LOOK UP: The Alberio binary star.
LOOK UP: The Alberio binary star. Contributed

DARK SKIES: More to stars than meets the eye

A VISUAL binary is one whose components can be resolved visually or photographically.

Visual binaries tend to be systems that are relatively close to us and whose components are widely separated in space - by at least a few hundred million kilometres and in many cases, much more than that.

Less than 1000 visual binaries are known, including well-known ones such as Alpha Centauri - the brightest star of the 'Pointers' and Alpha Crux - the 'Foot' star of the Southern Cross.

While both appear as a single bright star to the eye, both can be seen quite well as a 'double star' system, in a powerful telescope.

Alpha Centauri is also in a three-way system with a fainter red dwarf star - Proxima Centauri, which was just recently found to harbour a Jupiter-sized planet, orbiting this massive dying star!

Alpha Crux on the other hand, is a perfect set of 'twins'.

The orbital period of these two stars is about 1500 years and the separation is wide enough to fit quite a few Solar Systems, edge to edge.

Two other visual Binaries that are quite well known are the Turquoise and Crimson stars called Alberio (see image) and the 'Dog Star' Sirius in Canis Majoris, which has a smaller companion star orbiting its primary every 11.2 years.

Another nice double is Rigel in Orion.

A spectroscopic binary is a binary star in which the two components are so close together or so far from the Sun, that they cannot be resolved simply by looking at them, even through a powerful telescope.

Their binary nature can, however, be established because of the Doppler Shift - red means increasing distance and Blue approaching.

.All of this is based on its spectral lines which can tell astronomer, the star's age, size, surface temperature and to a degree, its distance in light years. So as the stars revolve around their common centre of gravity, they alternately approach and recede in the line of sight. This motion shows up in their combined spectra as a regular oscillation or doubling of the spectral lines.

Double-lined spectroscopic binaries have two sets of spectral features, oscillating with opposite phases while single-lined spectroscopic binaries have only one set of oscillating spectral lines, owing to the dimness of the secondary component. In most cases, the components of a spectroscopic binary are so close together that each is distorted into a nonspherical shape, making it quite hard to see in telescopes.

So the next time you look skyward at night, just think that two-thirds of them are in a relationship with another and half to three-quarters may not exist anyway, due to that star's death millions of years ago.

Book a stargazing night at the Observatory, email mao123@bigpond.com or phone James on 0427 961 391, or visit www.kingaroy- observatory.com.

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