Prominent in the southern night sky over the summer months is what is known as the Summer Triangle. This is a grouping of three of the brightest stars in the sky – Vega, Deneb and Altair. All three are shown in the attached diagram (based on SkyMap Pro software).
Vega is by far the brightest star in the tiny constellation of Lyra the lyre. Legend has it that Lyra was the lyre given by the god Apollo to Orpheus, and was placed in the sky after his death. Vega is the fifth brightest star in the sky. And of those visible from Devon, only Sirius and Arcturus are brighter. Deneb, in the constellation of Cygnus the swan, was dealt with in last month’s column. Altair is the most southerly of the three, and is the brightest star in the constellation of Aquila the eagle.
Stars vary enormously in the amount of matter they contain. The smallest are a little less than a tenth of the mass of our own Sun, and the most massivecan be up to a hundred times the mass of our Sun. The more massive a star is, the more its centre is squeezed by gravity, and the hotter its centre becomes. The hotter the centre of the star, the faster its nuclear reactions proceed, and the brighter the star shines. Deneb is a much more massive star than either Vega or Altair, so even though it is about a hundred times as far away as the other two, it seems to be about the same brightness. If it were the same distance away as the other two, it would appear far brighter than them.
Full Moon this month will be on Saturday 13 August.
There are two announcements of local interest. First, the Norman Lockyer Observatory Society in Sidmouth will be holding its annual Astronomy Fair on Saturday 6 August. This all-day event consists of a series of astronomical talks and exhibits, planetarium presentations and telescope dome tours. Entry is £5, and details can be found on the NLOS website at http://projects.exeter.ac.uk/nlo/Astronomy_Fair~EN.php
Secondly, September will see the resumption of meetings of the Torbay Astronomical Society, after its usual summer break. Full details of their September meetings will be provided in next month’s column.