September 29, 2015

NGC 7331, Stephan's Quintet (Sept. 2015)

NGC 7331 in its glory, and all the little galaxies.

NGC 7331 is about 48 million light years away (based on studies of its Cepheid variables).  The galaxies above and behind NGC 7331 are far in its background, but they are probably relatively near to each other.

The group of galaxies to the lower left is called Stephan's Quintet, after its discover.  It is also called Hickson 92, after a cataloger of odd galaxy groups.  And this one is odd.  The oval on the left (NGC 7320) is in front of the others.  In the Hubble image of this group, that oval galaxy is blue.  Its red-shift suggests that it is only 35 million light years away---somewhat near NGC 7331 and thus not a member of the group.  The other galaxies in the area, however, are closer to 270 million light years away, far in the background.  Those background galaxies are gravitationally connected to each other.

There is a small galaxy above Hickson 92, and one tidal tail from the 92 group reaches up toward it.  This galaxy, NGC 7320C, is also connected to the Hickson 92 group and probably moved through it millions of years ago, causing the tidal tail.

Telescope: Astro-Tech AT111EDT and William Optics AFR-IV (eff. f/5.6)
Camera and Exposure: SXVF-H9 (107x420"); Alnitak Flat-man flats
Filter(s): Astronomik CLS
Guiding: SX Lodestar and SX OAG
Mount: Takahashi NJP
Software: Nebulosity, PHD, SkySafari, Maxim DL, Photoshop CS3
Location: The Woodlands, TX