Star of the Month

Is there re­ally a green star in the sky?

Sky at Night Magazine - - CONTENTS -

The star Beta (`) Li­brae, sits at the top of the fairly in­dis­tinct con­stel­la­tion of Li­bra, the Scales. This is rea­son­ably well po­si­tioned, low in the south at 02:00 BST (01:00 UT) on 1 May, 01:00 BST (00:00 UT) on 15 May and 00:00 BST (23:00 UT) on 31 May. At mag. +2.6 it is a mid­dling bright­ness star that goes by the won­der­ful name of Zube­neschamali, mean­ing ‘north­ern claw’. Its coun­ter­part is mag. +2.8 Al­pha (_) Li­brae or Zubenel­genubi, which means ‘south­ern claw’. At the mo­ment, the bright planet Jupiter is lo­cated re­ally close to Zubenel­genubi. Al­though it seems odd to have two ‘claws’ in a con­stel­la­tion rep­re­sent­ing a set of bal­ance scales, this makes more sense when it’s re­vealed that Li­bra used to be part of neigh­bour­ing Scor­pius, the Scor­pion.

Zube­neschamali is a B8V dwarf star with a sur­face tem­per­a­ture around 12,300K. It’s around 130 times more lu­mi­nous and 4.9 times larger than our Sun, with 3.5 times the mass. It also spins over 100 times faster than the Sun, with a ro­ta­tional ve­loc­ity of 250 km/s. At an es­ti­mated age of 80 mil­lion years, this is a hot, young star. Its tem­per­a­ture sug­gests it should shine with a blue-white colour and to most ob­servers this is in­deed the case.

How­ever, in the past Zube­neschamali has been de­scribed as green in hue. If this were true Zube­neschamali would be the only green naked-eye star in the sky. Green is an odd colour for a star be­cause, even if a star’s out­put were to peak in the green part of the spec­trum, the nar­row range of wave­lengths from 500-570nm, which ap­pear as green, are eas­ily swamped by the wide yel­low and blue wave­lengths ei­ther side. Con­se­quently, to the hu­man eye, such stars tend to ap­pear white. Does Zube­neschamali buck the trend? Pop out­side and judge for your­self.

Small vari­a­tions in Beta (`) Li­brae’s mag­ni­tude hint at an as-yet undis­cov­ered com­pan­ion star

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