Constellations and Asterisms

Here in Aotearoa-New Zealand the sea surrounds us from all directions, the sky is darker than dark and the stars are very bright. This hemisphere is a magical water-world in which the night sky reflects. The Milky-Way descends here all the way to the horizon because of the lack of light pollution. Over the ocean, the only signposts of the night are the patterns of the stars. We call these ASTERISMS (roughly from ‘aster’ which in Latin means star) and are made of the brightest stars in a constellation.

Constellation versus asterismDEL
The Constellation of Delphinus is everything inside the white shape. The asterism of the dolphin is marked by the light blue-green line

Many cultures around the world had named their own constellations and stars since ancient times but once science advanced more, stars and deep sky objects were discovered that were invisible to the naked eye and a better classification system was needed. Astronomers decided to start by mapping the celestial sphere on the model we use on Earth – countries and regions. Although stars are at different distances from us, from Earth we see them as if they lay flat on a sphere, therefore we call the sky the celestial sphere.

Universal names were needed too so just like plants got their scientific name on top of their traditional name so did constellations.

One set of stars, the variable stars — which brighten and fade rather than shine steadily, became very popular with observers in late 1800 beginning of 1900. To map them, it was easier to assig them an area of the sky in which they reside, so it was important to agree where one constellation ends and the next begins.

Around 1930s, the International Astronomical Union (IAU) formally accepted the modern list of 88 constellations and adopted official constellation boundaries that together cover the entire celestial sphere. Eugène Delporte originally listed the 88 “modern” constellations on behalf of the IAU Commission 3 (Astronomical Notations), in Délimitation scientifique des constellations. (Delporte, 1930) (IAU 2020, Constellations)

The official name for the constellation of the Southern Cross is Crux.

Naming the brightest stars in constellations

While stars have many names, ancient names, modern catalogues names, now they can also be referred by their constellation name. For instance, the brightest star in Crux is Alpha Crucis. By convention, Alpha, Beta, Gamma etc (and the rest of the letter from the Greek alphabet) mark the stars in the order of their brightness, followed by the genitive form of the constellation, in this case Crucis, means ” of Crux” in Latin and Alpha Crucis means Alpha of Crux. For the stars in the Southern Cross these denominations gave further nicknames for these stars, so Alpha Crucis is also known as Acrux, Beta Crucis is Becrux, Gamma Crucis is Gacrux but Delta Crucis is still Delta Crucis.

About Asterisms

The modern constellations which are patches of the sky, just like countries are on Earth, take their name from the associated asterism’s image. Asterisms are dot to dot doodles – they’re the patterns on the celestial firmament.

Some asterisms are very small, like the Pleiades, some other stretch across many constellations, like it is here in New Zealand the waka of Tama Rereti. The same stars, can be part of one or more asterisms in the sky, such as the stars of the Pleiades are known here as Matariki in June in the morning when they herald the Māori New Year, and in November they are just the feathers of Te Waka O Tamarereti canoe. The same goes for the Southern Cross, it’s the anchor of the Tama Rereti canoe.

Te Waka O Tama Rereti, photo John Drummond, 2006. The asterism stretches from one side of the picture right to the the other side.

Te Waka O Tamarereti asterism is made of the following constellations: Scorpius, Lupus, Centaurus, Southern Cross, Vela, Carin, Puppis, Lepus, Orion and Taurus.

You can see all the constellations that make the asterism of Te Waka O Tama Rereti as patches in the sky and you can observe their boundaries marked by straight lines. The curved lines mark the paths of the Sun, Moon and planets. This image was made with Stellarium.

Read More on the IAU official page https://www.iau.org/public/themes/constellations/