hidden by the Moon and, on rarer occasions, by planets. A notable planetary occultation occurred on December 9, 1906, when Venus passed in front of the star, and the last such event by a planet was Jupiter's transit on May 13, 1971. On December 11, 2019, the system experienced a close conjunction with Mercury, their geocentric separation dropping below one arcminute. Beyond the telescope, the star has left a mark on national identity: it appears on the flag of Brazil, where it symbolizes the state of Maranhão. NASA featured an image of Beta Scorpii in its Astronomy Picture of the Day on May 21, 2009, bringing the system to a wider public audience. These occasional alignments and cultural adoptions remind us that even a young, multi-star system in a southern constellation can occupy a place in both the scientific record and the collective imagination.
More in Named Stars
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced