Whitby & District Astronomical Society

In this month's Sky Notes:

• Planetary Skylights
• Meteor Activity
• July 2016 Sky Charts

Planetary Skylights

Three major planets are now visible in the late evening sky, Jupiter, Mars and Saturn.

Jupiter continues to dominate the western sky as twilights deepens, although by the end of July it is only visible for a short while before setting.  Observations are always fascinating: the location of the Galilean moons, the banding across the disk, and when on view, the size and colour of the great red spot, all add interest through the eyepiece.  From the 12-14^th Jupiter resides quite close to sigma Leonis, whilst on the 13^th Europa passes in front of the Jovian disk with Ganymede on one side and Io and Callisto on the other.  Observe around 22:30h.  Our Moon is closest on the 8^th and 9^th

Saturn also resides low to the south, to the left of Mars, and having reached opposition last month remains a conspicuous object, its pearly white lustre contrasting nicely with ‘ruddy’ Mars.  A telescope is required to see the ring system, look for the Cassini gap in the rings as well as several moons nearby.  Titan, Saturn’s largest moon appears as a bright speck close by, orbiting anticlockwise twice a month.  Our Moon resides above Saturn on the 16^th.

Pluto reaches opposition on July 7^th, but at magnitude +14, you will definitely require quite a large scope (10-inch plus) to identify it for certain.  Pluto resides in the ‘teaspoon’ asterism which adjoins the ‘Teapot’ asterism in Sagittarius located not far above the south horizon around midnight. 

General location of Pluto in Sagittarius on 25-Jul-2016 at 23:00h	Teapot and Teaspoon Asterisms on 25-Jul-2016 at 23:00h

 

Meteor Showers

Looking South Mid-July - 22:30h	Looking North Mid-July - 22:30h
Looking East Mid-July - 22:15h	Looking West Mid-July - 22:30h
Overview 9th-July - 22:30h

Additional Image Credits:

• Planets and Comets where not otherwise mentioned: NASA
• Sky Charts: Stellarium Software

From mid-July, evenings gradually become darker once again, allowing more time for observers to explore and appreciate the summer constellations.  The brightest stars emerge around 22:15h, isolated jewels dotted around the heavens.  Of these, seven in particular catch the eye and three; Arcturus, Vega and Capella, are ranked in the top five brightest stars visible from our shores.

Let us begin at Arcturus, the brightest star visible for most of the year, second only to the winter star Sirius.  You will locate the golden hue of Arcturus - meaning ‘guardian of the bear’ quite high in the west, residing in the constellation of Bootes - the Herdsman.  At a distance of 37 light years Arcturus is the nearest example of an orange giant star, a ‘sun mass’ star in the latter stages of its life having converted all of its Hydrogen into Helium.  In the distant future our Sun will evolve through a similar stage before ending its days as a planetary nebula.  As twilight deepens further, note brilliant Capella in Auriga, found low above the N horizon.  At this time of year Capella is often mistaken for the ‘north’ star, the true pole star; Polaris in Ursa Minor, resides some 54 degrees above the North horizon and is considerably fainter.

Swing back round to the WSW to locate Spica in the constellation of Virgo, soon to depart our skies.  Like Capella, Spica is a spectroscopic double, each component bigger and more massive than our Sun and separated by just 17½ million km (11 million miles).

Our next destination is located not far above the South horizon.  Antares, meaning ‘rival of Mars’ due to its deep orange hue, is the chief star in Scorpius, the outline of which does resemble that of a scorpion, though it only partly rises over Britain.  Antares is a very tenuous ‘red super giant’ nearing the end of its life and is physically one of the largest stars readily visible to the naked eye, some 300 million miles in diameter.  Astronomers believe Antares is a supernova candidate, one of only a few such bright examples visible in the night sky, however they are uncertain as to when this catastrophic event will occur.  A word of caution when identifying Antares, Mars – the red planet, also resides low to the South, though is far brighter to the naked eye.  The conspicuous white star to the left of this ‘ruddy’ pairing is the planet Saturn.

High overhead to the south and southeast are the three stars of the ‘summer triangle’.  Rivalling Arcturus in brilliance, steely blue Vega, resides in the small geometric pattern of Lyra, the most Northern of the summer trio.  Vega is a young star, possessing a disk of debris that may coalesce to form a planetary system.  It lies just 26 light years away.  Our next stop, Altair can be located in the ESE in the constellation of Aquila the Eagle and marks the southern point of the triangle.  Still in the flush of youth, Altair is a star in a spin: an unusually rapid one, rotating in less than 6 hours and would appear rugby ball shaped up close.  Altair has the distinction of being closest of the bright summer stars visible from Britain, a mere 16 light years away.

Our final destination is currently located high to the ENE in the constellation of Cygnus.  Deneb is visually the least conspicuous of the ‘summer triangle’ and indeed our magnificent stellar seven.  Appearances though can be deceptive.  Its immense distance of around 1600 light years hide Deneb’s true nature, a stellar superstar, a genuine heavy weight, 25 times more massive and at least 150 thousand times more luminous than our Sun. 

Although just a few million years old, Deneb’s high initial mass dictates its rate of evolution, which is extremely rapid, converting energy at a prodigious rate so that even now all of its Hydrogen has been used up.  This ‘time bomb’ is ticking and in little more than 15 million years Deneb will evolve into a star almost twice the size of Antares, finally ending its brief life in cataclysmic fashion, as a supernova.  On that day our descendents will marvel at this spectacular stellar death, a wonder to all in our galaxy... and beyond.