Star of the Month
Pollux, one of the celestial twin stars in Gemini
Despite its beta status, mag. +1.2 Pollux (Beta (`) Geminorum) is the brightest star in Gemini and the 17th brightest star in the night sky. It is linked to mag. +1.6 Castor (Alpha (_) Geminorum) through mythology rather than any physical association. The stars provide the heads of the twins Castor and Pollux, who together form the constellation of Gemini. Castor lies at a distance of 51 lightyears while Pollux is closer at 33.8 lightyears.
On first appearance Castor and Pollux look fairly similar. It’s only on close inspection that it’s obvious that Pollux is brighter and shows a more orange-hued colour than its mythological twin. Pollux is an old giant star, the closest star of this type to our own Sun. It’s around twice as massive as the Sun and about nine times its radius.
It is known to have a gas planet in orbit around it with an estimated mass of 2.3 Jupiters. Known as Pollux b, it has a nearly circular orbit of radius 1.64 AU, taking 1.6 years to complete one circuit around Pollux. The IAU’s official name for Pollux b is Thestias, bestowed on it by public nomination in 2015.
Pollux has a spectral type of K0 III. The K0 part describes its position within the MorganKeenan spectral classification system. This is an important position because Pollux is regarded as one of the system’s ‘anchor’ stars: a star that has a key spectral type which hasn’t changed over the years. The ‘III’ identifies Pollux as a normal giant. Pollux has a weak X-ray emission similar to that of our Sun. Its magnetic field is particularly weak, with a strength measured below 1 Gauss, one of the weakest magnetic fields ever detected around a star.
The colour difference between Castor and Pollux is obvious and an interesting subject to reveal photographically. A particularly good way to do this is to capture both stars in one frame at long exposure so that they trail. This really emphasises their difference.