Distance vs Speed

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Charlie Mercer
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Distance vs Speed

Post by Charlie Mercer » Sun Oct 23, 2011 5:17 pm

Please excuse my ignorance as a stargazer only. My question is simply this: If a light year is a trillion miles, how is it possible to see anything listed as 300million light years away.? This just blows me away. :shock

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Ann
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Re: Distance vs Speed

Post by Ann » Sun Oct 23, 2011 5:40 pm

Charlie Mercer wrote:Please excuse my ignorance as a stargazer only. My question is simply this: If a light year is a trillion miles, how is it possible to see anything listed as 300million light years away.? This just blows me away. :shock
Well, basically that means that the light from a source that is 300 million light-years away was emitted 300 million years ago. We can never see a source that is 300 million light-years away as it is "today", but only as it was 300 million years ago.

Also note that the light from a light source that is 300 million light-years away will have been spread out over a very, very large "cone". That means that the light inside that light cone has been "diluted", so that there are only a few photons for for every, say, "light-day" inside that cone.

Or to put it simpler: If an object is 300 light-years away, the light that reaches us here on the Earth from that light source will be faint.

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Charlie Mercer
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Re: Distance vs Speed

Post by Charlie Mercer » Sun Oct 23, 2011 6:17 pm

Thanks very much Ann. That is exactly what has me stumped, " light will be spread out over a very large cone", so if it is in fact diluted and has fewer photons how is it possible to see individual stars in a distant galaxy say 150 million light years away. The pix we see from the Hubble are sometimes listed as farther away than even this.? Still stumped.

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Re: Distance vs Speed

Post by Beyond » Sun Oct 23, 2011 6:38 pm

Charlie Mercer wrote:Thanks very much Ann. That is exactly what has me stumped, " light will be spread out over a very large cone", so if it is in fact diluted and has fewer photons how is it possible to see individual stars in a distant galaxy say 150 million light years away. The pix we see from the Hubble are sometimes listed as farther away than even this.? Still stumped.
Time exposures collect more photons.
To find the Truth, you must go Beyond.

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Chris Peterson
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Re: Distance vs Speed

Post by Chris Peterson » Sun Oct 23, 2011 6:39 pm

Charlie Mercer wrote:Thanks very much Ann. That is exactly what has me stumped, " light will be spread out over a very large cone", so if it is in fact diluted and has fewer photons how is it possible to see individual stars in a distant galaxy say 150 million light years away. The pix we see from the Hubble are sometimes listed as farther away than even this.? Still stumped.
Instead of a cone, think of a sphere, since that's much more typical of the way light spreads- especially from a star. Every time you double the radius of a sphere (which is like getting twice as far from the star at its center), the surface area increases by a factor of four. In other words, for any given patch, you'll have only one-fourth as many photons hitting it. This is the basis of the inverse square law: get ten times further away, and you'll only have 1/100 the number of photons on a given size patch (like the pupil of your eye).

We can see a galaxy 150 million light years away because, even though our telescope aperture only captures a tiny, tiny fraction of the total photons that were originally produced, that original number was so huge that we still have thousands or millions left to be collected by the camera. The photon count may be diluted, but it isn't diluted to zero (and instrumentally, we can detect single photons, although it typically requires at least a few dozen in each pixel to get a statistically meaningful signal).
Chris

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Charlie Mercer
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Estimating distance

Post by Charlie Mercer » Mon Oct 24, 2011 4:25 pm

Tried the other day to get an answer to the thing that really stumps me but to no avail. Someone please help me out here. Some of the pix on the pix of the day archive list some galaxies as being millions of light years away. To me it would be impossible to SEE anything that far away, even with a powerful telescope. So how far does light travel in a year.? and...what formula is used to estimate these distances.? Finally....is there a forum in here that is educational that I could join.? A fascinating site to be sure.!

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Re: Estimating distance

Post by Chris Peterson » Mon Oct 24, 2011 4:32 pm

Charlie Mercer wrote:Tried the other day to get an answer to the thing that really stumps me but to no avail. Someone please help me out here. Some of the pix on the pix of the day archive list some galaxies as being millions of light years away. To me it would be impossible to SEE anything that far away, even with a powerful telescope. So how far does light travel in a year.? and...what formula is used to estimate these distances.? Finally....is there a forum in here that is educational that I could join.? A fascinating site to be sure.!
Asked and answered at http://asterisk.apod.com/viewtopic.php?f=30&t=25659

If you have more questions on the same topic, it would be best to continue them in that discussion.
Chris

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Re: Estimating distance

Post by bystander » Mon Oct 24, 2011 4:52 pm

Charlie Mercer wrote:....is there a forum in here that is educational that I could join.?
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