Its as big as Manhattan Island, is 10 trillion times denser than steel, and is hurtling our way at speeds over 100 times faster than a supersonic jet. An alien spaceship? No, its a runaway neutron star, called RX J185635-3754, forged in a stellar explosion that was visible to our ancestors in 1 million B.C. Precise observations made with the Hubble telescope confirm that the interstellar interloper is the closest neutron star ever seen. The object also doesnt have a companion star that would affect its appearance. Now located 200 light-years away in the southern constellation Corona Australis, it will swing by Earth at a safe distance of 170 light-years in about 300,000 years. What is its structure like? First, the typical mass of a neutron star is about 1.4 solar masses, and the radius is probably about 10 km. By the way, the mass here is the gravitational mass (i.e., what youd put into Keplers laws for a satellite orbiting far away). This is distinct from the baryonic mass, which is what youd get if you took every particle from a neutron star and weighed it on a distant scale. Because the gravitational redshift of a neutron star is so great, the gravitational mass is about 20% lower than the baryonic mass. The surface gravity is about 10^11 times Earths, and the magnetic field is about 10^12 Gauss, which is enough to completely mess up atomic structure: for example, the ground state binding energy of hydrogen rises to 160 eV in a 10^12 Gauss field, versus 13.6 eV in no field. In the atmosphere and upper crust, you have lots of nuclei, so it isnt primarily neutrons yet. At the top of the crust, the nuclei are mostly iron 56 and lighter elements, but deeper down the pressure is high enough that the equilibrium atomic weights rise, so you might find Z=40, A=120 elements eventually. At densities of 10^6 g/cm^3 the electrons become degenerate, meaning that electrical and thermal conductivities are huge because the electrons can travel great distances before interacting. Deeper yet, at a density around 4x10^11 g/cm^3, you reach the neutron drip layer. At this layer, it becomes energetically favorable for neutrons to float out of the nuclei and move freely around, so the neutrons drip out. Even further down, you mainly have free neutrons, with a 5%-10% sprinkling of protons and electrons.
Posted on: Mon, 17 Mar 2014 09:21:15 +0000
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