At That Distance; red and blue shift (APOD 12 July 2007)

[Saldyinie]

"At that distance, NGC 6384 spans an estimated 150,000 light-years."

Would it span a different number of light years at any other distance?

[BMAONE23]

I think they are refering to the relative apparent size (angular size) vs. approximate distance. If it were closer than the approximate distance then it will have a smaller actual size given it's relative apparent size.

So:

If it is as far away as expected, then given its apparent angular size (amount of the sky it covers) it should be 150,000 LY accross.

http://antwrp.gsfc.nasa.gov/apod/ap070712.html

[Saldyinie]

Excellent point.

Thanks!

[thebobgy]

It appears to me that our Galaxy's stars are superimposed over the 6384 Galaxy; if true then it should be stated in the discription don't you think. Thank you.

thebobgy

[DonB312]

http://antwrp.gsfc.nasa.gov/apod/ap070712.html

The entire point of this APOD is that we are seeing the galaxy "beyond the stars" of our own galaxy.

[MarinerMB]

It's a great picture but decieving as far as the listed distances go...the galaxy really should seem farther from sight.

[craterchains]

, , , It's all about perspective, , ,

[Indigo_Sunrise]

And now it's a shame, neither the link Floyd312 posted, nor the 'Archive' feature will allow the image to be accessed. It seems whatever happened to the site over the weekend affected images back to 9 Jul 07. Oh well, glad it's back!!!

[makc]

And now it's a shame, neither the link Floyd312 posted, nor the 'Archive' feature will allow the image to be accessed.

They will probably sort it out soon. Meanwhile, http://www.sai.msu.su/apod/ap070712.html

[emc]

Does the July 12 image represent blue shift and red shift star motion? If not how is this detected and are there any images in the archives? Thank you, Ed... ref... http://apod.nasa.gov/apod/ap070712.html

[William Roeder]

Red/Blue shifting is a very small thing. You can't see it with your eyes.

The normal way is to split the light with a prisim and see how the spectral absorption lines are shifted.

See http://apod.nasa.gov/apod/ap070624.html

[jimmysnyder]

Here is an explanation of red shift.

http://en.wikipedia.org/wiki/Redshift

I think most of the reddish color in nebulae is caused by starlight passing through clouds of hydrogen gas. That gas has a strong emission line in red. I expect that there is some difference in the red that you see from distant galaxies compared to that which you see from closer ones because of the shift in the red line. However, this effect is subtle and does not turn red to blue, just a slight change in red. I think the blue color is caused by reflection of starlight off of clouds of dust. I suppose that star formation occurs in dust clouds and so lots of blue indicates lots of star formation. I'm not sure about this so someone with more knowledge had better review this.

[emc]

Thanks for the responses!

Sorry for the dumb question... I did a little more research on Wikipedia and learned (actually I was reminded) that star color is indicative of the temperature. I had forgotten this and was simply enamored by the image of blue, yellow, and red stars in the foreground and was wondering what their relative motion would be to us.

I am curious how a prism would be used to measure individual star motion in a field of so many star lights.

[William Roeder]

I am curious how a prism would be used to measure individual star motion in a field of so many star lights.

Stars are red or blue because the star's temperature makes the majority of the light in that band. Our star (the sun) is yellow. All stars emit all colors.

The spectral absorption lines are independent of temperature. They are a function of elements and molecules at the surface.

[bystander]

Stars are red or blue because the star's temperature makes the majority of the light in that band. Our star (the sun) is yellow. All stars emit all colors.

The spectral absorption lines are independent of temperature. They are a function of elements and molecules at the surface.

Red and Blue shift has to to do with the apparent doppler shift of those absorption lines caused by relative motion and the vast disances involved.

However, the colors involved in the apod being discussed, have very little to do with shift. Some is probably due to stellar temperatures, and some due to absorption and reflection nebulae in that distant galaxy.

[emc]

I think I keep asking or stating really dumb thoughts... for example, the relative motion of stars to earth... I guess this is measured by observation, right? Not some elaborate velocity measuring device that indicates redshift or blueshift 'Doppler' effect... right??? Apologies for my ignorance, but I am not even remotely an astronomer. I am an artist and I am very grateful for any responses and especially the beautiful images!!!

[jimmysnyder]

the relative motion of stars to earth... I guess this is measured by observation, right? Not some elaborate velocity measuring device that indicates redshift or blueshift 'Doppler' effect... right???

There are a variety of methods used. For a few close stars, paralax is used. For stars at further distances, a variety of techniques are used. For the most distant galaxies, redshift doppler effect is used (there aren't any blueshifted galaxies at those distances) as a measure of distance according to the theory that the universe is expanding uniformly. Here is a more detailed explanation. I hope it meets your needs.

http://en.wikipedia.org/wiki/Cosmic_distance_ladder