The Spring Triangle is an astronomical asterism involving an imaginary triangle drawn upon the celestial sphere, with its defining vertices at Arcturus, Spica, and Regulus. This triangle connects the constellations of Boötes, Virgo, and Leo. It is visible rising in the south eastern sky of the northern hemisphere between March and May. George Lovi of Sky & Telescope magazine had a slightly different Spring triangle, including the tail of Leo, Denebola, instead of Regulus. Denebola is dimmer, but the triangle is more nearly equilateral. These stars form parts of a larger Spring asterism called the Great Diamond together with Cor Caroli. The stars of the Spring Triangle Arcturus (α Boötes) Arcturus is a giant orange star located in the constellation of Boötes. Located only 37 light-years away, it has an apparent magnitude of -0.05. It is the brightest star in the northern hemisphere and fourth brightest in the night sky. Due to its ability to be spotted easily, Arcturus was identified by ancient humans and tied to mythological ideals. The star was given its name from the ancient Greek Ἀρκτοῦρος (Arktouros), which translates to "Guardian of the Bear." This name was selected because of the stars proximity to Ursa Major and Ursa Minor, surmising the two bear constellations were guarded by Arcturus. Arcturus is thought to be around 6 to 8.5 billion years old, and has traveled up the red-giant branch of the Hertzsprung-Russell diagram, where it has expanded in size. The star is has a diameter of around 36 million km, making it around 26 times larger than the Sun. Despite this size difference, the mass of Arcturus is only 1.1 times that of the Sun. With a high speed of 122 km/s (or 270,000 mph) and movement not in the galactic plane most other stars move on, it is thought that Arcturus could have formed outside of the Milky Way. This star is the namesake of a group of 52 other stars which share this similar proper motion, named the Arcturus moving group or Arcturus stream. It has been proposed that these stars are remnants of an ancient dwarf satellite galaxy that was assimilated into the Milky Way long ago. See also Summer Triangle Winter Triangle Northern Cross External links Lions in the Sky and other Spring Treasures The Spring Triangle Shows Denebola instead of Regulus in the spring triangle Asterisms (astronomy)