Neutron stars are the densely compact cores remaining as one possible result of a supernova. These star remnants are immensely more compressed than their white dwarf counterparts, forcing electrons and protons together under extreme gravity to form tightly packed neutrons. By observing solar system 4U 1820-30 — a binary system containing a white dwarf and neutron star pair — Dr. Rosario Iaria of the University of Palermo and collaborators were able to measure the neutron star's compactness in relation to its radius using gravitational redshift.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
In this episode we’re talking about measurements of a neutron star. Neutron stars are very interesting and hold both a lot of information and a lot of imagination. Neutron stars are possible outcomes of exploding massive stars or supernovas. Unlike our sun, stars with about a dozen times more mass can continue core fusion of heavier elements including carbon, oxygen, neon, and even silicon, which produces iron, nickel, and cobalt. When that core gets too massive it collapses creating the supernova. But as we’ve discussed before core‑collapsed supernova are not really explosions as we normally think of them but rather a contraction and a bounce. As the core collapses gravity presses atoms together electrons are forced into the nucleus merging with protons to become electrically neutral neutrons. The neutrons themselves resist merging because of the strong nuclear force and the resulting rebound plus some help from neutrinos ejects the rest of the star in what we view as a supernova explosion.
Neutron stars can have a surprising and interesting feature. At least two‑thirds of all stars are in binaries or multiple systems and for massive stars that fraction is even higher. But even after exploding and losing the vast majority of their mass neutron stars can still be in binaries. Think about that for a moment. A star that begins with, say, ten times the mass of our sun will lose 80 to 90 percent of that in the supernova. This weakens its gravitational field by the same amount and of course all that material goes streaming past the companion. Even more remarkable is that there can be two stars both massive both having supernovas and still in binaries even close binaries. The so‑called Pulsi‑Taylor pulsar is two neutron stars orbiting each other with a period of only 7.7 hours. Russell Hulse and Joseph Taylor won the Nobel Prize in 1993 for the use of that compact binary to verify one of the predictions of general relativity.
Neutron stars are also notorious for being the most compact yet stable structures in our universe at least that we know of today. Their gravity is crazy strong approaching the speed of light and that affects their structure. On the other side like all astronomical objects they’re exposed to space and so they have a small atmosphere. Under that there is a solid crust made of quasi‑normal material electrons and atomic ions of iron nickel and other heavy elements. Inside of that the star is not solid but mostly considered to be a liquid of exotic material. This can only roughly be modeled as we can’t really create this in any lab. The relativistic heavy ion collider or RIC which recently ceased operations on Long Island smashed gold nuclei together at extremely high speeds to understand how matter behaves under high densities but they can only do it for a tiny amount of matter and only for a quick flash of time. So we have to go with models and they suggest that in the outer core neutrons form a frictionless superfluid and there are enough protons to form a superconducting superfluid which is a fluid without friction or electrical resistance. And in the inner core materials pressed so closely together that perhaps even quarks which neutrons are made of become free to flow.
That brings us to a paper by Rosario Iaria of the University of Palermo and collaborators published in the Astrophysical Journal. In it Dr. Iaria’s team used a special spectral feature under special conditions to determine a mass radius relation for neutron stars. The star is called 4U1820‑30 but let’s just call it 1820. This star like many neutron stars is still in a binary and the companion is a white dwarf which is also a compact star about the size of Earth. In this case the stars are close enough that 1820 the neutron star is slowly eating the white dwarf and the material forms a disc around 1820. That material slowly grows on 1820’s surface until it’s dense enough and hot enough to do carbon fusion. This creates a superburst of high energy radiation off of 1820’s surface sufficient to briefly remove its atmosphere. That’s the special condition. During that time Dr. Iaria’s team was able to view an iron spectral feature from near the neutron star surface and that’s the special feature. Photons that leave so close to a neutron star surface have to give up some of their energy to escape and this is called gravitational redshift. By measuring the gravitational redshift of the special iron line observed because of the special super outburst conditions they were able to measure the neutron star’s compactness at about 4.46 kilometers per solar mass. This means that a typical 1.4 solar mass neutron star would be 6.4 kilometers in radius and a 2.2 solar mass one would be 10 kilometers in radius.
I’ll close with a great quote from astronomer Frank Shue. A sugar cube of neutron star stuff on Earth would weigh as much as all of humanity. This illustrates again how much of humanity is empty space.