by Group 7
Gravitational Waves and Neutron Stars
In the department of Gravitational Waves, there are three main subjects that need to be explained, which are Gravitational Fields, Gravitational Waves and why are they so hard to detect; so let's start with Gravitational Fields.
What are Gravitational Fields?
A gravitational field exists in any region depending only on the particle’s mass and position. The gravitational field at a point is a vector quantity usually represented by the symbol g. The gravitational field at a point is defined as the force per unit mass that would act on a particle located at that point. If a test mass m is subject to a force F at some point, and F depends only on the particle’s mass and position, then the gravitational field at that point is defined as: g = Fm
What are Gravitational Waves?
Gravitational waves are 'ripples' in space-time caused by some of the most violent and energetic processes in the Universe. Albert Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity. Einstein's mathematics showed that massive accelerating objects (such as neutron stars or black holes orbiting each other) would disrupt space-time in such a way that 'waves' of undulating space-time would propagate in all directions away from the source. These cosmic ripples would travel at the speed of light, carrying with them information about their origins, as well as clues to the nature of gravity itself. The strongest gravitational waves are produced by cataclysmic events such as colliding black holes, supernovae (massive stars exploding at the end of their lifetimes), and colliding neutron stars. Other waves are predicted to be caused by the rotation of neutron stars that are not perfect spheres, and possibly even the remnants of gravitational radiation created by the Big Bang.
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Why are Gravitational Waves so hard to detect?
By the time gravitational waves reach us from the distant events that spawn them, they distort spacetime by an utterly minuscule amount. The distortion is many times smaller than the width of a proton, one of the particles in an atom’s nucleus. Measuring such minute changes in length is pretty much impossible for most instruments. But why are these waves so important? Gravitational waves are a new way of “seeing” what happens in space: We can now detect events that would otherwise leave little to no observable light, like black hole collisions. And with this latest detection, astronomers were able to combine gravitational waves with more traditional ways of seeing the universe, helping to untangle mysteries about the dense, dead objects known as neutron stars.
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Neutron Stars
Neutron Stars are formed when a massive star runs out of fuel and collapses. The core collapses crushing together every proton and electron into a neutron If the core of the collapsing star is between about 1 and 3 solar masses, these newly-created neutrons can stop the collapse, leaving behind a neutron star. (Stars with higher masses will continue to collapse into stellar-mass black holes. This collapse leaves behind the most dense object known, an object with the mass of a sun squished down to the size of a city. These stellar remnants measure about 20 kilometers (12.5 miles) across. One sugar cube of neutron star material would weigh about 1 trillion kilograms (or 1 billion tons) on Earth, about as much as a mountain.
Most of the neutron stars are undetectable simply because they don't emit enough radiation. However they can be detected if certain conditions are met. A handful of neutron stars have been found sitting at the centers of supernova remnants quietly emitting X-Rays. More often, though, are found spinning wildly with huge magnetic fields as pulsars and magnetars
Neutron stars can pack up their mass inside a 20 kilometer diameter. On average, the gravity on a neutron star can be 2 billion times stronger than gravity on Earth. In fact, it's strong enough to significantly bend radiation from the star in a process known as gravitational lensing, allowing astronomers to see some of the back side of the star.
That power from the supernova that birthed the neutron star gives an extremely quick rotation, causing it to spin several times per second. Neutron stars can spin as fast as 43 000 times per minute (716,7 full spins per seconds) slowing down over time.
If a neutron star is part of a binary system that survived the deadly blast from its supernova (or if it captured a passing companion), things can get even more interesting. If the second star is less massive than the sun then the star that has the bigger mass pulls the mass from its companion into a Roche lobe, a balloon-like cloud of material that orbits the neutron star. If the so-called companion stars have a mass 10 times bigger than the sun then instead of creating a Roche Lobe it transfers the material in the form of stellar wind. The material flows along the magnetic poles of the neutron star, creating X-ray pulsations as it is heated. By 2010, approximately 1,800 pulsars had been identified through radio detection, with another 70 found by gamma-rays. Some pulsars even have planets orbiting them.
Magnetars and Pulsars
Magnetars
Pulsars
Almost the entire Universe is a horrible and hostile place, apart from a fraction of a mostly harmless planet in a backwater corner of the Milky Way.
While living here on Earth takes about 80 years to kill you, there are other places in the Universe at the very other end of the spectrum. Places that would kill you in a fraction of a fraction of a second. And nothing is more lethal than supernovae and remnants they leave behind: neutron stars.
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Neutron stars are formed when stars more massive than our Sun explode as supernovae. When these stars die, they no longer have the light pressure pushing outward to counteract the massive gravity pulling inward. This enormous inward force is so strong that it overcomes the repulsive force that keeps atoms from collapsing. Protons and electrons are forced into the same space, becoming neutrons. The whole thing is just made of neutrons. Did the star have hydrogen, helium, carbon and iron before? That's too bad, because now it's all neutrons. You get pulsars when neutron stars first form. When all that former star is compressed into a teeny tiny package. The conservation of angular motion spins the star up to tremendous velocities, sometimes hundreds of times a second.
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But when neutron stars form, about one in ten does something really strange, becoming one of the most mysterious and terrifying objects in the Universe. They become magnetars.
Magnetars are neutron stars, formed from supernovae. But something unusual happens as they form, spinning up their magnetic field to an intense level. In fact, astronomers aren't exactly sure what happens to make them so strong.
One idea is that if you get the spin, temperature and magnetic field of a neutron star into a perfect sweet spot, it sets off a dynamo mechanism that amplifies the magnetic field by a factor of a thousand.
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But a more recent discovery gives a tantalizing clue for how they form. Astronomers discovered a rogue magnetar on an escape trajectory out of the Milky Way. We've seen stars like this, and they're ejected when one star in a binary system detonates as a supernova. In other words, this magnetar used to be part of a binary pair.
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And while they were partners, the two stars orbited one another closer than the Earth orbits the Sun. This close, they could transfer material back and forth. The larger star began to die first, puffing out and transferring material to the smaller star. This increased mass spun the smaller star up to the point that it grew larger and spewed material back at the first star.
Pulsars are spherical, compact objects that are about the size of a large city but contain more mass than the sun. Scientists are using pulsars to study extreme states of matter, search for planets beyond Earth's solar system and measure cosmic distances.
Pulsars also could help scientists find gravitational waves, which could point the way to energetic cosmic events like collisions between supermassive black holes. Discovered in 1967, pulsars are fascinating members of the cosmic community.
From Earth, pulsars often look like flickering stars. On and off, on and off, they seem to blink with a regular rhythm. But the light from pulsars does not actually flicker or pulse, and these objects are not actually stars.
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Pulsars radiate two steady, narrow beams of light in opposite directions. Although the light from the beam is steady, pulsars appear to flicker because they also spin. It's the same reason a lighthouse appears to blink when seen by a sailor on the ocean: As the pulsar rotates, the beam of light may sweep across the Earth, then swing out of view, then swing back around again. To an astronomer on the ground, the light goes in and out of view, giving the impression that the pulsar is blinking on and off. The reason a pulsar's light beam spins around like a lighthouse beam is that the pulsar's beam of light is typically not aligned with the pulsar's axis of rotation.
