How will the pattern of stars in the milky way change over time?

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Discussion Overview

The discussion centers on how the pattern of stars in the Milky Way galaxy will change over time due to the galaxy's rotation and the effects of cosmic expansion. Participants explore the implications of stellar motion, proper motion of stars, and the future visibility of stars in the context of both local gravitational binding and universal expansion.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions how the pattern of stars will change as the Milky Way rotates, seeking clarification on the direction of movement and the timescale involved.
  • Another participant explains that the relative motion of nearby stars is nearly random, with typical velocities around 50 km/s, suggesting a timescale of approximately 500,000 years for significant positional changes.
  • Some participants propose that due to the accelerating expansion of the universe, it may eventually be impossible to see any stars, although this claim is contested.
  • A participant challenges the idea that universal expansion affects stars within gravitationally bound systems like the Milky Way, asserting that local gravitational effects dominate.
  • There is mention of the potential for a "big rip" scenario to affect the galaxy, but it is noted that such an event is not currently indicated, and that the Milky Way will undergo significant changes due to the collision with Andromeda in about 4 billion years.
  • Another participant emphasizes the need to consider proper motion components of stars, citing Barnard's Star as an example of significant proper motion.
  • Some participants argue that while local gravitational binding is important, the expansion of space will eventually affect visibility on a larger scale, despite the Milky Way itself not expanding.

Areas of Agreement / Disagreement

Participants express disagreement regarding the implications of cosmic expansion on the visibility of stars within the Milky Way. Some assert that local gravitational binding negates the effects of universal expansion, while others argue that expansion will eventually render stars invisible. The discussion remains unresolved with multiple competing views present.

Contextual Notes

Participants reference various sources and examples to support their claims, but there are unresolved assumptions regarding the long-term effects of cosmic expansion and the specific dynamics of stellar motion within the galaxy.

alemsalem
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how will the pattern of stars in the milky way change as the galaxy rotates, will they be rotated in some direction or drifted or something?
and what is the timescale for that to happen?

Thanks!
 
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The relative motion of nearby stars (so nearly everything you can see in the sky) is nearly random. They will head towards some random fixed point in the sky, and get dimmer, while other stars come from some random position and get brighter. Typical relative velocities are ~50km/s or ~2 light years per 10000 years. For stars 100 light years away (see the brightest stars), this gives a timescale of ~500,000 years until they are at completely different positions. Some stars are much closer, and some stars are quicker, so they moved by a visible amount (for the naked eye) in the last ~2000 years. 61 Cygni, for example, is visible to the naked eye and moves with ~5arcseconds per year or 1° in ~700 years. This is two times the diameter of the moon.
 
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Eventually, due to an accelerating expansion of the universe, it will be impossible to see any stars!
 
schaefera said:
Eventually, due to an accelerating expansion of the universe, it will be impossible to see any stars!

That is not correct. Our local group of galaxies are gravitationally bound and the expansion of the universe does not effect the stars contained within them.




Damo
 
It would need a big rip to pull our galaxy apart*, and there are no signs of this. But even if with a big rip, our sun will reach the end of its life long before that. All other stars won't live forever either.

*well, the collision with Andromeda in ~4 billion years will significantly change its shape as well.
 
alemsalem said:
how will the pattern of stars in the milky way change as the galaxy rotates, will they be rotated in some direction or drifted or something?
and what is the timescale for that to happen?

Thanks!
One would need to find the proper motion components of many stars.

Here are insights.
. . . .
The highest proper motion of any star is that of the nearby Barnard's Star with a value of 10.3"/yr. It has a radial velocity of 108 km/s towards us and a transverse velocity of 88 km/s. This means it is moving closer to us and will be at its closest in 12,000 A.D.

. . . .
http://outreach.atnf.csiro.au/education/senior/astrophysics/proper_motion.html

http://en.wikipedia.org/wiki/Proper_motion
http://en.wikipedia.org/wiki/Stellar_kinematics

http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit1/motions.html
http://abyss.uoregon.edu/~js/ast122/lectures/lec08.html
http://csep10.phys.utk.edu/astr162/lect/motion/proper.html
http://burro.astr.cwru.edu/Academics/Astr222/Galaxy/Kinematics/solarmotion.html

http://heasarc.gsfc.nasa.gov/W3Browse/star-catalog/ppm.html

Atlas of stars - http://www.atlasoftheuniverse.com/12lys.html
 
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Ah yes, but even introductory books like Brian Greene explain that it isn't everything moving randomly, but space itself is expanding. So the gravitational boundedness only matters so long before the space between all of us expands and we can see no stars locally either.

A more scientific source: http://smithsonianscience.org/2011/04/big-bang-model/
 
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schaefera said:
Ah yes, but even introductory books like Brian Greene explain that it isn't everything moving randomly, but space itself is expanding.
This is irrelevant on the scale of our galaxy.

So the gravitational boundedness only matters so long before the space between all of us expands and we can see no stars locally either.
The milky way does not expand.

A more scientific source: http://smithsonianscience.org/2011/04/big-bang-model/
This just confirms that you cannot measure expansion of the universe within a galaxy - you need stars which get ejected so quick and travel so far away that they are not bound to the galaxy any more.
See Astronuc's links for scientific sources.
 
schaefera said:
Ah yes, but even introductory books like Brian Greene explain that it isn't everything moving randomly, but space itself is expanding. So the gravitational boundedness only matters so long before the space between all of us expands and we can see no stars locally either.

A more scientific source: http://smithsonianscience.org/2011/04/big-bang-model/

If you re-read that source, it states that after the merger of The Milky Way and Andromeda galaxies (he didn't bother to name the other galaxies in our local group as they are much smaller in comparison, but they will merge with the big 2 also), "The universe’s ever-accelerating expansion will send all other galaxies rushing beyond our “cosmic horizon,” sending them forever out of view."
Not our local group!



Damo
 

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