Star of the Month
Is there really a green star in the sky?
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