Double Stars: visual delights, observational challenges, astrodynamical testbeds

Tuesday 6th December, 2016

Our speaker was Christopher Taylor who is Director of Hanwell Community Observatory and he came to talk to us about "Double Stars: visual delights, observational challenges, astrodynamical testbeds".

He began by saying that of his 50 years observing the sky he had spent the past 20 years studying double stars and in 2016 he had managed 46 nights of detailed observing and recording. He explained that his reference for them was the Washington Double Star database, which lists 139,000 double stars and is compiled and updated nightly by two astronomers at the United States Naval Observatory.

To make his observations he uses a Newtonian telescope that was built in the 1930s or 1940s and has a mirror with a diameter of 12.5 inches that was made in 1908. He also uses a refractor telescope that has lenses instead of a mirror to focus the starlight and its main lens is only 4 inches across.

Christopher then said that most of the stars in the sky are actually members of double or multiple star systems and professional and amateur astronomers alike measure the relative distances and angles between double stars in order to work out how they orbit each other. There are three types of stars that appear in pairs in the night sky: optical doubles, visual binaries and non-visual binaries.

Optical doubles are stars that appear together by chance along a line-of-sight from Earth and so are not in orbit around each other. Visual binaries are stars that are in orbit around each other due to their mutual gravitational attraction and they can also be seen as separate stars when viewed through a telescope. Non-visual binaries are stars in a true orbit around each other but they cannot be seen as two distinct objects using just a telescope. Special instruments such as spectroscopes, which split starlight into a spectrum, need to be used to confirm that this third group is in fact two stars in orbit around each other. Fortunately, as technology advances in telescope design the non-visual binaries can become visual binaries.

In our current winter sky, the brightest star called Sirius is actually a double star system consisting of the brilliant white star Sirius A and a dimmer star Sirius B. As the main white star is so bright it is difficult to see its orbiting companion which is 63,000 times fainter although in 2019 they will be at maximum separation.

A much easier target is that of the stars Mizar and its companion Alcor which can be seen with the naked eye for those with sharp eyesight in the bend in the handle of the "Big Dipper" or "Plough" pattern of stars. Viewing Mizar with binoculars or a telescope will confirm that this indeed a pair of stars.