NGC 891 - The Outer Limits Galaxy

Designations:	NGC 891, Caldwell 23, H V 19, IRAS02193+4207, UGC1831, MCG7-5-46, CGCG538-52, PGC09031, Outer Limits Galaxy
Object Type:	Spiral Galaxy
Constellation:	Andromeda
RA:	02 hr 22 min 36.0 sec (2000.0)	Dec:	+42° 21' 00" (2000.0)
Magnitude:	9.9 (SB 13.7)	Size:	13.5 x 2.8 arcminutes
Distance:	31 million light years
Discoverer:	Caroline Herschel 1784

One of Autums Gems and in my opinion, the finest of the edge-on galaxies. NGC 891 is easy to find being on the same declination as Gamma Andromeda and approximately 3.5° to the east of this star. Alternately it can be found by "star hopping" from the open cluster M34, which is a similar distance away on its other side i.e. east. In an 8 inch (20 cm) telescope its elongated spindle shape is obvious and lies roughly east-west. The dark dust lane is much harder to spot - it is in fact not dark at all, just less bright! Increased magnification and averted vision helps but in a 16 inch (40 cm) telescope it becomes more obvious. NGC 891 is a classical edge-on spiral galaxy and even though its spiral arms are invisible from our viewpoint, its prominent dust lane and central bulge makes its spiral classification certain. It is thought to be part of a group that includes nearby galaxies NGC 1023 and 925. The current estimate of its distance is 31 million light years. NGC 891 has been extensively studied over most of the electromagnetic spectrum and is noted for extensive "extraplanar gas" i.e. gas far from its galactic plane. Recent high quality images taken with the large 3.5 meter WIYN telescope on Kitt Peak revealed a network of hundreds of dust clouds and streamers above and below this galaxies plane. How this matter is propelled out of the disc is the puzzle. Some of the dust streamers observed suggested a supernova driven fountain or chimney phenomena thought to be tracing the violent disc-halo interface. However other dust clouds suggested a lower energy origin. Alton et al. (1988), using the newly commissioned Sub-millimeter Common-User Bolometer Array, produced the deepest images yet of any galaxy in this waveband. They confirmed the presence of dust chimneys extending over 6000 light years.

Telescope:	TMB 152 F8 APO refractor
Focal Length:	1200 mm
Mount:	Takahashi NJP 160
Camera	SBIG ST10XME for Luminance Data and Starlight XPress MX916 for color data
Guider:	SBIG ST402ME for autoguiding the luminance image and SBIG STV autoguider (color exposures).
Exposures:	6 10-minute Luminance exposures and 2 10-minute exposures for each color (red, green, blue)
Location:	Cicero, IN
Software:	CCDSoft for image acquisition, processed with CCDStack and Photoshop CS2. AstroArt for image aquisition with the Starlight XPress MX916 camera.