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How bright is the Milky Way at night?
How bright is the Milky Way at night?
That silvery expanse of nebulosity, rich in magnificent and colourful stars so closely packed together as to appear as grains of silicates glinting in a bed of oceanside sand across the vast firmament of the black sky on a moonless night. But if you had to use math to give an analytical estimate of that total Milky Way brightness, what value would you assign it? How would you work the sums? AA Institute formulated an original methodology using natural logarithmic equations, and published his results online thus: The Milky Way galaxy's absolute magnitude, accepted in most official journals as an astronomical constant, is -20.5. That figure is based on the assumption that the *entire* galaxy is viewed face on, as one integrated object, from a standard distance of 10 parsecs (32.6 light years). Now, since we are located in one of the spiral arms of the Milky Way not far from the galactic plane and only get an "edge-on view" looking inwards towards the centre of the Milky Way, we see only 50% of the galaxy's total brightness stretching across the night sky (since the remaining 50% is on the *other side* of the dense galactic core, and not directly in view to us). Now, the standard formula for evaluating the brightness ratio, R, between any two objects of magnitudes M1 and M2 is given by:- R = 10^[0.4*(M1-M2)] Hence, this formula can be used to "reduce" the Milky Way galaxy's total absolute magnitude of -20.5 by 50% to give a figure of -19.7, representing the "portion" that we see stretching across our night sky. Since we are located at a distance, d, of about 8,200 parsecs from the galactic centre [Source: Handbook of the British Astronomical Association], the apparent magnitude, m, of the bulk of this "portion" can be calculated from:- m = M - [5 - 5 * log10(d)] = -19.7 - [5 - 5 * log10 (8200)] = - 5.1 Hence, the net integrated magnitude of the "visible" Milky Way stretching across our night skies ought to be about -5.1. However, there are various dark, intervening clouds of interstellar gas and dust, such as the "Cygnus Rift", the "Coal Sack" near Crux, many dark clouds in Sagittarius looking towards the centre of the galaxy, etc. which contribute to dimming the overall light reaching Earth from the broader Milky Way. Hence, if one makes a 10% (0.1- magnitude) allowance for light extinctions owing to such obscuring interstellar media, one will arrive at a net magnitude of -5.0. This would be one way that I would *analytically* estimate the Milky Way's total integrated brightness as -5.0 magnitudes. This figure of course relates to a full 360-degree view of the whole Milky Way. In actual practice, from a particular location on the Earth's surface, only a fraction of this total brightness will be experienced by an observer depending on various factors such as how low the horizons are, which particular quadrant of the galaxy is on view (e.g. the Cygnus region is much brighter than the Auriga region), airglow and light extinctions due to the Earth's own atmosphere which depends on the observer's elevation above mean sea level, etc... AA Institute http://uk.geocities.com/aa_spaceagent/ [Sourced from: Astronomy Physics forum, November 7, 2004 ] Above from a myspace blog evidently run by an abdul ahad fan: http://blog.myspace.com/index.cfm?fu...ogID=300459132 |
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How bright is the Milky Way at night?
Dr. Doolittle wrote:
How bright is the Milky Way at night? The same as it is during the day... John. |
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How bright is the Milky Way at night?
"John" wrote in message ... Dr. Doolittle wrote: How bright is the Milky Way at night? The same as it is during the day... John. Works for me..... Beeb |
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