Astronomy Benalla
Reports - 2015
Whirlpool Galaxy m51 & companion galaxy ps07 (Hubble) Black Hole Butterfly Nebula (Hubble image) Sombrero Galaxy (Hubble) Home of Astronomy Benalla Carina Nebula Pillar - ps49 (Hubble image) Most photos on this site can be zoomed by clicking the photo
Astronomy Benalla  General Meeting Presentations - Wednesday 18th february 2015 Our Nearest Star                         Presenter: Rupe Cheetham   Our nearest Star – the Sun  a  Class G2 star The Sun’s surface is very dynamic. It is perpetual and variable in its motion and at times even more so. The size, location  and brightness of the various features we view, can reach peaks and troughs on a minute to minute, hour to hour, day to day  and month to month basis; besides having a general 11 year sunspot cycle.   The Sun’s  diameter is 110 times that of the Earth and consists of a Core (25 % of its diameter) at 15million deg K,  surrounded by a Radiative Zone reaching out to 86% of the diameter, then a Convective Zone out to 100 %, Then there is  the Photosphere, like a thin skin, a Chromosphere that is the Sun’s atmosphere; followed by the ultra-low density Corona at  1 to 3 million Kelvin with a variable depth of many millions of kilometres.    I will concentrate on the  Sun’s surface and  atmosphere that we see. Photosphere This is the Sun’s surface we see and has a rice grain texture (granules) that are about 500km deep with a  temperature of about 5,800K, and is orange when viewed through H-alpha filtered telescopes.  Granules  I have also mentioned there are numerous small light areas on the photosphere that are about 1000km across,  may have polygonal shape and are separated by inter-granular lanes about 400K cooler. Granules have a life time of around  20 minutes and are made up of fragmentation of granules. It is postulated that the granules are convection cells of rising hot  gas and the inter-granulation lanes are descending cooler gas. Near sunspots, the granules are elongated.    Chromosphere Is the Sun’s atmosphere above the photosphere from about 500km to about 9,000km thick.  Its temperature  rises from 4,400K  at 500km to about 6,000K at 2000km and rapidly rises above that height.  Prominences Are easily seen in H-alpha on the Sun’s dark limb silhouetted against the black sky, they have a temp of  about 10,000K  typical of the Sun’s chromosphere with densities of 100 times higher than the inner corona. There are active  prominence’s lasting days, and quiescent prominences lasting for a month (one solar rotation). They can take the shape of  jets, plumes, loops, hedgerows and detached. They closely follow the magnetic inversion lines. (often marked by filaments)  They are more frequent during the Sun’s rising solar cycle.   Filaments Are long tongues of relatively cool material (10,000K) suspended in top of the chromosphere and the much  hotter solar corona (2,000,000K) and show up as dark lines against the Sun’s orange face comprising of granules. At the  Sun’s limb they appear as prominences or loop prominences and sometimes seen near large flares.    Flares  Are a sudden release of a huge amount of energy in the Sun’s inner corona 1/4 Sun’s diameter i.e. about  175,000km; lasting several hours and are associated with active Sun spots. Most of the energy is in the first few seconds or  minutes. They emit radiation over the whole spectrum, gamma to radio waves; also at high speed (70% speed of light) with  particles (electrons, protons, and atomic nuclei) reaching the Earth in 15 minutes. The temperatures reach 10 to 20 million  Kelvin They are classified increasingly as C,M, & X classes in the emitted soft x-rays.  Hyder Flares are associated with large thicker filaments and are slower to peak than normal flares i.e. 30 to 60 minutes, and  may last several hours. Energy released can be up to 1027 joules. Sunspots  Are the dark areas we see using H-alpha filters or white light filters when viewing the Sun’s photosphere, are  cooler than their surrounds, and are associated with very strong magnetic fields. They vary in size from 300km diameter to  > 100,000km diameter and are about 1600K cooler (i.e.. 4,600K) than the photosphere’s granules we see at about 5800K.   Well-developed sunspots have darker interior ‘umbra’ and lighter outer ‘penumbra’ which amount up to 70 % of the spots  area.  They are surrounded by faculae with higher temperatures. The more complex in appearance and magnetic field  structure sunspots, can give rise to flares.  Faculae Are brighter and hotter 6,100K patches on the Sun’s photosphere that appear before sunspots form and remain  after the spots vanish. They are best seen close to the Sun’s darkening limb  Plages  Are brighter and hotter patches in the chromosphere above faculae and are regions of particularly strong magnetic  fields.  Can be seen with H-alpha and Calcium K filters.    Spicules  Are small (500km diameter) spike-like jets in the chromosphere lasting about 15 minutes then fading. They can  be seen clustering on the Sun’s limb in H-alpha light. On the Sun’s disc they form radiating patterns called Rosettes.    Coronal Mass Ejection  (CME)  Is a large scale ejection of matter from the Sun’s corona at speeds of 10- 3000km/s. Their  mass is about 1010 tonnes. (100 billion tonnes). Their frequency varies with the sunspot cycle. At Solar Minimum they may  occur 1 per week, to 2 or 3 per day at Solar Maximum. Their energy takes 1 to 4 days to reach the Earth subject to their  speed and causes electromagnetic disturbance to our upper atmosphere resulting in auroras in our polar regions; can also  cause disturbance with HF radio etc. (NASA many views!!!)  2015 is the UN Year of Light. Our Light and energy ultimately comes from the Sun.   Acknowledgements     Oxford Dictionary of Astronomy  Royal Swedish Academy NASA Wikipedia.
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