Alpha Eridani, the End of the River

Achernar is Alpha Eridani, the brightest star of Eridanus, the long winding constellation of the celestial river, and it marks the river's far southern end, its name, from akhir al-nahr, means the end of the river. It is a hot blue star of the B class at about 140 light years, and it is one of the most oblate stars known: it spins so rapidly that it is flung into a markedly flattened, squashed shape, its equatorial diameter far exceeding the distance between its poles. It lies so far south that it never rises for observers across most of Europe and the northern United States.

Achernar's oblateness is extreme even among fast-rotating stars: its equatorial diameter has been measured at more than fifty percent greater than its polar diameter, making it one of the most flattened stars known, a shape so extreme that standard stellar-structure models have struggled to reproduce it without invoking additional physical effects such as internal mixing or a surrounding equatorial disc of shed material. Achernar's far-southern declination means it was entirely unknown to the ancient Mediterranean astrologers and entered the astrological catalogue only once European navigators began systematically mapping the southern sky from the sixteenth century onward.

Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with Almagest (Mathematike Syntaxis)Claudius Ptolemy