Seasonal Asterisms of The Night Sky

Seasonal Asterims of the Night Sky
Credit: Ben McGee

Astronomers are fond of organizing the stars into recognizable patterns in order to better navigate their way around the night sky. While the International Astronomical Union (IAU) recognizes 88 modern constellations depicting animals (42), inanimate objects (29) and humans/mythological characters (17), many of their shapes are often difficult to discern leading scientists to reimagine simpler shapes that may better act as heavenly landmarks.

Such reimagined patterns of stars are called asterisms, and while not constellations in their own right, their stars may either belong to a single constellation, such as the Big Dipper, or alternatively may borrow a number of stars from different constellations, such as the Winter Hexagon.

Needles to say, asterisms come in many shapes and sizes, but one thing they all share in common is that they are particularly helpful in allowing stargazers to navigate across the night sky. Especially useful in this regard are the following four beautiful seasonal asterisms:

The Spring Triangle

The Spring Triangle  is visible in the northern hemisphere between March and May, and consists of the following three stars:

The Spring Triangle-Arcturus in the constellation of Boötes is a red-orange giant star located 36.7 light years away with an apparent magnitude of −0.04. Arcturus means “Guardian of the Bear” in Greek, and it is the northern celestial hemisphere’s brightest star, as well as the 4th brightest star overall in the entire night sky.

-Virgo in Spica is a blue-white supergiant located 260 light years away with an apparent magnitude of 1.04. Spica means “ear of grain” in Latin, and is the 16th brightest star in the night sky, although it is fact a close binary star system.

-Denebola in Leo is a white main sequence star located 35.9 light years away with an apparent magnitude of 2.11. It is relatively young star being less than 400 million years
old, and has a mass of 2.10 sol. Denebola is a shortened version of the Arabic phrase “dhanab al-asad” meaning “tail of the lion”, and it is the 62nd brightest star in the sky.

The Summer Triangle

The Summer Triangle

The Summer Triangle is visible during spring, summer, and autumn, although it is most prominent during the summer time. The Milky Way also appears to run through the asterism, tracing a line through the stars Deneb and Albireo in the constellation of Cygnus. The asterism is comprised of the following three stars:

-Deneb in Cygnus is a white supergiant located 3,550 light years distant with an apparent magnitude of 1.25. As we have learned, “Deneb” means tail in Arabic, in this case referring to the tail of the swan. It is the night sky’s 19th brightest star.

-Vega in Lyra is a bluish-white dwarf star located 25 light years away with an apparent magnitude of 0.03. Its name is derived from the Arabic for “swooping eagle”, and it is the 5th brightest star in the sky.

-Altair in Aquila is a white main sequence dwarf star located 16.6 light years away with an apparent magnitude of 0.77. Its name is derived from the Arabic phrase meaning “the flying eagle”, and is the 12th brightest star in the sky.

The Great Square of Pegasus

The Great Square of Pegasus represents the body of the winged-horse Pegasus, and is visible in the northern hemisphere from July to January. The asterism is also a gateway to locating the constellation Andromeda off the square’s left corner, and the Andromeda Galaxy (M31), which at 2.5 million light-years distant is the furthest object you can see with he unaided eye. The Great Square of Pegasus consists of four stars, namely:

The Great Square of Pegasus

-Scheat is a red giant variable star located 178 light years away with an apparent magnitude of between 2.4 and 2.9. It is found in the top right corner of the “square” and means “the leg” in Arabic.

-Alpheratz is a blue subgiant and the brightest star in the “Great Square of Pegasus”. It is found in the left-hand corner of the asterism, although it is actually part of the  constellation Andromeda, where it is also the brightest star. This binary system is 97 light-years from the Sun, and shines with an apparent magnitude of 2.06. Its name means “the navel of the horse” in Arabic.

-Markab is a blue-white star 133 light years distant that shines with an apparent magnitude of 2.48. It is found in the right-hand bottom corner of the Great Square of Pegasus, and means “the saddle of the horse” in Arabic.

-Algenib is a blue-white variable star located 391 light-years distant with an apparent magnitude of 2.84. It is found in the left-hand corner of the asterism, and its name derives from the Arabic for “The Wing”.

The Winter Triangle (& Hexagon)

The Winter Triangle (& Hexagon)

The Winter Triangle is visible not only in winter, but also in spring during the early evening, and autumn in the early morning. The Winter Triangle consists of three stars, namely Betelgeuse in Orion, Sirius in Canis Major, and Procyon in Canis Minor, with the Milky Way running through the middle of this arrangement of stars. The latter two stars mentioned (Sirius, Procyon) also form part of an even bigger asterism of six stars known as the Winter Hexagon, with the additional stars including Rigel in Orion; Aldebaran in Taurus; Capella in Auriga; and Pollux in Gemini.

The Winter Triangle stars consists of:

-Betelgeuse in Orion is a red supergiant star located 642.5 light years distant with an apparent magnitude of 0.42. Its name is derived from the Arabic phrase meaning “hand of the central one”, and is the 9th brightest star in the sky.

-Sirius in Canis Major is located 8.16 light distant and is a blue-white star that shines with an apparent magnitude of -1.46, although its color does seem to change when low in the sky. Sirius is in actual fact a binary star system consisting of Sirius A and Sirius B, although the much larger Sirius A accounts for most of the apparent brightness we observe back on Earth. Sirius means “scorching” in Greek, and is the brightest star in the night sky.

-Procyon in Canis Minor is a white star located 11.5 light-years away with an apparent magnitude of 0.4, although it is actually a binary system consisting of
Procyon A (white main-sequence) and a fainter companion Procyon B (white dwarf). Procyon means “before the dog” in Greek, referring to its apparent rising before Sirius the “Dog Star”, and it is the night sky’s 8th brightest star.