# VERY simple question about the universe!

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#### falkor

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When we see a star 10 light years away, we are seeing how that star looked 10 years ago, but are we also seeing the star as it looked when it was closer to us? On average, how far apart do 2 stars drift in 10 years across the expanding universe?<br /><br />There you go: I bet you've never even heard a simple question like that before, so I wouldn't be surprised if most of you need to get down deep into some text book to answer first yes/no part of the question.<br /><br />In the earliest hubble deep field view, how far were the galaxies apart from each other? How far apart are galaxies now?

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#### heyscottie

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Since any star that is moving toward us or away from us is moving at less than 'c' relative to us (exception noted later), we are only ever seeing it as it looked at some particular time. However, if it is moving at some appreciable fraction of 'c' relative to us, we will see it aging faster (if it is moving toward us) or slower (if it is moving away from us).<br /><br />In the case of reaches of the universe that are expanding from us at or faster than 'c', the question becomes moot, as the light is red-shifted to DC, and we never see them at all.<br /><br />So the short answer is that we never get to see an object as it looked at more than one time. Of course, there's an exception to this, too. If we look at an object whose light has been gravitationally lensed such that the light took two or more paths to get to us, we will indeed see the object at more than one age in different areas of the sky. As far as I know, we haven't found any scenarios where the two ages were very different from one another.

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#### SpeedFreek

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<i>(Simple answer)</i><br /><br />The hubble constant is around 71 km/s per 3,262,000 ly. So the speed of expansion at 10 light years would be something around 21.7 mm per second, I think.<br /><br /><br /><i>(More complicated answer)</i><br /><br /><b>But!</b> if the two stars are part of a gravitationally bound system, i.e. if they are within a galaxy, then there is no increase in distance. The expansion of the universe is only theorised to occur at a measurable rate <i>outside</i> of gravitationally bound systems, i.e in between the galactic clusters, increasing the distance between those clusters.<br /><br />The gravity of our galaxy overwhelms any metric expansion at the interstellar scale. <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

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#### qso1

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Falkor:<br />When we see a star 10 light years away, we are seeing how that star looked 10 years ago,<br /><br />Me:<br />Lets use a real example...Tau Ceti, 11 light years. If we look at Tau Ceti 50 years ago, the difference in distance is not very significant to what it would be looking at it today. Tau Ceti was 11.9 light years distant 50 years ago and is still approximately that distance today. The most extreme example of drift is Barnards star. Barnards is about 6 light years distant and its relative position in the sky over a 50 year period is apparent. Even so, what we see coming from Barnards star 50 years ago vs today would not be significantly different.<br /><br />http://www.solstation.com/stars/barnards.htm<br /><br />The link has all the skinny on Barnards and its apparent motion.<br /><br />Falkor:<br />In the earliest hubble deep field view, how far were the galaxies apart from each other? How far apart are galaxies now?<br /><br />Me:<br />The earliest deep field was 1995 or 13 years ago. In 13 years, these galaxies will not have moved enough for us to detect their motion even with the most sensitive instruments we have today. Galaxies are huge and distant galaxies too distant to see appreciable motion change in human lifetimes. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

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