Stars and accelaration.

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aditiudupa

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Why do stars expand and contract?<br />If all the things in space are moving away from one another, how is their accelaration increasing rather than decreasing?
 
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Saiph

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stars expand and contract because they are no longer in "hydrostatic equilibrium".<br /><br />Basically, their inner fusion process no longer precisely counteracts the inwards pressures of gravity. At various points in the stars lifetime the fusion cycle shifts processes (due to changing amounts of elements in the core). When this occurs, the output drops, increases, whatever.<br /><br />Due to the change in the energy output, the outer envelope of the star contracts or expands, this changes the overall pressure and temperature at the core (which regulates the rate of fusion).<br /><br />If the core got to hot, the envelope expands, reducing the pressure and temperature exerted on the core, slowing the fusion process...which allows the envelope to slow to a stop, then collapse back. The now collapsing envelope increases pressure and temperature, which increases the fusion rate, to slow and stop the collapse.<br /><br />The cycle repeats over and over so long as the expansion or contraction phases overshoot the equilibrium point. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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Saiph

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your second question doesn't really relate to the first.<br /><br />But, objects are carried away from eachother due to the expansion of the universe, according to the BB model. There are other models, but they haven't faired as well, and aren't "mainstream" so I'll focus on this one.<br /><br />The rate they are carried away from eachother, depends on how much space between them is expanding. It is currently believed that the expansion is uniform everywhere btw.<br /><br />Now the expansion is an increase in distance, per unit distance. This means that over time, every inch grows to be larger, say 1 inch larger every million years (figure is made up). That means that every million years a distance that was 1 inch, is now 2 inches, unless some outside factor interfered (like gravity, but we'll get to that).<br /><br />So, if there was 1 inch between you and I, in a million years, there would be two. If there was originally 2 inches between you and I, there would be 4 inches seperating us in a million years, as each of those two inches, increased by an inch each, for a net gain of 2 inches.<br /><br />Now, this is why further objects appear to be receeding faster. In the first case, where you and I were 1 inch apart, we seperated by 1 inch over a million years. In the second case, where we were initially seperated by 2 inches, we've traveled 2 inches apart in a million years (twice as fast!).<br /><br />Basically, expansion "forces" get stronger with distance. The further apart you are, the greater the effect.<br /><br />Okay, so that's the basics without any interfering effects.<br /><br />Retarding forces: The catch, of course, is that there are other forces in play. Gravity for instance, attracts objects to eachtother in opposition to the expansion. The space still expands, but gravity pulls the objects together through that space (sorta like pulling someone upstream using a rope, the stream still does it's thing, but the person goes the wrong way). Also, gravity gets <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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