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HEAPOW: Weighing a Neutron Star (2010 Oct 18)
GSFC: Eclipsing Pulsar Promises Clues to Crushed Matter
How much matter can you squeeze into a small space? On earth the densest natural element
is iridium (or perhaps osmium), with a density of about 22.5 grams per cubic centimeter. Beyond earth things get weirder. Black holes
floating around our Galaxy have regions called singularities where the density is (perhaps) infinite, but such singularities are screened by the event horizon
(with perhaps some notable exceptions
) so they're impossible to study from the safety of our Universe. Some of the densest things we know about that we can study are neutron stars
, the dead cinders left behind by the explosive death of a massive star. Nature kindly provides these objects, where effectively the entire mass of the sun is squeezed down to a tiny object that would fit inside the Washington DC beltway
. A neutron star is about 10 trillion times denser than iridium. It's similar, in fact, to the density of the atomic nucleus. No one really understands how the extreme densities of neutron stars make matter behave, or whether such enormous pressures at the center of a neutron star produces types of matter seen nowhere else in the Universe. So neutron stars are objects of intense interest for astrophysicists. Recently astronomers have uncovered a key neutron star system which provides perhaps the best measure of the mass of a neutron star. This object, called Swift J1749.4-2807, was discovered by the Swift
space observatory in June of 2006. In April of this year it exploded in X-rays, and during this outburst the X-ray emission was observed by the RXTE
space observatory. RXTE made a detailed study of the variation of the X-ray emission with time and found an important result: the the neutron star is actually eclipsed by the companion star. By studying these eclipses, astronomers can obtain an extremely precise neutron star mass measurement, once the companion star is identified and studied. The image above is an artist's impression of Swift J1749.4-2807.
- 17 Aug 2010
http://asterisk.apod.com/vie ... 31&t=20675
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bystander wrote: HEAPOW: Weighing a Neutron Star (2010 Oct 18)
On earth the densest natural element
is iridium (or perhaps osmium), with a density of about 22.5 grams per cubic centimeter.
<<Osmium is an extremely dense, blue-gray, hard but brittle metal that remains lustrous even at high temperatures. Due to its hardness, brittleness, low vapor pressure (the lowest of the platinum group metals), and very high melting point (the fourth highest of all elements), solid osmium is difficult to machine, form, or work. Osmium is generally considered to be the densest known element, slightly denser than iridium. Calculations of density from the space lattice may produce the most reliable data for these elements, giving a density of 22.562±0.009 g/cm3 for iridium versus 22.587±0.009 g/cm3 for osmium. The extraordinary density of osmium is a consequence of the lanthanide contraction.
Osmium has the highest melting point and the lowest vapor pressure in the platinum family. Osmium has a very low compressibility. Correspondingly, its bulk modulus is extremely high, reported between 395 and 462 GPa, which rivals that of diamond (443 GPa). However, the hardness of osmium is lower than diamond, only 4 GPa.>>
<<Lanthanide contraction is a term used in chemistry to describe the decrease in ionic radii of the elements in the lanthanide series which results in smaller than otherwise expected ionic radii for the subsequent elements starting with 72, Hafnium
. The ionic radius drops from 102 pm for cerium to 86.1 pm for lutetium. About 10% of the lanthanide contraction has been attributed to relativistic effects.
There is [also] a general trend of increasing Vickers hardness, Brinell hardness, density and melting point from cerium to lutetium (with ytterbium being the most notable exception). Lutetium is the hardest and densest lanthanide and has the highest melting point.>>