Adrian Dening's Stars Over Somerset

Wednesday, 13 July 2022 17:00

Monday 18th to Sunday 24th July 2022

Last week we focussed on the constellation of Hercules that is located high up towards the south if you are out observing just after it gets dark.
Beside Hercules is the constellation of Lyra and that's where we are going next.

The most obvious feature of Lyra is the bright star Vega. The star is only 25 light years away from us and is the third-brightest star in the night sky of the northern hemisphere. Vega is only about 1/10th the age of our own Sun and it is classed as a "variable star" because its brightness varies slightly. Curiously, it rotates rapidly at a speed of 236Km/s and this makes it bulge in the middle.

Everyone is familiar with Polaris, the "Pole Star", that marks the point in the night sky everything else appears to rotate around. Well that isn't quite true and hasn't always been the case.

When astronomers set-up an equatorial telescope mount, they have to make a slight adjustment because Polaris isn't exactly in the centre.

The Earth "wobbles" slightly on its axis and its position relative to the stars, or "axial precession" to get technical, changes over a period of 22,500 years. Vega was actually the northern pole star around 12,000 BC and will be again around the year 13,727AD.

Slightly to the left of Vega is the "Double Double" star Epsilon Lyrae. With binoculars a binary star is seen, but when observed with a telescope, each of these two stars is revealed to be a double star - hence the name. Binary stars orbit around each other due to mutual gravitational forces where double stars just appear close together when viewed from the Earth.

Axial Precession diagram courtesy of Wikipedia
Epsilon Lyrae photograph courtesy of Nikolay Nikolov
Screenshots courtesy of Stellarium
Copyright Adrian Dening and Radio Ninesprings 2022