July & August 2020

by Roger Davies

T he summer months bring into view one of the great signposts of the sky: the summer triangle.

Three bright stars: Vega in the small constellation of Lyra (the Lyre), Altair in Aquila, the Eagle, and Deneb in Cygnus, the Swan, form a grand triangle more-or-less overhead at 01.00 in mid-July and 23.00 in mid-August. To find them, see the interactive star chart here.

The summer triangle of Vega, Deneb and Altair are high in the sky in July and August. Image source from here.

Vega is the brightest of the three stars (it is the fifth brightest star in the heavens) and just 25 light years from the Sun. It is the standard on which measurements of the brightness of stars are based, and as such it has been studied in great detail. In the 1980s data from the IRAS satellite showed that Vega emitted more light in the infrared part of the spectrum than was expected from an ordinary blue star. This intriguing discovery is attributed to a disk of asteroid-like debris orbiting around Vega, similar to the Kuiper belt, which is beyond Neptune, in our own Solar System. In the following decades many examples of debris disks surrounding stars have been discovered.

Altair is the next brightest star, about half as bright as Vega, and even closer, 17 light years away. Deneb appears half as bright again but it is much more distant — over 3500 light years away. Although it appears to be faintest of the three stars it is intrinsically by far the most luminous radiating perhaps as much as 200,000 times as much light as the Sun. All three stars have a bluish hue and span the sky overhead spectacularly in the summer months.

The Milky Way passes through the constellation of Cygnus. The Kepler spacecraft stared at the field shown for several years monitoring the brightness of 150,000 stars looking for transits caused by exo-planets. It found over 2,500 such worlds in the small field of view shown. Image source from here.

Now you've found Cygnus, on a dark night, and after a few minutes allowing your eyes to accommodate to the darkness, you will see the Milky Way crossing the sky through this constellation. Kepler, the NASA mission launched in 2009, stared in the direction of Cygnus for several years (see figure) monitoring the brightness of over 150,000 stars. It was looking for the characteristic dip in brightness caused when a planet passes directly across the line-of-sight between the telescope and a star, a so-called transit. This method of discovering planets proved to be extraordinarily successful, Kepler discovered over 2,500 and showed that 'exo-planets', are very common, in fact the norm for stars of all types. The study of exo-planets has opened up the prospect of analysing the composition of their atmospheres and in a few cases water has been discovered. The ultimate goal is to look for biomarkers in the atmospheres of other planets as indicators of biological activity. This is an enormous interdisciplinary effort involving biochemists, atmospheric scientists and biologists as well as astronomers. It will probably require new space missions and terrestrial telescopes to detect even the first hints of biological activity. Surely one of the central questions of twenty-first century science must be: is there life elsewhere in the Universe?

Two of the gas giant planets, Jupiter and Saturn, move into 'opposition' this month on July 14th & 20th respectively — that means they are opposite the Sun in the sky, which is the best time to see them. They are both bright and close together in the constellation of Sagittarius, but are not very high in the sky. They rise in the south-east around 10pm but are best seen between mid-night and 1.00am when they will be due south. Even then they reach an elevation of only 17° so you'll need a good southern horizon to see them.

For early risers Venus is a morning object visible in the eastern sky before sunrise and on July 17th it will be close to the waning crescent Moon — they are separated by 5-6 Moon diameters — but you'll need to up at 4am to get a good view!

New Moon is 20th July and 19th August so the few days around these dates are the best times to see the Milky Way but Jupiter and Saturn can be seen on any clear night in July and August.

August brings the Perseid Meteor shower peaking on 11th and 12th when perhaps we'll be treated to one meteor per minute. The meteors arise as the earth passes through debris left behind in the orbit of comet Swift-Tuttle, discovered in 1862. The meteors will radiate from the constellation Perseus, but can appear anywhere in the sky. For the best view find a dark location and wait until after midnight — but the Perseids are sufficiently numerous and bright you can see them all night.

Happy stargazing!

SEE ALSO by Roger Davies:
Stargazing • February 2021
Stargazing • November 2020
Stargazing • September 2020
Comet Neowise • July 2020
Stargazing • July & August 2020
Stargazing • June 2020
Stargazing • May 2020
Stargazing • April 2020

Roger Davies is a resident of the village of Kidlington, Oxfordshire.

Roger is the Philip Wetton Professor of Astrophysics and Director of the Hintze Centre for Astrophysical Surveys at the Department of Physics, University of Oxford,

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