Triangulum (The Triangle) & M33 Galaxy

Sky Navigation & Astrophysical Context

Nestled into 132 square degrees between the foot of Andromeda (Almach) to the north and the horns of Aries (Hamal and Sheratan) to the south, Triangulum is one of Ptolemy's 48 classical constellations—known to the ancient Greeks as 'Deltoton' (from its resemblance to the capital Greek letter Delta, Δ, and the Nile River Delta) and in Babylonian tablets as MUL.APIN ('The Plough,' the very first constellation listed in the 1000 BCE Babylonian astronomical compendium).

Its narrow isosceles triangle is formed by Beta Trianguli (V = +3.00, an A5 IV subgiant binary at 39 parsecs surrounded by a debris disk) and Gamma Trianguli (+4.01, just 2.4° southeast of Beta) at the eastern base, pointing 7° west-southwest to the apex star Mothallah (Alpha Trianguli, meaning 'Head of the Triangle,' V = +3.42, a close F6 IV spectroscopic binary).

Four degrees west-northwest of Mothallah (Alpha Trianguli), and exactly mirrored across Mirach (Beta Andromedae) from the Andromeda Galaxy (M31), glows the Triangulum Galaxy (Messier 33 / NGC 598): at ~840 kiloparsecs (2.73 million light-years), M33 is the third-largest galaxy in the Local Group and a face-on spiral spanning 70 × 42 arcminutes (wider than two Full Moons). Its outer spiral arm houses NGC 604, a 1,500-light-year-wide H II star-forming region nearly forty times the linear diameter of the Orion Nebula that is easily visible inside M33 in backyard telescopes.

Equatorial Center Position & Transit Altitude

Triangulum is centered near Right Ascension 02h 10m 48s, Declination +31° 28′ 48″ (The Triangle).

Practical Star-Hopping & Observing Tips

• Spot the compact isosceles triangle of Beta (+3.00), Gamma (+4.01), and Alpha Trianguli (+3.42) halfway between Almach (Gamma Andromedae) and Hamal (Alpha Arietis).
• Center 10×50 binoculars 4° west-northwest of Alpha Trianguli (Mothallah) under dark, transparent skies to detect the 1°-wide oval glow of the Triangulum Galaxy (Messier 33, V = +5.7).
• In a 150 mm+ telescope on M33, look 12 arcminutes northeast of the galaxy's core to resolve the bright extragalactic H II nebula NGC 604, and split the gold-and-blue double star Iota Trianguli (6 Trianguli, separation 3.9 arcseconds).

Notable Celestial Objects in & Near Triangulum

Scientific Sources Cited

• IAU Official Constellation Boundaries & Designations (IAU 88 Recognized Constellations): Delporte, E. 1930, Délimitation scientifique des constellations (IAU / Cambridge University Press) — International Astronomical Union (IAU)
• SIMBAD Astronomical Database (CDS) (2026-10 release): Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database) — Centre de Données astronomiques de Strasbourg