NGC 7331 – Wide Field
Key Scientific Data
Object type: Unbarred spiral galaxy – SA(s)b
Constellation: Pegasus
Other designations: Caldwell 30, UGC 12113, PGC 69327
Distance from Earth: approximately 45–50 million light-years
Diameter: approximately 120,000 light-years
Apparent size: approximately 10.5 × 3.7 arcminutes
Apparent magnitude: approximately 9.5–10.4, depending on the photometric band
Inclination: approximately 75–77°
Right Ascension: 22h 37m 04s
Declination: +34° 25′
Discovered by: William Herschel, 1784
Nucleus: hosts a supermassive black hole
Light-travel time: approximately 45–50 million years
Description
NGC 7331 is a large spiral galaxy located in the constellation Pegasus, approximately 45–50 million light-years from Earth.
With a diameter of around 120,000 light-years, it is comparable in size to our own Milky Way. We see NGC 7331 at a steep angle, almost edge-on. This perspective reveals its bright central region, extended galactic disc and spiral structure crossed by prominent lanes of interstellar dust.
In colour images, the different shades also reveal something about its stellar populations. The yellowish central regions are dominated mainly by older stars, while the spiral arms contain younger, hotter stars that give portions of the disc their characteristic bluish appearance. Reddish regions trace clouds of ionised hydrogen associated with ongoing star formation.
At the heart of NGC 7331 lies a supermassive black hole, as is the case in many large galaxies, including our own Milky Way.
A Twin of the Milky Way?
NGC 7331 is sometimes described as a “twin of the Milky Way” because the two galaxies share broadly comparable dimensions, stellar populations and overall spiral structure.
The comparison, however, should not be taken too literally. The Milky Way is a barred spiral galaxy, with an elongated bar of stars crossing its central region, whereas NGC 7331 is generally classified as an unbarred spiral galaxy.
Observing NGC 7331 therefore provides something close to an external perspective on a galaxy broadly resembling our own — a view of the Milky Way that, from our position inside it, we can never experience directly.
Curiosities
NGC 7331 was discovered by William Herschel in 1784, more than two centuries before modern astronomical imaging made it possible to reveal its structure in such remarkable detail.
One of its most interesting characteristics concerns its central bulge. Observations indicate that part of the central stellar population rotates in the opposite direction to the galactic disc, an unusual feature that may preserve clues about the galaxy’s past evolution.
NGC 7331 is also located in an exceptionally rich area of the sky. In a wide-field image such as this one, the main galaxy becomes only the most prominent member of a much deeper extragalactic landscape.
A Field Full of Galaxies
The wide field is perhaps the most fascinating aspect of this image.
Immediately around NGC 7331 are several much smaller-looking galaxies, including NGC 7335, NGC 7336, NGC 7337 and NGC 7340. Together with NGC 7331 they form the apparent grouping commonly known as the Deer Lick Group.
Their apparent proximity is deceptive. NGC 7331 lies approximately 45–50 million light-years away, while the smaller galaxies surrounding it are much more distant background systems, hundreds of millions of light-years from Earth. The group is therefore largely an effect of perspective rather than a single gravitationally bound family of galaxies.
Elsewhere in the field lies another remarkable concentration: Stephan’s Quintet, one of the most famous compact groups of galaxies in the sky.
Four of its five apparent members are physically associated at a distance of roughly 280–300 million light-years. Their gravitational interactions have dramatically distorted their structures, producing tidal features and complex interactions between stars and intergalactic gas.
The fifth apparent member, NGC 7320, is actually a foreground galaxy at only about 40 million light-years from Earth. Its presence within the Quintet is an extraordinary coincidence of perspective.
But these famous groups represent only a fraction of what is visible here. The annotated version of the image reveals dozens upon dozens of additional galaxies, many catalogued only with PGC designations. Some appear as tiny elongated streaks, others as faint elliptical glows or miniature spiral systems.
Each of these apparently insignificant patches of light is an entire galaxy containing millions or billions of stars.
Almost all of the sharp individual points of light scattered across the photograph, by contrast, are foreground stars belonging to our own Milky Way.
The image therefore spans an extraordinary range of cosmic distances: nearby stars within our Galaxy, NGC 7331 tens of millions of light-years away, and numerous background galaxies extending hundreds of millions of light-years into deep space.
What initially appears to be a simple star field is actually a view through several different layers of the Universe.
Looking Back in Time
Perhaps the most fascinating aspect of this image is invisible: different parts of the photograph show the Universe at different moments in its history.
The light from NGC 7331 has travelled through space for approximately 45–50 million years. We therefore see the galaxy not as it exists today, but as it appeared tens of millions of years ago.
The effect becomes even more dramatic when looking towards Stephan’s Quintet. The light from its main interacting galaxies has travelled for roughly 280–300 million years before reaching Earth.
Within the same photograph, we are therefore simultaneously seeing NGC 7331 as it existed around 50 million years ago and much more distant galaxies as they appeared hundreds of millions of years earlier.
When the photons from NGC 7331 began their journey towards us, human beings did not yet exist. When the light now reaching us from the distant members of Stephan’s Quintet began its journey, the first dinosaurs had not yet appeared on Earth.
After crossing intergalactic space for tens or hundreds of millions of years, a tiny fraction of those ancient photons finally reached Earth and completed their extraordinary journey on the sensor of the telescope that captured this image.
One photograph therefore does not show a single moment in time: it contains several different chapters in the history of the Universe.
Image Data
Total integration time: 19h
Integration by filter:
LP: 4h 40m (56 × 300")
Red: 4h 33m 20s
Green: 5h 13m 20s
Blue: 4h 33m 20s
Equipment:
Telescope: William Optics RedCat / Cat 51 III WIFD
Camera: ToupTek ATR2600M
Mount: 10Micron GM2000 HPS II
Filters: Optolong Blue 2", Optolong Green 2", Optolong L-Pro 2", Optolong Red 2"
Accessories: ToupTek AAF Electric Focuser, WandererAstro WandererCover V4-EC
Software: Adobe Photoshop, Pleiades Astrophoto PixInsight, Stefan Berg Nighttime Imaging 'N' Astronomy (N.I.N.A. / NINA)
For further details and the full-resolution image, visit AstroBin: