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Stellar Astronomy: Star Facts – What is a Neutron Star?

written by: VinceSummers•edited by: George Adcock•updated: 4/22/2011

A supernova event is the explosion of a star. What remains afterwards? One possibility is a neutron star—what is a neutron star? What causes a neutron star to form, and what amazing properties characterize it?

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    Star Qualifications - Star "Pre-Approval"

    Isolated Neutron Star RXJ185635-3754 - F. Walter State University of New York at Stony Brook) and NASA 

    A supernova is an exploding star. Although the outer portion of a supernova shoots away from the star's original location, itsNeutron Star RCW103 - NASA/CXC/Penn State/G.Garmire et al  central portion is blown inward, producing a dense core. If the mass is about 1.3 to 2.1 times that of our sun, it will have reduced to a size approximately 12 miles across. Gravity will have exerted sufficient force so the electrons of its atoms have tunneled into its protons. This results in neutrinos and neutrons. The neutrinos escape the pull of gravity, leaving the neutrons—hence, the name “neutron star." If the mass is much greater, a black hole results.* On the other hand, if the initial mass is less, the result is a white dwarf.

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    Formation and Properties

    Although the inner portion of the star implodes inward, its angular momentum is maintained. This greatly increases rate of rotation, as is illustrated by a spinning ice-skater. As an ice-skater spins and pulls in her arms, her body rotation speeds up. For some neutron stars, rotation rate may be even hundreds of times per second! This, coupled with surface smoothing due to intense gravity, molds a neutron star into an oblate spheroid. The core of a neutron star in a sense resembles an oversized nuclear particle.

    Since the magnetic field rotates with the core, the neutron star loses energy, slowing gradually. Contrariwise, if there is loose matter within its gravitational field, it may form into an accretion disk, which gradually feeds the neutron star. This speeds up rotation. If the axis of a neutron star is aligned appropriately with earth, its rotating magnetic field will be detected as discrete, time-regulated pulses—hence the term Pulsar. For interesting statistics associated with the properties of a neutron star, see the article “Nothing but the Facts about Neutron Stars."

    But what does the inside of a neutron star “look like?"Spinning Neutron Star (Pulsar) with Accretion Disk - NASA-Dana Berry 

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    Interior of a Neutron Star

    The interior of a neutron star is not definitely known, but is being anticipated based on computer-models. Included are an atmosphere, a crust, and various layers. The crust may contain atomic matter—iron because of its great binding energy, or lighter nuclei— in matrix form with some free-moving electrons. The layer beneath may be liquid. Much as gravity produces layers with different properties for each layer in other objects, the same is surely the case for neutron stars. There have been complex representations made, based on molecular modeling. See the references for an artist’s rendering.

    * Some theorize the formation of a “quark star" at masses between those necessary to produce a neutron star or a black hole.Neutron Star Cross-section - NASA/Marshall Space Flight Center 

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    References and Resources

    University of Maryland - “Introduction to Neutron Stars," by N. Coleman Miller -

    Neutron Star Interior Image -

    The Internet Encyclopedia of Science - Neutron Star -

    National Radio Astronomy Observatory - Artist’s Conception of Supernova 1986J -