![]() ![]() Each emits several hundred thousand times the luminosity of the Sun. ![]() Properties īoth components of 9 Sgr are massive main sequence class O stars, and both have effective temperatures above 40,000 K. The two stars have been resolved using the Hubble Space Telescope Fine Guidance Sensor and with ground-based interferometry using the Very Large Telescope PIONIER instrument. Earlier clues such as nonthermal radio emission and periodic spectral line profile variations had prompted the detail search for a companion. Lack of x-rays, low orbital velocities, and similar spectral types merging to a combined spectrum of O4V, mean that 9 Sgr was only confirmed to be a binary in 2012. The large separation means that the stellar winds of the two stars do not impact strongly and so the pair are not a strong source of x-rays. The orbit is eccentric and the separation between the stars varies from 11 AU to 27 AU. System ĩ Sagittarii is a binary system with the longest known period for a pair of class O stars at 9.1 years. On this basis, it is thought to be a member of the NGC 6530 open cluster. A 2021 study of the binary system derives a distance of 1,310 ☖0 pc. Erosion of the front of the molecular cloud apparently caused by 9 Sgr suggests that it lies in front of the cloud, but studies of 9 Sgr as a binary star give a distance of 1,790 pc. Recent studies derive a distance around 1,250 pc for the M8 region. The distances to 9 Sgr, M8, and NGC 6530 are uncertain, but generally estimated to be between 1,200 and 1,800 parsecs. This ionised region lies in front of a denser molecular cloud. ĩ Sgr itself is surrounded by an ionised H II region about 30 light years across including the reflection nebulae NGC 6523 and NGC 6533. 9 Sgr is the main source of ionisation for much of the visible nebulosity in the region, although the young O star Herschel 36 ionises the dense Hourglass Nebula region. It lies close to the open cluster NGC 6530. It was not catalogued by Bayer with a Greek letter, but was listed by Flamsteed as number 9 in the constellation Sagittarius. The scattering of stars on the left is NGC 6530.ĩ Sgr is a naked eye star lying in the direction of the Lagoon Nebula (Messier 8), very close to the central condensation of the Hourglass Nebula around Herschel 36. ![]() O4V((f))z (O3.5V((f+)) + O5–5.5V((f)) )ĩ Sgr is the brightest star in the image, just left of the intense Hourglass Nebula core of the Lagoon Nebula. ![]()
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