Go to the edge with NGC 891

NGC 891 is a showpiece edge-on spiral galaxy that is an irresistible target for imagers. Image: Bob Fera.

NGC 891 comfortably takes its place in the pantheon of those special edge-on galaxies such as the Sombrero (Messier 104) in Virgo and NGC 4565 in Coma Berenices. It’s arguably the finest inclined spiral visible in the autumn sky. This showpiece object has a SA(s)b classification (see below), but its almost fully edge-on presentation makes a definitive designation problematic. Astronomers believe it bears some resemblance to our own Milky Way galaxy.

NGC 891 is located 3.5 degrees east of magnitude +2.1 Almach (gamma [γ] Andromedae), the beautiful double star. It also lies 3.5 degrees west of M34, the nice open cluster lying across the border in Perseus. NGC 891 has the advantage of never setting (circumpolar) in UK skies. In late November it culminates high overhead (altitude of over 76°) at about 10.30pm GMT. 

NGC 891 is a very attractive proposition for amateur imagers, with deep images easily capturing the prominent obscuring lane of dust and interstellar gas that runs along the entirety of its 13.5 arcminute-long major axis. Unlike the aforementioned NGC 4565, no prominent bulge is present. Although shining at a relatively bright magnitude +10, on a transparent and moonless night you’ll probably need a four-inch telescope to spot it as a needle-like slither of light and the greatly increased light grasp and resolution of a 300mm (12-inch) ‘scope to glimpse the dust lane.

The galaxy is also peppered with many foreground stars, which adds to its visual allure. A stunning sight from a dark country sky, NGC 891 is a sight to behold!

NGC 891 lies in the far east of Andromeda. AN graphic by Greg Smye-Rumsby.

Galaxy cluster ACO 347

If you’re observing through a large telescope or wide-field imaging, then you might come across a gathering of faint smudges of light under a degree east of NGC 891. This is Abell 347 (ACO 347), part of the the Perseus–Pisces Supercluster, a huge filament of galaxy clusters that is perhaps the most obvious supercluster in the sky, which is not surprising considering its massive wall of galaxies extends almost 300 million light years across the cosmos.

It is thought to lie 250 million light years away and, despite its vast distance it still covers 60 degrees of sky, from the Perseus cluster (Abell 426) at its extreme eastern end, through Abell 347 and 262 in Andromeda and the NGC 507 and NGC 383 galaxy groups in northern Pisces, and finally to some smaller and more obscure galaxy groups at the western end, such as the NGC 7515 group just below the Great Square of Pegasus.

Abell 347 is hard to observe with anything less than a 300mm (12-inch) telescope. It’s not a rich cluster by any means, thought to contain at most 50 galaxies lying 240 million light years away. Large amateur telescopes and images reveal seven NGC galaxies, four brighter than fourteenth-magnitude within a 20 arcminute field, with around a dozen overall covering a degree. 

The brightest galaxy in ACO 347 is thirteenth-magnitude NGC 910, a common-all-garden round elliptical with a bright core that spans some 2 x 2 arcminutes across. This should be seen in a 300–350mm (12- to 14-inch) telescope from our shores. NGC 911 and NGC 909 are slightly fainter and smaller ellipticals, with NGC 906 being a tougher prospect as a face-on, barred spiral (class SBab).

Greg can you do a graphic based on this please

Galaxy morphological classification system

The Hubble morphological classification system was determined in 1926 by the great American astronomer Edwin Hubble, who broadly divided galaxies into spirals (S), ellipticals (E), lenticulars (S0) and irregulars (Irr), further classifying spirals according to the tightness of the spiral arm structure and degree of a central bulge, and whether a central bar is present. The more tightly wound specimens with a pronounced bulge were classed Sa, progressing through Sb and Sc as the arms become more open with an ever-smaller bulge (equivalent barred spirals are denoted as SBa to SBc). 

Gérard Henri de Vaucouleurs, the great twentieth-century French astronomer, devised an extension to the Hubble system, first described in 1959. Simply, it retains the four basic Hubble classes but divided galaxies on the presence or lack of a central bar; class SA for no bar, with SB for a barred spiral and SAB for an intermediate class denoting the presence of a weak bar. 

He also added further divisions and information for spiral arms and ring-like structure (r (ring present), s (no ring) and rs (transitional).

Ellipticals, featureless-looking galaxies where star formation has long ceased, are classed according to their degree of flatness; E0, being the most round examples, through to E7, cigar-shaped specimens. Lenticulars are an intermediate class between elliptical and spiral galaxies, exhibiting a prominent bulge surrounded by a featureless disc. Generally, there is little on-going star formation.

NGC 891 and the galaxy cluster ACO (Abell 347). The bright star dead centre is magnitude +6.7 HIP 11185, with spiral galaxy NGC 906 to its left and elliptical NGC 910 below NGC 906. Image: Ronald Brecher.