NGC 7331: Odd One Out

Galaxies in Pegasus, the Winged Horse, have a certain reputation: They’re challenging. With famous members like Stephan’s Quintet and Einstein’s Cross, you could be forgiven for thinking that only observers with large scopes need apply.

There’s at least one exception, however: NGC 7331 (Caldwell 30), a big, beautiful spiral galaxy in the northern reaches of the constellation. To find NGC 7331, draw a line from Mu (μ) Pegasi to Eta (η) Pegasi and straight on for another 4½°. The galaxy also forms the northeastern corner of an imaginary right triangle with Eta Peg and Pi2 (2) Peg.

With a visual magnitude of 10.4, NGC 7331 won’t necessarily be an easy catch, especially in light-polluted skies. But observers in only moderately dark skies have spotted it with 7×50 binoculars. So far I’ve needed at least 10×50s to pull it out, and 15×70s are better still — but when is that not true? It helps that the galaxy is big, 10 × 4, which is comparable to the largest Virgo galaxies. The large apparent size (as these things go) corresponds to ample proportions in real life. With a diameter of 130,000 light-years, NGC 7331 is larger than our own Milky Way.

NGC 7331 isn’t just unusually bright for a Pegasus galaxy, it’s also an astrophysical oddball. Several recent studies suggest that the galaxy’s central bulge rotates in the opposite direction of the spiral arms.

I have one more bit of homework for you: Go online and find a high-res photo of NGC 7331. One of my favorite was taken by Vicent Peris at the Calar Alto Observatory in Spain — it looks like the cover of a sci-fi novel, but it’s real. How wonderful that we can plumb the depths of intergalactic space with handheld instruments.

All comments must follow the Sky & Telescope Terms of Use and will be moderated prior to posting. Please be civil in your comments. Sky & Telescope reserves the right to use the comments we receive, in whole or in part, and to use the commenter’s username, in any medium. See also the Terms of Use and Privacy Policy.

COMMENT