NGC 6309, Box Nebula
NGC 6309 is a planetary nebula of unusual appearance. Loops similar to the spiral arms of a galaxy can be seen. This structure, which is rather rare in PN, can also be seen, for example, in images of NGC 4361 in Corvus. The current classification scheme by Vorontsov-Velyaminov describes NGC 6309 as a PN with an abnormal shape (Type VI) and an irregular disk with traces of a ring (+Type IIIb). The latter is more of a misclassification, perhaps due to the loops.
The nebula's faint central star was examined more closely in 1995 with the Hubble Space Telescope (HST). The incredibly high resolution of the HST images showed for the first time that the Box Nebula actually has two central stars! The distance between the two stars is less than a second of arc, so they can hardly be separated with conventional telescopes from the earth's surface.
|Designations||PN G009.6+14.8: NGC 6309, PK 9+14.1, ARO 66, He 2- 206, Sa 2-181, VV 96, VV' 171|
|Right Ascension (J2000.0)||17h 14m 04s|
|Declination (J2000.0)||-12° 54' 37"|
|Dimensions||15.5" (optical), 30.6" (radio)|
|Radial Velocity||-47.6 km/s ± 2.8 km/s|
|Expansion Velocity||34.0 km/s (O-III)|
|C-Star Designations||AG82 234, CSI -12-17112 0, HD 155752|
|C-Star Magnitude||16.58 mag (B filter)|
How to find the Box?
The Box Nebula is located in the Ophiuchus constellation, which is favorable in the sky during the months of May to July. To view the PN in a telescope, a "one-step star hop" from Sabik (η Ophiuchi) just under 3 degrees north and slightly east is enough to be exactly 1.5 degrees west of the star ν Cauda Serpentis. In a wide-field eyepiece, the small nebula should then appear next to a mag 8.6 star.
200 mm aperture: The Box Nebula appears as a small oblong nebula, located north of a mag 8.6 foreground or background star that is physically unrelated to the nebula. Even with a magnification of 68x you can spot the elongated shape. At 226x the shape is clearer and a kind of «backbone» can be seen: the nebula is a bit brighter along the longitudinal axis. The nebula looks similar to a distant galaxy looking almost edge on. However, the lateral loops (see POSS shot above) remained invisible even under dark skies and when using an O-III filter! With an aperture of 200 mm, the use of a nebula filter on NGC 6309 does not improve the contrast and is therefore not recommended here. The faint central star (13 mag according to Hynes ) did not appear even under good observation conditions. This is probably due to the fact that the star is difficult to distinguish from the comparatively bright central part of the nebula.