Star of the Month

Alde­baran - the fiery eye of Tau­rus, the Bull

Sky at Night Magazine - - CONTENTS -

Alde­baran (Al­pha (_) Tauri) is one of the more prom­i­nent stars of the au­tumn and win­ter skies. It is an or­ange gi­ant star, shin­ing at mag. +0.9 with a slight vari­abil­ity of 0.2 mag­ni­tudes. Alde­baran is par­tic­u­larly easy to lo­cate – just ex­tend the line of Orion’s Belt north­west. It ap­pears to be associated with the V-shaped Hyades open clus­ter, ly­ing at the end of the V’s south­ern arm, yet this is an­other in­stance where ap­pear­ance is de­ceiv­ing – the 153 lightyear-dis­tant clus­ter is roughly twice as far as Alde­baran, which is 65.3 lightyears away.

Alde­baran is 425 times more lu­mi­nous than the Sun, and the most lu­mi­nous star within 100 lightyears of Earth. It is also one of the clos­est giants, with a di­am­e­ter es­ti­mated to be 44 times that the Sun. If you re­placed our Sun with Alde­baran, it would have an ap­par­ent di­am­e­ter of around 22º.

Its close prox­im­ity and large phys­i­cal di­am­e­ter means Alde­baran presents a disc

large enough to mea­sure. Even so, this is very small at just 0.021 arc­sec­onds. A gas gi­ant planet sev­eral times the size of Jupiter may or­bit the star: its ex­is­tence was sug­gested in 1993, but re­mained doubt­ful un­til a 2015 study pro­vided long-term ev­i­dence to back up the claim. From Earth, a num­ber of faint stars

ap­pear close to Alde­baran. One of these, Al­pha Tauri B, ap­pears to have a sim­i­lar dis­tance and mo­tion to Alde­baran. How­ever, be­ing faint at mag. +13.6 and only sep­a­rated from its bright host by half an ar­cminute, it is dif­fi­cult to con­firm or deny whether it is a true, phys­i­cally bound com­pan­ion.

Alde­baran’s name trans­lates as ‘fol­lower’, a ref­er­ence to it chas­ing the Pleiades through the sky

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