NGC 7331 (Galaxy)
A large spiral galaxy in Pegasus, tilted so its disc looks like a long oval. Its distance comes from Cepheid variable stars in its disc: about 49 million light-years.
Galaxy in Pegasus. Magnitude 9.4.
Can you see it from Guyana?
From Georgetown (6.8° N) it climbs to 62° above the northern horizon at its highest; from Lethem (3.4° N), to 59° above the northern horizon. It is highest at midnight on the night of 28 Aug, in the middle of its season. The page works out tonight's rise, highest point and set for both towns in Guyana time (UTC-4), and where the Moon is.
What you'll see
- Your eyes: Not visible to the eye. It lies 8.5° north-west of Scheat.
- 10x50 binoculars: Just detectable in binoculars from a dark site, as a faint smudge.
- A small telescope (80 to 100 mm): A bright oval core in a long halo, about 2.5 times as long as wide; its arms need a larger telescope.
- A camera: Tiny on camera: it needs 2,000 mm or more of focal length on a tracking mount.
How big, how bright, how far
- Size on the sky: 9.3′ × 3.8′, 30% of the Moon's width (the Moon is about 31′ wide).
- Brightness: magnitude 9.4. Fainter than the faintest stars in Georgetown (4.3) and Lethem (6.0): binoculars or a telescope.
- Distance: 49 million light-years. The light reaching your eye left it about 49 million years ago.
- True size: 130,000 light-years across its catalogued outline; light crosses it in 130,000 years.
Apparent size from OpenNGC (Mattia Verga), CC-BY-SA-4.0.
What it is made of
- Type
- Spiral galaxy, tilted to us
Measured: What its visible mass is made of: stars and gas, not dark matter.
- Stars: 81.8%, from its infrared light
- Atomic gas: 13%, hydrogen, from 21 cm radio maps
- Molecular gas: 5.2%, from carbon monoxide maps
Each mass is given to a tenth of a dex, so each share is good to about a quarter of itself. Dark matter is not counted. Leroy et al. 2008, AJ 136, 2782, Table 4 (VizieR J/AJ/136/2782), NGC 7331: log stellar mass 10.9, atomic gas 10.1, molecular gas 9.7 (Suns). Shares do not depend on the distance the paper adopted.
Also in it, not measured:
- Spiral arms: lit by young stars
- Dark matter halo: not in this tally; seen only by its gravity
How we know: Distance from Cepheid variable stars: NED-D (NASA/IPAC Extragalactic Database, redshift-independent distances), median of 19 Cepheids estimates: 15.00 Mpc, 14.50-15.30 Mpc (16th-84th percentile); e.g. 2008A&ARv..15..289T, 2006ApJS..165..108S, 2006A&A...452..423P; retrieved 2026-10-21. True size: the apparent size times the distance (apparent size: OpenNGC (Mattia Verga, CC BY-SA 4.0), commit 4e2398a2).
More deep-sky objects: M110 (Galaxy) · M32 (Galaxy) · Andromeda Galaxy (M31) · Fireworks Galaxy (NGC 6946) · Triangulum Galaxy (M33) · NGC 891 (Edge-on Galaxy)