Constellations and star names

The IAU divides the whole sky into 88 constellations, agreed in 1922, with boundaries published in 1930.

Pictures made of unrelated stars

Every culture that watched the sky joined bright stars into figures: hunters, animals, ships, rivers, carts. NASA Space Place points out that the stars in one figure are usually not connected at all. In Orion, Betelgeuse lies roughly 550 to 700 light-years away while Rigel, on the other side of the figure, is more than 800. They only look close because they lie in the same direction. Seen from somewhere else in the galaxy, Orion would fall apart.

The figures we use in Europe and the Americas come mostly from ancient Greek and Roman sky lore, gathered in Ptolemy's star catalogue in the second century. Korea, China and Japan had their own system of hundreds of smaller star groups arranged like a state, with a palace near the pole and officials, markets and roads spread across the sky. The IAU's own constellation pages now set such Chinese star groups side by side with the European figures.

88 official areas of sky

By the nineteenth century star charts were crowded with new figures, some invented to flatter kings or patrons. To bring order, the International Astronomical Union at its first General Assembly in Rome in 1922 agreed a list of 88 constellations covering the whole sky, each with a three-letter abbreviation: Ori for Orion, UMa for Ursa Major, Cru for Crux. Boundaries came next. Eugène Delporte of the Royal Observatory in Brussels drew them along lines of right ascension and declination, they were approved at Leiden in 1928 and published in 1930. Since then a constellation is officially an area of sky, not a stick figure. Every point in the sky belongs to exactly one constellation.

That is why the lines in the model are only one choice. The IAU defines boundaries, not lines, so every book and app joins the stars a little differently. The lines here were drawn for this site from the catalogue stars, for 21 well-known constellations.

Asterisms: famous shapes that are not constellations

Some of the best-known star patterns are not constellations at all. The Big Dipper, Bukdu Chilseong in Korean, is seven stars inside the much larger constellation Ursa Major, the Great Bear. Such a pattern is called an asterism. The Summer Triangle joins three bright stars from three different constellations: Vega in Lyra, Deneb in Cygnus and Altair in Aquila. In winter, the Winter Hexagon links Sirius, Procyon, Pollux, Capella, Aldebaran and Rigel. The IAU itself uses these asterisms to introduce the seasons' skies.

How stars get their names

Many bright stars carry old names, most of them Arabic in origin: Aldebaran, Altair, Deneb, Betelgeuse. In 1603 Johann Bayer began labelling stars in each constellation with Greek letters, usually in rough order of brightness, so Sirius is Alpha Canis Majoris. The order is only rough: in Orion, Alpha (Betelgeuse, magnitude 0.50 in the catalogue) is fainter than Beta (Rigel, 0.12). Catalogues add numbers too; this course's star data use HR numbers from the Bright Star Catalogue. In 2016 the IAU set up a Working Group on Star Names, which published its first list of 227 approved names that November and has since catalogued several hundred; the names in the model follow that list.