ρ Ophiuchi region (N is down): Antares is the bright yellow star, σ Scorpii the red star, and M4 the globular cluster in between. ρ Ophiuchi is the close pair in blue nebulosity near the centre of the image.
Rho Ophiuchi (ρ Ophiuchi) is a multiple star system in the constellationOphiuchus. The central system has an apparent magnitude of 4.63. Based on the central system's parallax of 9.03 mas, it is located about 360 light-years (110 parsecs) away. The other stars in the system are slightly farther away.
The central pair is known as Rho Ophiuchi AB. It consists of at least two blue-colored subgiants or main-sequence stars, designated Rho Ophiuchi A and B, respectively. Rho Ophiuchi AB is a visual binary, and the sky-projected distance between the two stars appears to be 3.1″, corresponding to a separation of at least 344 astronomical units (au). However, the actual separation is larger, and the two take about 2,400 years to complete an orbit.
Several other stars are located close to Rho Ophiuchi AB. HD 147932 is located 2.5 arcminutes away (at least 17,000 au), and is known as Rho Ophiuchi C. HD 147888 is located 2.82 arcminutes away (at least 19,000 au), and is known as Rho Ophiuchi DE. Stars C and D are both B-type main-sequence stars, and D itself is another binary with an orbital period of around 680 years.
Rho Ophiuchi is the namesake of the Rho Ophiuchi cloud complex. It is a nebula of gas and dust, which the Rho Ophiuchi system is embedded in. It is one of the easiest star forming regions to observe, as it is one of the nearest, and it is visible from both hemispheres.
The interstellar extinction (AV) of Rho Ophiuchi is measured to be 1.45 magnitudes, meaning the dust and gas in front of Rho Ophiuchi absorbs light from the system, making it appear 1.45 magnitudes dimmer than it should be. Additionally, gas and dust also scatters more higher-frequency light, leaving the light appearing more reddish. The interstellar reddening (EB−V) of Rho Ophiuchi has been measured to be 0.47 magnitudes.
^ abDucati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". 2237. Bibcode:2002yCat.2237....0D. Cite journal requires |journal= (help)
^Houk, N.; Smith-Moore, M. (1988). "Michigan Catalogue of Two-dimensional Spectral Types for the HD Stars. Volume 4, Declinations −26°.0 to −12°.0". Michigan Catalogue of Two-dimensional Spectral Types for the HD Stars. Volume 4. Bibcode:1988mcts.book.....H.
^ abNicolet, B. (1964). "Catalogue of homogeneous data in the UBV photoelectric photometric system". Astronomy and Astrophysics Supplement Series. 34: 1–49. Bibcode:1978A&AS...34....1N.