# Some confusion over light

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#### mikeyboy

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<p>If the light we see from stars&nbsp;travels millions of light years, presumably they were in a different place when the light was originally "transmitted".&nbsp; </p><p>Whilst light travels in a straight line,&nbsp;I assume that the calculations for predicting where a&nbsp;star will be in the future is quite difficult since gravitational effects of other planets (and maybe stars?) must come in to play to some extent.</p><p>In those millions of years, even the smallest of gravities must have an effect, no matter how insignificant that must be amplified over such large distances.</p><p>If we are still discovering planets previously unknown to us, how is it that we can know where a particular star is going to be at any particular time?</p><p>&nbsp;</p><p>&nbsp;</p>

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#### aphh

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<p>The closest stars are only a few light years away, and a typical observed star might be 25 000 LY away. In the scale of the universe those are but a blink away, so predicting the star's motion and future location can be done.</p><p>The star's velocity in space is measured using two axis, the line of sight axis measures star's speed in relation to us or whether the star is approaching us or going away. This is called radial velocity. This can be measured using technique called doppler effect (redshift and blueshift).</p><p>The other component of the star's velocity is it's movement perpendicular to us, which is called tangential velocity.</p><p>Combining these two components gives us the star's proper motion, or how much in total the star is going to move. Repeating the measurements annually, or once in a decade, should give us quite good accuracy in predicting the star's future location (that we can't see yet).</p><p>I found a pretty good page about star velocity: http://www.astronomynotes.com/starprop/s8.htm </p>

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#### weeman

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>If the light we see from stars&nbsp;travels millions of light years, presumably they were in a different place when the light was originally "transmitted".&nbsp; Whilst light travels in a straight line,&nbsp;I assume that the calculations for predicting where a&nbsp;star will be in the future is quite difficult since gravitational effects of other planets (and maybe stars?) must come in to play to some extent.In those millions of years, even the smallest of gravities must have an effect, no matter how insignificant that must be amplified over such large distances.If we are still discovering planets previously unknown to us, how is it that we can know where a particular star is going to be at any particular time?&nbsp;&nbsp; <br />Posted by mikeyboy</DIV><br /><br />Millions of light years is more of an intergalactic scale, rather than a galactic scale. Most things that we&nbsp;can observe&nbsp;on a galactic scale (within our galaxy) are within 20-25 thousand light years. We simply cannot detect planetary systems that are on the order of millions of light years away. </p><p>Aphh explained it very well. We can also assume that if a star is within our own galaxy, it will move in a relatively consistent orbit around the galactic center, just as our solar system is doing right now. This especially goes for star systems that are within the habitable zone of our galaxy; stars within a couple hundred light years of the galactic center are not the best candidates. Our best bet is to find other stars like our own that are at about the same distance from the galactic center as our sun. </p> <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity.</strong><strong>" -Albert Einstein </strong></font></p> </div>

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#### Ranjha

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Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>If the light we see from stars&nbsp;travels millions of light years, presumably they were in a different place when the light was originally "transmitted".&nbsp; Whilst light travels in a straight line,&nbsp;I assume that the calculations for predicting where a&nbsp;star will be in the future is quite difficult since gravitational effects of other planets (and maybe stars?) must come in to play to some extent.In those millions of years, even the smallest of gravities must have an effect, no matter how insignificant that must be amplified over such large distances.If we are still discovering planets previously unknown to us, how is it that we can know where a particular star is going to be at any particular time?&nbsp;&nbsp; <br />Posted by mikeyboy</DIV><br /><br />I agree...accept for the minor effect of a problem that normally distroys physics.....GRAVITY.&nbsp; That twinkling light we see had to pass through many heavy objects. It's trajectory being deviated.&nbsp; Since light "does not travle striaght but is bent by gravity"&nbsp; The position of the star is difficult to know, all we know is weather it's moving close or away from us.&nbsp; Therefore it's even more difficult to predict the future prediction..I would suggest some reading on quantum mech.&nbsp; <div class="Discussion_UserSignature"> <p><font size="2" color="#cc33cc"><font color="#000000">"I'm normally not a praying man, but if you're up there, please save me Superman."<br /></font><span style="font-size:11px;color:#339999">By: Homer Simpson<br /></span></font></p> </div>

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#### DrRocket

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I agree...accept for the minor effect of a problem that normally distroys physics.....GRAVITY.&nbsp; That twinkling light we see had to pass through many heavy objects. It's trajectory being deviated.&nbsp; Since light "does not travle striaght but is bent by gravity"&nbsp; The position of the star is difficult to know, all we know is weather it's moving close or away from us.&nbsp; Therefore it's even more difficult to predict the future prediction..I would suggest some reading on quantum mech.&nbsp; <br />Posted by Ranjha</DIV></p><p>You might want to read up on electrodynamics and gravitation rather than on quantum mechanics, they are more important to this question.&nbsp; Light from stars does NOT pass through many heavy objects on the way to us.&nbsp; In fact if it were to be absorbed by even one heavy object, it would not get to us -- you can't see through a star any more than you can see through a door.</p><p>Gravitatinal lensing is only a factor for light if it passes very close to very massive bodies, and even then it only changes the path of the light, and by a small amount for most objects.&nbsp; Most light that reaches us is not particularly affected by gravitatinal lensing in any case.&nbsp; Most of space is, well, space.</p><p><br /><br />&nbsp;</p> <div class="Discussion_UserSignature"> </div>

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#### derekmcd

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Since light "does not travle striaght but is bent by gravity"...<br /> Posted by Ranjha</DIV><br /></p><p>Technically, in this context, light does&nbsp; travel in a straight line.&nbsp; It follows a geodesic path through the curvature of spacetime. <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-wink.gif" border="0" alt="Wink" title="Wink" /> </p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>

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#### mikeyboy

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<p><span style="font-size:11pt;font-family:'Calibri','sans-serif'">Thanks.&nbsp; So how far away is the star which is the furthest from us yet detected?</span></p>

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#### derekmcd

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Thanks.&nbsp; So how far away is the star which is the furthest from us yet detected? <br /> Posted by mikeyboy</DIV></p><p>If you consider supernovae as 'stars' then those distances reach several billion light years distance.</p><p>The most distant Cepeid variable star would be at a distance of over 50 million ly distance&nbsp;</p><p>http://www.seds.org/messier/more/m100_hst2.html</p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>

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#### DrRocket

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>If you consider supernovae as 'stars' then those distances reach several billion light years distance.The most distant Cepeid variable star would be at a distance of over 50 million ly distance&nbsp;http://www.seds.org/messier/more/m100_hst2.html <br />Posted by derekmcd</DIV></p><p>And if you accept the detection of galaxies as also detectin of stars then the distance is quite far, on the order of 13 billion light years (light travel time for the purists).&nbsp; Individual stars are generally not detected, except for the odd supernova, at those distances.&nbsp; But, where you find a galaxy you can generally conclude that there is a star or two around somewhere.&nbsp; </p><p><br />http://hubblesite.org/newscenter/archive/releases/2004/07/</p><p><img src="http://imgsrc.hubblesite.org/hu/db/2004/07/images/a/formats/small_web.jpg" alt="" /></p> <div class="Discussion_UserSignature"> </div>

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