Starship Asterisk*

by **jimmysnyder** » Fri Jul 20, 2007 1:22 pm

emc wrote: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

by **emc** » Fri Jul 20, 2007 12:38 pm

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!!!

by **bystander** » Thu Jul 19, 2007 9:24 pm

William Roeder wrote: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.

by **William Roeder** » Thu Jul 19, 2007 7:47 pm

emc wrote: 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.

by **emc** » Thu Jul 19, 2007 4:40 pm

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.

by **jimmysnyder** » Thu Jul 19, 2007 12:04 pm

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.

by **William Roeder** » Wed Jul 18, 2007 7:28 pm

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

by **emc** » Tue Jul 17, 2007 12:22 pm

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

by **makc** » Mon Jul 16, 2007 1:44 pm

Indigo_Sunrise wrote: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

by **Indigo_Sunrise** » Mon Jul 16, 2007 1:22 pm

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!!!

by **craterchains** » Sat Jul 14, 2007 9:24 am

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

by **MarinerMB** » Sat Jul 14, 2007 1:16 am

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

by **DonB312** » Fri Jul 13, 2007 3:44 pm

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.

by **thebobgy** » Fri Jul 13, 2007 3:14 pm

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

by **Saldyinie** » Thu Jul 12, 2007 7:37 pm

Excellent point.

Thanks!

by **BMAONE23** » Thu Jul 12, 2007 7:25 pm

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

by **Saldyinie** » Thu Jul 12, 2007 5:58 pm

"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?

Starship Asterisk*

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

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Re: More light - less noise

Re: More light - less noise

More light - less noise

Red and blue shift (APOD 12 July 2007)

Distances

12 July, 2007 NGC 6384: "Beyond the Stars" picture

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