A Down to Earth Guide to the Cosmos by Mark Thompson

Author:Mark Thompson
Language: eng
Format: epub
ISBN: 9781448126910
Publisher: Transworld

July: Northern Hemisphere Sky

Looking at the sky we can see why our ancestors believed the night sky was perfect and constant, but it is not until you examine it more closely that you realize it is changing. Take the stars in the easily recognized formation to the south-east called the Summer Triangle: they appear to remain the same year after year. The points of the triangle are marked by Altair in Aquila to the south, Deneb in Cygnus further north-east and Vega in Lyra due west of Deneb. To the casual observer Vega seems to be the brightest, followed by Altair and then Deneb as the faintest, yet in reality Deneb is one of the most luminous stars known, but at a distance of 1400 light years from us its brightness is diminished significantly.

The most southerly of the stars in the Summer Triangle is Altair, which sits in the constellation of Aquila and is a pale yellow star just over 16 light years away. Because of its proximity to us it moves fast enough with respect to other stars for its position to change by around twice the diameter of the full moon in only 5000 years. The movement of close stars like Altair is caused by their motion through space, but stars can display an apparent motion due to the Earth’s movement around the Sun. This ‘parallax’ shift was first measured for a star called 61 Cygni in the mid-1800s by Friedrich Bessel, who determined its distance to be 10.4 light years, just 1 light year off our modern-day value of 11.4 light years. It is a subtle 5th magnitude star found 10 degrees (one fist’s width at arm’s length) to the south-east of Deneb in Cygnus, forming a triangle with Gamma Cygni at the centre of the cross.

Just to the north of Altair is a red giant star over 400 light years away, and north of this is the faint and discreet constellation called Sagitta, which represents an arrow shot by Hercules. The four brightest stars in the constellation are between 3rd and 4th magnitude, making them easy to spot from dark skies. Scan the skies to the north-west of Sagitta and there is a beautiful cluster of stars known as the Coathanger, and as its name suggests the brightest stars form the shape of an upside-down coathanger. Over thirty stars make up this loose open cluster and it is a real treat for binocular observers.

Further north still is another faint constellation, Vulpecula, which is made up of three stars fainter than 4th magnitude forming a shallow triangle with its brightest star, Anser, at the northern point of the triangle. Through binoculars, this star looks like it has a fainter companion, called 8 Vulpeculae, but this is just a line-of-sight effect, with nearly 200 light years separating the two. Through binoculars or the wide field of a finder telescope, the star at the eastern end of the three brightest stars, 13 Vulpeculae, forms a semi-circle with its flat side to the south.

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