NGC 6231

Lore & Background

The cluster was discovered by Giovanni Batista Hodierna before 1654, who listed it as Luminosae in his catalogue of deep sky observations, included in his book De Admirandis Coeli Characteribuse published in 1654 at Palermo. It was independently observed by other astronomers after Hodierna, including Edmond Halley (1678), Jean-Philippe de Cheseaux (1745–46), and Abbe Lacaille (1751–52).

NGC 6231 forms the head of the False Comet, a wider collection of stars from Scorpius OB1 running northward from Zeta Scorpii and NGC 6231 roughly halfway toward Mu Scorpii. The tail is formed by two clusters, Collinder 316 and Trumpler 24. Trumpler 24 is surrounded by the emission nebula IC 4628, also known as the Prawn Nebula, where the tail appears to fan out. The cluster is also sometimes known as The Northern Jewel Box, due to its similar appearance to NGC 4755, the Jewel Box cluster, which is further south in the sky.

In Chinese astronomy, NGC 6231 is usually identified as Shengong, a dressing room. In older sources, Shengong corresponds to the star Mu2 Scorpii, or another faint star or patch of stars nearby; the name has been adopted by the IAU Working Group on Star Names for the star HD 153072. Still other sources identify Shengong as Zeta Scorpii.

Reader's Guide

NGC 6231 is notable as a young open cluster that anchors the Scorpius OB1 association, a swath of young bluish stars. Its brightest star, Zeta1 Scorpii, is one of the most radiant stars known in the galaxy. The cluster includes three Wolf-Rayet stars, rare massive stars in a late evolutionary stage. NGC 6231 was used to measure the binary fraction of B-type stars, indicating that B-type stars are commonly found in binary systems, but not as commonly as in O-type stars. The star Zeta1 (HR 6262) is a member of the Scorpius OB1 association, while its brighter apparent partner, Zeta2 (HR 6271), is only 150 light-years from Earth and so is not a member. The cluster's role as the head of the False Comet, with its tail formed by Collinder 316 and Trumpler 24 near the Prawn Nebula, makes it a distinctive landmark in the southern sky. Its alternative name, the Northern Jewel Box, reflects its visual resemblance to the more southerly Jewel Box cluster.

Did You Know?

• NGC 6231 is also known as the Baby Scorpion Cluster.
• The cluster forms the head of the False Comet, with a tail made of Collinder 316 and Trumpler 24.
• Zeta1 Scorpii, a member of the association, is one of the most radiant stars known in the galaxy.

Stellar Youth and Galactic Position

NGC 6231 is a remarkably young open cluster, with estimates placing its age between two and seven million years. This makes it a relatively recent addition to the stellar landscape of the Milky Way, situated within the neighboring Sagittarius Arm. The cluster is currently closing the distance to our Solar System at a velocity of 22 kilometers per second, a fact that underscores its dynamic relationship with our local galactic neighborhood. Perhaps most strikingly, the cluster harbors three Wolf-Rayet stars—HD 151932, HD 152270, and HD 152408—representing an advanced evolutionary stage for massive stars and signaling the intense stellar activity within this young grouping. Astronomers have also leveraged NGC 6231 to study the binary fraction among B-type stars, arriving at a figure of 52 percent with a margin of uncertainty of eight percent. This finding confirms that B-type stars frequently exist in paired systems, though they do not reach the same prevalence of binarity seen among their more massive O-type counterparts. The cluster's position half a degree north of Zeta Scorpii in the southern sky anchors it firmly within the constellation of Scorpius.

A Long History of Discovery

The first recorded identification of NGC 6231 belongs to the Sicilian astronomer Giovanni Batista Hodierna, who catalogued the cluster before 1654. In his deep-sky observation catalogue, Hodierna listed it under the designation Luminosae, a name that would later be incorporated into his book De Admirandis Coeli Characteribus, published in Palermo in 1654. Despite this early documentation, the cluster did not remain exclusive to Hodierna's records. Over the following century and a half, several other prominent astronomers independently turned their instruments toward this region of the Scorpius sky. Edmond Halley noted the cluster in 1678, followed by Jean-Philippe de Cheseaux during observations made between 1745 and 1746. The French astronomer Abbe Lacaille added his own independent observation between 1751 and 1752. This succession of rediscoveries across more than a century illustrates both the challenges of naked-eye and early telescopic astronomy in the southern sky and the enduring visual appeal of this compact grouping of young, luminous stars nestled near the heart of the Scorpion's body.

Cultural Names and the False Comet

Beyond its formal designations as NGC 6231 and Caldwell 76, the cluster carries several evocative common names that reflect its visual character and cultural context. In the southern sky, it is sometimes called the Baby Scorpion Cluster, a nod to its position within the constellation of Scorpius. More poetically, NGC 6231 serves as the head of the False Comet, a broader stellar formation drawn from the Scorpius OB1 association that stretches northward from Zeta Scorpii and the cluster itself roughly halfway toward Mu Scorpii. The tail of this False Comet is composed of two additional clusters, Collinder 316 and Trumpler 24, with the latter encircled by the emission nebula IC 4628, popularly known as the Prawn Nebula, where the tail appears to spread outward. The cluster has also earned the nickname The Northern Jewel Box because of its visual resemblance to NGC 4755, the better-known Jewel Box cluster located further south. In Chinese astronomical tradition, the cluster is typically identified as Shengong, meaning a dressing room, a name that the IAU Working Group on Star Names has since adopted for the star HD 153072.

The Scorpius OB1 Association and Zeta Scorpii

NGC 6231 does not exist in isolation; it is embedded within the Scorpius OB1 association, a sprawling region of young, bluish stars that dominates much of the constellation Scorpius. The association's brightest member is Zeta1 Scorpii, catalogued as HR 6262, a star of spectral type O8 with an apparent magnitude of 4.71. This makes Zeta1 not only the most luminous star within the association but also one of the most radiant stars known anywhere in the Milky Way. Its apparent companion, Zeta2 Scorpii (HR 6271), presents an interesting contrast: despite appearing brighter to the observer, it lies only 150 light-years from Earth and is therefore not a true member of the association. The entire complex of NGC 6231 and its parent association occupies the neighboring Sagittarius Arm of the Milky Way, placing it in a region of active star formation. The cluster's youth, estimated at two to seven million years, is consistent with the ongoing stellar birth processes characteristic of OB associations, where massive stars are still in their earliest evolutionary phases and where objects like Wolf-Rayet stars can already be identified among the population.