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 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 million years

Description

NGC 7331 is a large spiral galaxy located in the constellation Pegasus, approximately 45 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 beautifully reveals its bright central region, surrounded by an extended galactic disc and elegant spiral arms crossed by prominent lanes of interstellar dust.

In colour images, the different shades also tell us something about the stars within the galaxy. The yellowish central regions are dominated by older and generally cooler stellar populations, while the spiral arms contain many young, massive and hot stars, giving some areas their characteristic blue appearance. Reddish regions are associated with clouds of ionised hydrogen where new stars are actively being formed.

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 often described as a “twin of the Milky Way” because the two galaxies share broadly comparable dimensions, mass, stellar populations and overall spiral structure.

The comparison, however, should not be taken too literally. The Milky Way is now known to be a barred spiral galaxy, with an elongated structure of stars crossing its central region. NGC 7331, by contrast, is generally classified as an unbarred spiral galaxy.

Observing NGC 7331 therefore gives us something close to an external perspective on a galaxy 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 unusual characteristics concerns its central bulge, the dense concentration of stars surrounding the nucleus. Observations indicate that part of this central stellar population rotates in the opposite direction to the galactic disc — an intriguing feature that may provide clues about the galaxy's past evolution and interactions.

Deep photographs of NGC 7331 also reveal several apparently smaller galaxies surrounding it. Among them are NGC 7335, NGC 7336, NGC 7337 and NGC 7340. Although they appear close to NGC 7331 in the sky, most are actually much farther away, creating a striking perspective effect.

This region of Pegasus is particularly rich in galaxies. Less than half a degree away lies the famous Stephan's Quintet, one of the best-known compact groups of galaxies in the night sky.

Looking Back in Time

Perhaps the most fascinating aspect of this image is something that cannot be seen directly.

The light collected to create this photograph has travelled through space for approximately 45 million years.

Looking at NGC 7331 therefore means literally looking into the past. The galaxy shown here is not NGC 7331 as it exists today, but as it appeared around 45 million years ago.

When these photons began their journey towards Earth, human beings did not yet exist. They travelled across intergalactic space for tens of millions of years before a tiny fraction of them finally completed their extraordinary journey on the sensor of the telescope that captured this image.

Image data

Total integration time: 21h 55m

Integration by filter:

• Luminance/Clear: 5h (60 × 300")
• Red: 6h 29m 45s
• Green: 5h 45m 30s
• Blue: 4h 39m 45s

Equipment:

• Telescope: Artesky ARTEC 250 Pro
• Camera: Player One Poseidon-M Pro
• Mount: 10Micron GM2000 HPS II
• Filters: Antlia V-Pro Luminance 2",  Antlia V-Pro Red 2",  Antlia V-Pro Green 2", Antlia V-Pro Blue 2" 
• Accessories: Pegasus Astro Prodigy Microfocuser, Player One Phoenix Wheel 7×2", 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:

https://app.astrobin.com/i/9otc2l