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The Neutron Star

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Wade Hampton III

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The Neutron Star

PostSun Aug 19, 2018 2:30 am

Alex Fleming, Graduate Research Assistant at Texas A&M University
(2016-present) asks...

How can a beam of light travel around the Earth roughly 7 times
in a second and a neutron star can supposedly rotate 1,000 times
in a second? Wouldn’t the rotational speed of the neutron star
exceed the speed of light?

Answered Fri · Upvoted by David Vanderschel, PhD Mathematics &
Physics, Rice (1970) and Paddy Alton, PhD in Astrophysics.


The reason we know that a neutron star CAN spin up to 716 times
in a second is because we have measured this in the fastest
spinning pulsar on record. A pulsar is a neutron star that has
a jet of high energy photons going off in two directions at
opposite poles. These photons are created along the direction
of the terrifyingly strong magnetic field generated by the neutron
star. The magnetic poles, just like on Earth, can occur on an axis
other than the axis of rotation. And as such, the pulsar acts kind
of similar to a lighthouse, sweeping a beam of light in a circle.
We can see a pulsar when we just happen to be in the direct path
of this beam of light. When we see this light, we can simply count
the number of pulses that we see in a second and we know how rapidly
the neutron star is rotating.
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Now to answer your question. I've read a lot of the given answers
that simply say how large a neutron star is. The reason we know how
large the neutron star is actually is BECAUSE people first asked
the very question you asked. The speed of light is actually an upper
limit that theoretical nuclear astrophysicists used to take a first
guess at approximating the radius of a neutron star. We know THAT
a neutron star can rotate at 700 times a second. Since the speed of
light is the fastest anything can go, that means that a perfectly
spherical object with an equatorial tangential velocity of the
speed of light and spinning 700 times a second would have a radius
of about 40 miles. This is a crude estimate, but it does tell us
that we cannot have a radius larger than this.

We can also use the mass of the neutron star to set a lower limit
on this radius. Within some minimum radius, a neutron star of a
given mass will collapse into a black hole. Knowing the mass, and
how fast its spinning can give us a definite range. With the help
of LIGO and other gravitational wave detectors, we can hopefully
in the future get better ideas of the upper and lower limits of
the mass of neutron stars.
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These two numbers, speed of rotation and mass, can help give us a
definite range of the size of a neutron star. To figure out the
EXACT radius of a given neutron star of a given mass is actually
a tremendous challenge, and still an ongoing problem in theoretical
nuclear astrophysics. We have to create models that use nuclear
physics and special and general relativity to take as inputs a set
of conditions we think exist inside neutron stars and then have
our model spit out a guess for the mass or radius. If the mass is
too large for too small a radius, it would collapse into a black
hole, the model is rejected. If the radius is so large that it
would break the cosmic speed limit, this too must be thrown out.
We have other conditions that limit the range further, but the
mass and rotational velocity are enough to make a good first guess.

So finally to answer your question, would the rotational speed of
a neutron star exceed the speed of light? No, because astrophysicists
based our best guess of the radius of a neutron star with the
speed of light in mind.
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Wade Hampton III

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Re: The Neutron Star

PostSun Oct 21, 2018 2:20 pm

PSR J1748-2446ad is the fastest-spinning pulsar known, at 716 Hz, or 716 times
per second. This pulsar was discovered by Jason W. T. Hessels of McGill University
on November 10, 2004 and confirmed on January 8, 2005. It has been calculated
that the neutron star contains slightly less than two times the mass of the Sun,
within the typical range of neutron stars. Its radius is constrained to be less
than 16 km. At its equator it is spinning at approximately 24% of the speed of
light, or over 70,000 km per second. The pulsar is located in a globular cluster
of stars called Terzan 5, located approximately 18,000 light-years from Earth in
the constellation Sagittarius.

The location of PSR J1748-2446ad in the night sky. The pulsar is located in the
center of the yellow square. It is too faint in this image to be visible against
the background.
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Fast Little Fella
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It is part of a binary system and undergoes regular eclipses with an eclipse
magnitude of about 40%. Its orbit is highly circular with a 26-hour period.
The other object is about 0.14 solar masses, with a radius of 5–6 solar radii.
Hessels states that the companion may be a "bloated main-sequence star, possibly
still filling its Roche Lobe". Hessels goes on to speculate that gravitational
radiation from the pulsar might be detectable by LIGO.
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Wade Hampton III

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Re: The Neutron Star

PostTue Mar 26, 2019 11:10 pm

Asked by Jonathan Fox...

Neutron stars are created when a star around eight to ten
times the mass of our Sun runs out of fuel. The outward
pressure generated by fusion reduces rapidly, allowing
gravity to pull the star in on itself and trigger a
supernova, where the outer layers of a star’s atmosphere
get blown into space. The remaining matter continues to
collapse under gravity, forcing electrons and protons to
be squashed together and become neutrons. The neutron star
will have less mass than its parent star (typically about
1.4-times the mass of the Sun), but this mass will be
confined by gravity to a region of approximately
20 kilometers (12 miles) across, leading to an
incredibly dense object. It is this density (a
teaspoon full of neutron star would have a mass
of about a billion tons) that truly defines a
neutron star.
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A neutron star, pictured here next to Manhattan,
New York, for scale, are highly dense objects.
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Wade Hampton III

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Re: The Neutron Star

PostSat Apr 06, 2019 7:28 am

Playing With Neutron Stars....

Nelson Cunnington, Enthusiast...wants to bring a cubic meter of
neutron star material to Earth.....

W-e-l-l....

There are a number of challenges to this scenario that we can’t
solve with our technology....

A cubic meter of neutronium would mass around 4 × 1017 kg/m³.
So that would be 400 trillion tonnes, or about the mass of
an asteroid 45 km across.
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The gravity in the center of each face would be around 11
million g, and at each corner 3.5 million g. One hundred
meters away, the gravity would be only 267 g, and at a
kilometer 2.7 g. At 10 km, you would probably not notice
the change in your local vertical. So the first thing
that would happen is that everything around the cube would
be drawn to it, very quickly. At a hundred metres the fall
would take a fraction of second; at 1650m stuff would start
rolling towards it as though down a 45° slope at 1.4 g.
When the matter hit the surface (traveling at about a
kilometer a second or less) it would be compressed by the
extreme gravity into a form known as condensed matter,
where the nucleons are in contact and the electrons freely
travel among them. This is very dense itself, about a million
tonnes per cubic meter. A hemisphere of granite a kilometer in
radius would be about 750,000 tonnes, so your cube would acquire
a surface of condensed matter less than half-a-meter thick.
There would be a great deal of energy released as this happened,
equivalent to a hundred kiloton-scale nuclear weapon, but spread
out over several seconds.
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Presumably, you are holding up the mass with tractor beams. If
you released the tractor beams as you realized what an utter
mistake you had made bringing this mass to Earth, it would fall
toward the Earth at one gee. Hitting the surface, it wouldn’t
pause, being so much denser than the rock on which you built your
high-density physics laboratory that it would slip through it like
a steel weight through high-altitude air. Presumably there would
be a terminal velocity for neutronium through rock, which would
lessen as the cube got deeper, but my math isn’t up to working
it out. Assuming it didn’t slow down, in about twenty-one minutes
it would have fallen to the center of the Earth, and then begun
to climb again so that it would appear at the antipode from the
site of your former laboratory twenty-one minutes later. By this
time it should have accumulated quite a thickness of condensed
and normal matter, though, so it wouldn’t necessarily get to the
surface again, instead oscillating back and forth at a decaying
rate until it slowed enough to settle at the center. Before that,
though, as it falls through the surface of the Earth, the tidal
forces of the extreme mass passing will trigger a local earthquake,
quite likely at the high end of the magnitude scale, which will
level buildings for kilometers around.
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But perhaps as well as the tractor beams you accidentally switch
off the force-field that contains the neutronium cube? As others
have mentioned, stupendous though it is, the mass of a one-meter
cube isn’t enough to keep the neutronium in a stable state, and
it will start to disintegrate immediately. The half-life of a
free neutron is about ten minutes, leading to it decaying into
a proton, an electron and a neutrino. These particles are stable,
but the sheer quantity of the first conversions will lead to a
gigantic explosion, turning the cube and everything around it
into plasma. An explosion that would make our most powerful
nuclear weapons look like firecrackers.

Better to perform this kind of experiment OFF the Earth!
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Wade Hampton III

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Re: The Neutron Star

PostMon Apr 08, 2019 2:22 am

In the aftermath of a 8 – 20 solar mass star’s demise, we find a weird
little object known as a neutron star. Neutrons stars are incredibly
dense, spin rapidly, and have very strong magnetic fields. Some of
them we see as pulsars, flashing in brightness as they spin.
Neutrons stars with the strongest magnetic fields are called
magnetars, and are capable of colossal bursts of energy that
can be detected over vast distances.
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More Bank For Buck!
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https://www.youtube.com/watch?v=RrMvUL8HFlM

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