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Offline syhprum

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Do Neutron stars cool down
« on: 26/12/2008 12:46:37 »
Collased stars start their Neutron star phase with surface temperatures of hundreds of thousand degrees K.
Do they ever cool down, if so how long does it take ?


 

Offline LeeE

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Do Neutron stars cool down
« Reply #1 on: 26/12/2008 15:00:21 »
Yes they'll cool down, by virtue of EM radiation.  Although neutron stars aren't perfect black-body radiators, they'll come pretty close.

http://en.wikipedia.org/wiki/Black_body_radiation

As for how long it'll take, unless someone has already done it, you'll have to do the maths.
 

Offline syhprum

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Do Neutron stars cool down
« Reply #2 on: 26/12/2008 20:16:23 »
Athough it is relatively simple to calculate the cooling time knowing the initial mass and temperature I wondered if the normal rules applied to what is effect a giant nucleus and whether Neutron stars cooled to such a temperature that they were no longer visible could constitute a significant part of the invisible mass of galaxies
 

Offline Mr. Scientist

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Do Neutron stars cool down
« Reply #3 on: 26/12/2008 20:30:44 »
As for the time required, you'd be safe to say about 5 billion years would be sufficient for a reasonable-sized neutron star. But, as you might tell, it really depends on the mass.
 

Offline Chemistry4me

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Do Neutron stars cool down
« Reply #4 on: 27/12/2008 09:53:16 »
I heard this in one of the podcasts, but how hot is X-RAY hot? Never heard of it before.
 

Offline yor_on

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Do Neutron stars cool down
« Reply #5 on: 27/12/2008 11:13:14 »
Ok, it's a fair question.
I don't know how long that might take to cool a Neutron star.

But if we look at our Earth it's about 4.5 to 4.8 billions year old.
http://image.gsfc.nasa.gov/poetry/ask/a10597.html
And it still has an average heat of somewhere around about 4100 to 4200 Centigrades at its inner core.
Radioactivity in the Earth's interior contribute some little of that heat but mostly it was created as the planet was 'pressed' into shape:)
http://www.madsci.org/posts/archives/1997-03/856964891.Es.r.html

A Neutronstar is a star who have stopped radiate light/waves as its core has collapsed inward.
"The types of compact stars are:
Black holes, in which the physics at the center is unknown.
White dwarfs, in which gravity is opposed by electron degeneracy pressure;
(And finally...) Neutron stars, in which gravity is opposed by neutron degeneracy pressure and short-range   repulsive neutron-neutron interactions mediated by the strong force;"
http://en.wikipedia.org/wiki/Gravitational_collapse

Which means that the electron degeneracy pressure is what hinders white dwarfs from collapsing  but when this is less than the gravitational pressure (its invariant mass) the stars 'inverse beta decay' will begin to create neutrons, as I've understood it.

Neutrons "which are subatomic particles with zero electrical charge and roughly the same mass as protons.
Neutron stars are very hot and are supported against further collapse because of the Pauli exclusion principle.
This principle requires that no two neutrons can occupy the same quantum state simultaneously."
http://en.wikipedia.org/wiki/Neutron_star

The heat inside a neutron star stabilizes around 1000 000 centigrades rather fast.
Compare that too Earths meager 4200 centigrades:)
Remember also that the pressure (Density) inside a neutron star is immensely much higher than the pressure the Earth can 'produce' at its inner core.

"The gravitational field at the star's surface is about 21011 times stronger than on Earth.
The escape velocity is about 100,000 km/s, which is about one third the speed of light."
http://en.wikipedia.org/wiki/Neutron_star

And now its time for you to make your 'computations' :)

--------

seemed I wrote 'millions' instead of 'billions' when I mentioned the age of our Earth.
I hate making mistakes.
 
Although i do them all to often, especially when it comes to the fairer sex:(
But to make up, I will give you earths 'escape velocity', just to make those equations ahhh, simple?
(Hey, don't look at me like that, I'm no Einstein:)

-------------
"
If you throw an object straight up, it will rise until the the negative acceleration of gravity stops it, then returns it to Earth. Gravity's force diminishes as distance from the center of the Earth increases, however. So if you can throw the object with enough initial upward velocity so that gravity's decreasing force can never quite slow it to a complete stop, its decreasing velocity can always be just high enough to overcome gravity's pull. The initial velocity needed to achieve that condition is called escape velocity.

From the surface of the Earth, escape velocity (ignoring air friction) is about 7 miles per second, or 25,000 miles per hour. Given that initial speed, an object needs no additional force applied to completely escape Earth's gravity.

Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor


Escape velocity is defined to be the minimum velocity an object must have in order to escape the gravitational field of the earth, that is, escape the earth without ever falling back.

The object must have greater energy than its gravitational binding energy to escape the earth's gravitational field. So:

1/2 mv2 = GMm/R

Where m is the mass of the object, M mass of the earth, G is the gravitational constant, R is the radius of the earth, and v is the escape velocity. It simplifies to:

v = sqrt(2GM/R)

or

v = sqrt(2gR)

Where g is acceleration of gravity on the earth's surface.

The value evaluates to be approximately:

11100 m/s
40200 km/h
25000 mi/h


So, an object which has this velocity at the surface of the earth, will totally escape the earth's gravitational field (ignoring the losses due to the atmosphere.) It is all there is to it.

Answered by: Yasar Safkan, B.S. Phsyics Ph.D. Candidate, M.I.T.

"

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Yep, it's time to put forward that trusty 'slide calculator'.
Electricity be damned:)

« Last Edit: 27/12/2008 14:16:16 by yor_on »
 

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Do Neutron stars cool down
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