This image features the Seyfert galaxy NGC 6951, which is seen through a region of the Cepheus Flare, one of several extensions out to higher galactic latitudes of the Milky Way's obscuring dust first mapped (and named) in 1934 by Edwin Hubble. This galaxy is a barred spiral about 75 million light-years away and about 100,000 light-years in diameter. With Hallowe'en coming up, I also adapted a starless version of the image, suitably drained of colour, to include here as a mouseover, my wife Loula having convinced me that it looks kinda spooky that way.😈

NGC 6951 (also catalogued as NGC 6952) contains an impressive starburst ring that tightly circles the nucleus, at a radius of only 2,000 light years. I was pleasantly surprised to find that a fair amount of structure in the ring has been resolved, given that its angular diameter is just 10 arc-seconds; the bright knots in the ring have been identified as young star clusters. The ring's bluish tint also clearly shows up, a tell-tale sign (rarely found in amateur images of this galaxy) of the starburst activity. The galaxy's prominent bar drives material towards its centre, which piles up to form the starburst ring in a region known as the inner Lindblad resonance [†]. NGC 6951 has been studied repeatedly by professional astronomers as a laboratory for understanding the relationship between bars, spiral density waves, circumnuclear rings, and active nuclei, thanks to the galaxy's low inclination, which gives a clear view of the entire starburst ring, and its relative proximity.

The 1934 paper by Hubble mentioned above was an epic study of the distribution of some 44,000 galaxies (or "extra-galactic nebulae" in the contemporary parlance) across the sky and in distance. The actual distribution of galaxies, and what it would imply about the nature of the universe, had been the subject of a "Great Debate" for more than 100 years before Hubble's paper conclusively settled the argument [‡]. Today we take it for granted that obscuring dust in the Milky Way explains why few galaxies are found near the galactic plane (what Hubble referred to as the "Zone of Avoidance"), and that there is no end to the distribution in space of observable galaxies, but these were fundamental open questions until Hubble's 1934 work.

The image spans about 31'x26', at a plate scale of about 0.47"/pixel, and is the result of just over 19 hours of integration, split evenly between luminance and RGB colour, taken over the course of thirteen nights from mid-July to mid-August. I threw out an additional 12 hours of exposures according to an aggressive set of quality cuts, especially on the FWHM of star profiles in the luminance channel, in order to obtain the sharpest and deepest image possible with an acceptable level of noise.

[†] A somewhat accessible qualitative description of resonant orbits in disk galaxies can be found in these lectures (see Fig. 1.10 and the related text).

[‡] A brief but very informative account of Hubble's career, including his 1934 paper on galaxy distributions, is given in this tribute by Alan Sandage, written for the 1989 centennial of Hubble's birth.