Starship Asterisk*

In regard to the "The result is strong evidence that NGC 2818 is only by chance found along the line of sight to the star cluster and so may not share the cluster's distance or age" in the explanation, the information brought up through the "Supplemental" that is brought up through the "Hubble image is" link is useful and easily missed.

PS. The 'Paige Ponders' in the "Supplemental" has a 1950s or so look to me. :P

[quote="Chris Peterson"]
It works just like with stars, except instead of measuring the Doppler shift of absorption lines, you measure the shift of emission lines.[/quote]

Got it. I'm going to assume, then, that the emission spectra are distinct and separate enough that there's little chance of crossover or mistake.

I appreciate your help!

[quote="Al Denelsbeck"]So, how does this work for nebula? Is there a 'standard candle' that applies, or some way in which we have a spectral marker, to measure redshift from that? [/quote]
It works just like with stars, except instead of measuring the Doppler shift of absorption lines, you measure the shift of emission lines.

[quote]On a related note, when filtering for hydrogen emissions in the images, does redshift mess this up at all? Is there a different filter that has to be used for nebula receding at a higher velocity?[/quote]
Only for objects at cosmological distances. The shift for nearby, fairly slow bodies is spectroscopic, but typically inside the passband of photometric and photographic filters.

All right, a complicated question here.

The description says that the nebula's own velocity is different from that of the open star cluster. I know that the stars can be measured in redshift because the absorption line patterns define their type, and the shift in where those lines 'should be' tells us the redshift and thus the velocity.

So, how does this work for nebula? Is there a 'standard candle' that applies, or some way in which we have a spectral marker, to measure redshift from that? It seems a little odd to me that we can measure its velocity but not its distance.

On a related note, when filtering for hydrogen emissions in the images, does redshift mess this up at all? Is there a different filter that has to be used for nebula receding at a higher velocity?

I hope I'm not provoking something that requires a major essay, but this struck me as curious.

Awesome....looks like an aquarium, with fish...but that is OK...Hydrogen and Oxygen make WATER....


:---[===] *

4 light years across width worth of wow.

Truly incredible joint-effort production here.

I'm personally struck between favoring planetary nebula more, or super nova, in terms of inspiring imagery and pure awe and fascination. The last two APOD's in particular have been pretty mind-blowing.

Wow! Looks like someone poked a hole in the black of space and let some light in.

[url=http://apod.nasa.gov/apod/ap140713.html][img]http://apod.nasa.gov/apod/calendar/S_140713.jpg[/img] [size=150]Planetary Nebula NGC 2818 from Hubble[/size][/url]

[b] Explanation: [/b] NGC 2818 is a beautiful [url=http://heritage.stsci.edu/2000/28/pne.html]planetary nebula[/url], the gaseous shroud of a [url=http://hubblesite.org/newscenter/archive/releases/1997/38/background/]dying sun-like star[/url]. It could well offer a glimpse of the future that awaits our own Sun after spending another 5 billion years or so steadily using up hydrogen at its core, and then finally helium, as fuel for [url=http://hyperphysics.phy-astr.gsu.edu/Hbase/astro/astfus.html]nuclear[/url] fusion. [url=http://adsabs.harvard.edu/abs/1984ApJ...287..341D]Curiously[/url], NGC 2818 seems to lie within an open star cluster, NGC 2818A, that is some 10,000 light-years distant toward the southern constellation [url=http://www.hawastsoc.org/deepsky/pyx/index.html]Pyxis[/url] (the Compass). At the distance of the [url=http://asterisk.apod.com/viewtopic.php?f=24&t=18009]star cluster[/url], the nebula would be about 4 light-years across. But accurate velocity measurements show that the nebula's own velocity is very different from the cluster's member stars. The result is [url=http://arxiv.org/abs/0710.2900]strong evidence[/url] that [url=http://adsabs.harvard.edu/abs/2012ApJ...751..116V]NGC 2818[/url] is only by chance found along the line of sight to the star cluster and so [url=http://amazinganimalstories.com/wp-content/uploads/2014/01/o-JERK-CAT-WANTS-ALL-THE-FOOD-facebook.jpg]may not share[/url] the cluster's distance [url=http://apod.nasa.gov/apod/ap990305.html]or age[/url]. The [url=http://heritage.stsci.edu/2009/05/]Hubble image is[/url] a composite of exposures through narrow-band filters, presenting emission from [url=http://periodic.lanl.gov/7.shtml]nitrogen[/url], [url=http://periodic.lanl.gov/1.shtml]hydrogen[/url], and [url=http://en.wikipedia.org/wiki/Oxygen]oxygen[/url] atoms in the nebula as red, green, and blue hues.

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