Starship Asterisk*

Thank you starsurfer and Chris for your help with my query. :)

[quote="tomatoherd"]So, does it seem the MW structurally more resembles the Pinwheel than Andromeda? And within the Pinwheel, it looks like the arms fork or branch a lot, with prob. only two close in to the galactic center.[/quote]
The Milky Way is quite different from Andromeda. It is a barred spiral, with a pair of arms coming off the bar (as is usual). Within this broad structure lie two or more additional minor arms, which may be independent of the two major arms, or may be related forks. It's very difficult to tell from our inside view.

So, does it seem the MW structurally more resembles the Pinwheel than Andromeda? And within the Pinwheel, it looks like the arms fork or branch a lot, with prob. only two close in to the galactic center.

[quote="DavidLeodis"]The "luminance" in the explanation is a link to the definition of luminance. I've long assumed that luminance simply meant light that was not a specific colour (such as red, green and blue filters are specific). The definition of luminance does not seem to mean what I thought it meant (not that I really understand the definition :? and a bit :oops: ). I would be grateful if someone could please give an easier to understand definition of what luminance means in astrophotography.[/quote]
In astrophotography (and imaging in general) "luminance" usually refers to the intensity channel of the data ("intensity" being another word with a technical meaning different from how it is used by imagers). So it is, as you suggest, just the image without color information attached- essentially, what you get if you convert a color image to B&W.

More technically, however, photometricists- people interested in extracting useful information from light, and those concerned with the energy of light, use a somewhat bewildering collection of rigorously defined concepts, including intensity, luminance, luminous intensity, flux, irradiance, and many more. These consider things like the angle of emission, area of collection, variation in energy with wavelength, and other factors that are usually not of concern to imagers.

[quote="DavidLeodis"]The "luminance" in the explanation is a link to the definition of luminance. I've long assumed that luminance simply meant light that was not a specific colour (such as red, green and blue filters are specific). The definition of luminance does not seem to mean what I thought it meant (not that I really understand the definition :? and a bit :oops: ). I would be grateful if someone could please give an easier to understand definition of what luminance means in astrophotography.[/quote]
The luminance filter used for astrophotography (usually marked as L) is a clear filter that is used for the definition of detail and brightness and contrast information for a particular imaging target. Most images have the luminance exposure longer than the RGB data for the sake of improved detail.

[float=right][img3="[b][color=#0000FF][size=135]Combined infrared, visible, & x-ray images of The Pinwheel Galaxy M101 showing that both young & old stars are evenly distributed along the tightly-wound spiral arms.[/size][/color][/b]"]https://upload.wikimedia.org/wikipedia/commons/3/3b/NASA-SpiralGalaxyM101-20140505.jpg[/img3][/float]
[quote="tomatoherd"]

Art: I'm sure computers worked hard to compile these hypothetical Milky Way arms out of stellar positions/concentrations, but why, if they are true, does the MW have so many arms? I don't recall seeing any spiral galaxy on APOD with that many....[/quote]

[list]Perhaps you've never looked that hard.[/list]

The "luminance" in the explanation is a link to the definition of luminance. I've long assumed that luminance simply meant light that was not a specific colour (such as red, green and blue filters are specific). The definition of luminance does not seem to mean what I thought it meant (not that I really understand the definition :? and a bit :oops: ). I would be grateful if someone could please give an easier to understand definition of what luminance means in astrophotography.

Art: I'm sure computers worked hard to compile these hypothetical Milky Way arms out of stellar positions/concentrations, but why, if they are true, does the MW have so many arms? I don't recall seeing any spiral galaxy on APOD with that many....

[quote=" http://messier.seds.org/m/m020.html"]

<<The Trifid Nebula's distance is rather uncertain, with values between 2,200 light years (Mallas/Kreimer; Glyn Jones has 2,300) and about 7,600 light years (C.R. O'Dell 1963). The Sky Catalog 2000 gives 5,200 light years, a value which is also used by Archinal and Hynes (2003), and which we adopt here. The WEBDA database has 3140, the Hubble Press Release of Jeff Hester (STScI-PRC99-42) gives "about 9000" light years.>>[/quote][float=right][img3="[url=https://en.wikipedia.org/wiki/Carina%E2%80%93Sagittarius_Arm][size=145][b][color=#FF0000]Carina–Sagittarius Arm[/color][/b][/size][/url][size=100][b] or [/b][/size][url=https://en.wikipedia.org/wiki/Scutum%E2%80%93Centaurus_Arm][size=145][b][color=#00AF00]Crux-Scutum–Centaurus Arm[/color][/b][/size][/url] :?:"]https://upload.wikimedia.org/wikipedia/commons/1/1d/Milky_Way_Arms.svg[/img3][/float][quote=" http://asterisk.apod.com/viewtopic.php?f=9&t=34995&p=246251"]

As seen here, the Trifid lies only [b][color=#FF0000]5,500 light-years away[/color][/b] [in the [b][color=#FF0000]Carina–Sagittarius Arm[/color][/b]] and is about 30 light-years across.[/quote][quote="APOD Robot"]

[url=https://www.youtube.com/watch?v=xe3lYHcmbEI]The nebula[/url] lies about [b][color=#00AF00]9,000 light years away[/color][/b] [in the [b][color=#00AF00]Crux-Scutum–Centaurus Arm[/color][/b]] and the part [url=http://www.robgendlerastropics.com/M20-Subaru-HST.html]pictured here[/url] spans about 10 light years.[/quote]

[b][size=150][color=#0000FF]The Flux Capacitor Nebula[/color][/size][/b]
[img]http://fsa.zedge.net/scale.php?img=Ni84LzMvMC8xLTYyMzM4MzUtNjgzMDY4MS5qcGc&ctype=1&v=4&q=81&xs=620&ys=383&sig=5a47e1cc6bc098303a812205aa2ed0c22dcf9649[/img]
:D

Since this is Sunday this is a repeat?

Well, it is a spectacular repeat!!!

:---[===] *

[url=http://apod.nasa.gov/apod/ap151011.html][img]http://apod.nasa.gov/apod/calendar/S_151011.jpg[/img] [size=150]In the Center of the Trifid Nebula[/size][/url]

[b] Explanation: [/b] Clouds of glowing gas mingle with dust lanes in the [url=http://messier.seds.org/m/m020.html]Trifid Nebula[/url], a star forming region toward the [url=http://www.astro.wisc.edu/~dolan/constellations/extra/constellations.html]constellation[/url] of the Archer ([url=http://chandra.harvard.edu/photo/constellations/sagittarius.html]Sagittarius[/url]). In the center, the three prominent [url=http://apod.nasa.gov/apod/ap030706.html]dust[/url] lanes that give the [url=http://apod.nasa.gov/apod/ap110513.html]Trifid[/url] its name all come together. [url=http://apod.nasa.gov/apod/ap071226.html]Mountains[/url] of opaque dust appear on the right, while other dark filaments of dust are visible threaded throughout the nebula. A [url=http://adsabs.harvard.edu/abs/2001ApJ...562..446R]single massive star[/url] visible near the center causes much of the [url=http://apod.nasa.gov/apod/ap980828.html]Trifid's glow[/url]. The Trifid, also known as [url=http://en.wikipedia.org/wiki/Trifid_Nebula]M20[/url], is only about 300,000 years old, making it among the youngest [url=http://apod.nasa.gov/apod/emission_nebulae.html]emission nebulae[/url] known. [url=https://www.youtube.com/watch?v=xe3lYHcmbEI]The nebula[/url] lies about 9,000 [url=http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question19.html]light years[/url] away and the part [url=http://www.robgendlerastropics.com/M20-Subaru-HST.html]pictured here[/url] spans about 10 light years. The [url=http://www.robgendlerastropics.com/M20-Subaru-HST.html]above image[/url] is a composite with [url=http://en.wikipedia.org/wiki/Luminance]luminance[/url] taken from an image by the 8.2-m ground-based [url=http://subarutelescope.org/Introduction/outline.html]Subaru Telescope[/url], detail provided by the 2.4-m orbiting [url=http://hubblesite.org/the_telescope/]Hubble Space Telescope[/url], color data provided by [url=http://www.martinpughastrophotography.id.au/]Martin Pugh[/url] and image assembly and processing provided by [url=http://www.robgendlerastropics.com/]Robert Gendler[/url].

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