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The Sky This Month - September 2013

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Written by Gary Boyle on
Post Date: 
Sun, 2013/09/01

Look High To The Eagle In The Sky

Where has the time gone? We are already into September. And it is this time of year we can start observing about 8:30 p.m. local time. As we begin our observing run, the famous Summer Triangle greets us high overhead. The three suns that make up this asterism are Vega in the constellation Lyra the Harp, Deneb in the Cygnus the Swan and Altair in Aquila the Eagle and is the starting point for this month’s article.

Altair is referred by astronomers as the sweet sixteen star as that is its distance in light years (ly) from us. So if you are sixteen years old and reading this column, the light that you see from Altair tonight left around the time you were born, hence 16 ly away. By comparison Vega is 25 ly and Deneb is an amazing 1,600 ly from us. As you can see, objects are always seen in the past. Even the Moon is one and a third light seconds with galaxies being measures in the millions of light years.

Altair is a very an odd star, oddly shaped to be more exact. This dwarf star has a surface temperature of 7,700 Kelvin (K) and is classified as an A7 star, making it 2,000 K hottest than our Sun. In fact it is ten times brighter than our daytime star. It is considered a main-sequence star as it is fusing hydrogen in helium at its core. What is amazing is its rotation. Our Sun revolves once a month at a leisurely speed of 2 km/sec whereas Altair spins once in about ten hours and its radius is 1.8 that of our Sun. With a spin rate of 210 km/sec rate, no wonder Altair is egg shaped and not round.

The constellation of Aquila is home to no less than eleven planetary nebulae. While galaxies exhibit lovely spiral arm structure and open clusters are difference from one to another, planetary nebulas have character. Amongst the colour of a particular object, they portray unique patterns of stellar composition. Magical portraits are produced from the dying star’s forces pushing against its expanding envelope of gas. An example of a simple bubble is NGC 6781. The nebula is located some 2,500 ly away and is about 2 ly in width. It is listed as magnitude 11.5 but a good six inch telescope should reveal it.

A more unique looking structure can be found in NGC 6741 where the end result is more like a twisty sea shell. Although is small eight arc second length makes it hard to visually see is detail. NGC 6751 is dubbed the ‘glowing eye’ and for good reason. There is a lot of fine detail associated with this relic. Estimated to be around 6,500 ly from earth, NGC 6751 central star has collapsed to become a white dwarf with at surface temperature of an astonishing 140,000 K.

On August 14th only a couple of days after the peak of the Perseid meteor shower Koichi Itagaki of Yamagata, Japan, discovered a new star that was not there the night before. Located in Delphinus, it was estimated this star was about 17th magnitude before the great event and suddenly increased in brightness by 100,000 times. The nova peaked on August 16 and is slowly dimming so you should try to locate it as soon as you can. Now keep in mind the original star is intact. Unlike a supernova when the star is almost completely destroyed. A nova simply blows its lid. The process is a white dwarf slowly building up an accretion disk from a close companion outer atmosphere. Over the course of a hundred years or so the young star continues to steal from the big old dying sun. When sufficient heat (20 million K) and pressure is built up, the envelope around the white dwarf pops like a balloon into a tremendous flash that lingers for a few weeks.

Saturn is sinking further and further into the western sky and will disappear into the Sun’s glare in another couple of months. The ringed planet now sets at 11:30 p.m. local time so you still have time to show the neighbours this jewel. On the other hand mighty Jupiter is up in the eastern sky in the constellation Gemini around 3 a.m. local time. Check the 2013 RASC Observers Handbook page 234 for timing of Jupiter’s moons as they across the surface. The red planet Mars is making its morning comeback and is nestled in the Beehive Cluster (M44) on the morning of the 8th at 22:00 UT. This would make a great digital moment.

Well long awaited Comet ISON is finally emerging from behind the sun and solar glare. That is the good news. The so-so news is ISON appears to be two magnitudes fainter that what is should be on its approach path  and to its November 29 close encounter with the sun. At this point it is hard to predict if the icy mountain will have a sudden outburst to brighten further and save earlier expectations of a daytime comet or will be a great dark sky visitor. Only time will tell. Either way the excitement is there.

The Fall Equinox will occur on the 22nd at 20:44 UT thus marking equal hours of day and night. From this point on we will notice observing hours lengthening till Winter solstice. As during the time around the Spring Equinox, this is the magic time to catch a glimpse and even photograph the Zodiacal light. During the first two weeks look for an arc in the eastern sky before dawn. You will be looking at dusty debris along the plane of the solar system. This arc is lit by sunlight bouncing off the particles in space. The Zodiacal Lights are as faint as the Milky Way and can extend about 30 degrees in height.

This month’s new moon (lunation 1122) is listed for the 5th at 11:36 UT and the full Harvest Moon will occur on the 19th at 11:13 UT.

Until next month, clear skies everyone.

Gary Boyle