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Identifying galaxies within 1 MPc



 
 
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  #51  
Old January 4th 07, 07:01 PM posted to rec.arts.sf.science,sci.astro,alt.books.isaac-asimov
Karsten Kretschmer
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Default Identifying galaxies within 1 MPc

Am Thu, 04 Jan 2007 07:50:23 -0500 schrieb Yousuf Khan:
What's a good maximum range for a pulsar beam? Hundreds of light-years?
Thousands? Tens of thousands? Etc.


http://www.atnf.csiro.au/research/pulsar/psrcat/
  #52  
Old January 4th 07, 09:47 PM posted to rec.arts.sf.science,sci.astro,alt.books.isaac-asimov
Erik Max Francis
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Default Identifying galaxies within 1 MPc

Yousuf Khan wrote:

Erik Max Francis wrote:
One issue we haven't mentioned is despite being good beacons, pulsars
aren't perfect. Far enough away, even though in the Galaxy, and
they'd be hard to detect, so if you were far enough from our solar
neighborhood, even you were in the (feeble at that distance) beam of
those reference pulsars you might not be able to detect it.


What's a good maximum range for a pulsar beam? Hundreds of light-years?
Thousands? Tens of thousands? Etc.


From skimming papers on pulsar distances on Google, it looks like the
effective range is ~6 kpc, or perhaps ~2 kly.

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  #53  
Old January 4th 07, 11:56 PM posted to rec.arts.sf.science,sci.astro,alt.books.isaac-asimov
Steve Willner
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Default Identifying galaxies within 1 MPc

In article ,
Erik Max Francis writes:
From skimming papers on pulsar distances on Google, it looks like the
effective range is ~6 kpc, or perhaps ~2 kly.


Dropped a decimal the 6 kpc \approx 20000 LY. I agree with the
number, which is of course based on present Earth technology.

I think the point of the Pioneer and Voyager plaques was that any
advanced Galactic civilization would have a comprehensive map of
pulsars throughout the Galaxy and know their behavior over long spans
of time. _Given that information_, the probe's origin could be
located both in space and time.

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  #54  
Old January 5th 07, 01:00 AM posted to rec.arts.sf.science,sci.astro,alt.books.isaac-asimov
Erik Max Francis
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Default Identifying galaxies within 1 MPc

Steve Willner wrote:

Dropped a decimal the 6 kpc \approx 20000 LY. I agree with the
number, which is of course based on present Earth technology.


Right. I zigged, when I should have zagged.

I think the point of the Pioneer and Voyager plaques was that any
advanced Galactic civilization would have a comprehensive map of
pulsars throughout the Galaxy and know their behavior over long spans
of time. _Given that information_, the probe's origin could be
located both in space and time.


I think the greater point about the plaques is that no one expects
anyone else to find it, let alone decipher it. Its representation,
along with the golden record put on the _Voyager_ probes, and in
addition to that the unrelated Arecibo message, were really for our own
entertainment and as a publicity stunt rather than as a serious attempt
to communicate with extraterrestrials. For the probes, they'll be
drifting in interstellar space and will be incredibly unlikely to ever
be found even by superadvanced civilizations. For the Arecibo message,
it was beamed to a distant globular cluster, far enough away that they'd
be essentially unable to trace back the source, and globular clusters
consist of old population II stars, not exactly conducive to
technological intelligences.

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  #55  
Old January 5th 07, 04:06 AM posted to rec.arts.sf.science,sci.astro,alt.books.isaac-asimov
Joseph Lazio
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Default Identifying galaxies within 1 MPc

"YK" == Yousuf Khan writes:

YK Do the pulsars' axes also precess, like the Earth's does? Or are
YK they just too massive and spinning too fast for that?

Maybe.

Others have already pointed out forced precession, in which a torque
exerted by a nearby object causes the precession (analogous to how the
Moon and other objects in the solar system can cause the Earth's
rotational axis to precess over time).

There's also free precession, caused by an asymmetry of the object.
One might think that this should not happen to a pulsar, given that
its interior is liquid and should damp any asymmetries, but there's at
least one report of this possibly being detected in PSR B1828-11 by
Stairs et al.

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  #56  
Old January 12th 07, 06:03 PM posted to rec.arts.sf.science,sci.astro
[email protected]
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Default Identifying galaxies within 1 MPc


John Schilling kirjutas:
On 10 Jan 2007 14:50:25 -0800, "bbbl67" wrote:

On Jan 5, 2:36 am, Erik Max Francis wrote:
And, has been pointed out, if there's motion of the internal fluids
inside the rotating neutron star, that could also result in precession
and all kinds of lower-order rotational behaviors.


Now, I know the interior of a neutron star has been described as a
fluid. But I find it hard to believe that something so dense that its
neutrons are touching could be a fluid. Can the neutrons slip past one
another, somehow?


Of course. What would stop them?

In order for a thing to *not* be a fluid, something has to hold each
particle in a particular spot, and that something has to be the strongest
force at work in the system. In ordinary solid materials, a particular
arrangement of the electromagnetic force does this just fine.

In a neutron star, you've got A: neutrons, which are B: compressed to
such a density that gravity overwhelmes electromagnetism. If even
electromagnetism applied, which it doesn't because neutrons are
electrically neutral.


But you also have something that counteracts the gravity and stops the
neutrons from being compressed to decreasing volume and black hole.
Neutrons must repel each other.

And this means that the best way to minimize the repulsion is occupying
specific positions in space in a well-packed framework. Which would be
solid. Any change of shape would increase the mutual repulsions
somewhere.

And unless there's something funky going on in quantum gravity that
we haven't figured out yet, gravity is not a force for holding things
in a particular spot. Gravity is just fine with having particles flow
along equipotential surfaces, and actively *wants* particles to flow
downhill.



  #57  
Old January 12th 07, 06:37 PM posted to rec.arts.sf.science,sci.astro
kuo
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Default Identifying galaxies within 1 MPc

On Fri, 12 Jan 2007 09:03:29 -0800, chornedsnorkack wrote:
John Schilling kirjutas:
On 10 Jan 2007 14:50:25 -0800, "bbbl67" wrote:


Now, I know the interior of a neutron star has been described as a
fluid. But I find it hard to believe that something so dense that its
neutrons are touching could be a fluid. Can the neutrons slip past one
another, somehow?


In order for a thing to *not* be a fluid, something has to hold each
particle in a particular spot, and that something has to be the strongest
force at work in the system. In ordinary solid materials, a particular
arrangement of the electromagnetic force does this just fine.


In a neutron star, you've got A: neutrons, which are B: compressed to
such a density that gravity overwhelmes electromagnetism. If even
electromagnetism applied, which it doesn't because neutrons are
electrically neutral.


But you also have something that counteracts the gravity and stops the
neutrons from being compressed to decreasing volume and black hole.
Neutrons must repel each other.


And this means that the best way to minimize the repulsion is occupying
specific positions in space in a well-packed framework. Which would be
solid. Any change of shape would increase the mutual repulsions
somewhere.


That's not a bad thought. A bunch of well lubricated ball bearings
being packed into a sphere just barely big enough to fit them will
pack into a rigid formation.

However, this gets into the difference between a "solid" and a "liquid".
A liquid like water is nearly incompressible because the molecules are
packed in almost as much as possible--but there's enough random motion
to keep them "jiggling" and thus able to easily slide past each other.

Similarly, a neutron star might theoretically compress into a rigid
"solid", but with any sort of temperature the neutrons will "jiggle"
enough to be a fluid.

Isaac Kuo

  #58  
Old January 12th 07, 07:22 PM posted to rec.arts.sf.science,sci.astro
George W Harris
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Default Identifying galaxies within 1 MPc

On Fri, 12 Jan 2007 11:37:41 -0600, kuo wrote:

: And this means that the best way to minimize the repulsion is occupying
: specific positions in space in a well-packed framework. Which would be
: solid. Any change of shape would increase the mutual repulsions
: somewhere.
:
:That's not a bad thought. A bunch of well lubricated ball bearings
:being packed into a sphere just barely big enough to fit them will
ack into a rigid formation.

Neutrons aren't analogous to ball-bearings, though.
They're fuzzy, without well-defined borders.
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exquisite and beautiful in its structure, in comparison with
which the physical world is turbid and confused."

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