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Topic Name: Arecibo Observatory finds Neutron Stars can be Considerably more Massive and also difficult to form Black Holes

Category: STAR (Space, Telecommunications & Radioscience)

Research persons: Paulo Freire

Location: Cornell University, United States

Details

Neutron stars and black holes aren’t all they’ve been thought to be.

In fact, neutron stars can be considerably more massive than previously believed, and it is more difficult to form black holes, according to new research developed by using the Arecibo Observatory in Arecibo, Puerto Rico. Paulo Freire, an astronomer from the observatory, will present his research at the American Astronomical Society national meeting in Austin on Jan. 11.

The Arecibo Observatory is managed by Cornell University for the National Science Foundation.

In the cosmic continuum of dead, remnant stars, the Arecibo astronomers have increased the mass limit for when neutron stars turn into black holes.

“The matter at the center of a neutron star is highly incompressible. Our new measurements of the mass of neutron stars will help nuclear physicists understand the properties of super-dense matter,” said Freire. “It also means that to form a black hole, more mass is needed than previously thought. Thus, in our universe, black holes might be more rare and neutron stars slightly more abundant.”

When the cores of massive stars run out of nuclear fuel, their enormous gravitation then causes their collapse then becomes a supernova. The core, typically with a mass 1.4 times larger than that of the sun is compressed into a neutron star. These extreme objects have a radius about 10 to 16 kilometers and a density on the order of a billion tons per cubic centimeter. Freire says that a neutron star is like one single, giant atomic nucleus with about 460,000 times the mass of the Earth.

Astronomers had thought the neutron stars needed a maximum mass between 1.6 and 2.5 suns in order to collapse and become black holes. However, this new research shows that neutron stars remain neutron stars between the mass of 1.9 and up to possibly 2.7 suns.

“The matter at the center of the neutron stars is the densest in the universe. It is one to two orders of magnitude denser than matter in the atomic nucleus. It is so dense we don’t know what it is made out of,” said Freire. “For that reason, we have at present no idea of how large or how massive neutron stars can be.”

From June 2001 to March 2007, Freire used Arecibo’s “L-wide” receiver (sensitive to radio frequencies from 1100 to 1700 MHz) and the Wide-Band Arecibo Pulsar Processors – a very fast spectrometer on the Arecibo telescope – to examine a binary pulsar called M5 B, in the globular cluster M5, which is located in the constellation Serpens. Like a lighthouse emits light, a pulsar is a strongly magnetized neutron star that emits large amounts of electromagnetic radiation, usually from its magnetic pole. As in the case of a lighthouse, distant observers perceive a sequence of pulsations, which are caused by the rotation of the pulsar. In the case of M5 B, these radio pulsations arrive at the Earth every 7.95 milliseconds.

These radio pulsations were scanned by the wide-band spectrometers once every 64 microseconds for 256 spectral channels, and then recorded to a computer disk, with accurate timing information. The precise arrival time of the pulses were then used by the astronomers to accurately measure the orbital motion of M5 B about its companion. This allowed the astronomers to estimate the mass (1.9 solar masses) of the pulsar.

Note for Neutron Star
A neutron star is formed from the collapsed remnant of a massive star; i.e. a Type II, Type Ib, or Type Ic supernova. Models predict that neutron stars consist mostly of neutrons, hence the name. Such stars are very hot, as supported by the Pauli exclusion principle indicating repulsion between neutrons. A neutron star is one of the few possible conclusions of stellar evolution.
A typical neutron star has a mass between 1.35 and about 2.1 solar masses, with a corresponding radius between 10 and 20 km — 30,000 to 70,000 times smaller than the Sun. Thus, neutron stars have overall densities of 8.4×1016 to 1×1018 kg/m³, which compares with the approximate density of an atomic nucleus of 3×1017 kg/m³. The neutron star's density varies from below 1×109 kg/m³ in the crust increasing with depth to above 6 or 8×1017 kg/m³ deeper inside.
In general, compact stars of less than 1.38 solar masses, the Chandrasekhar limit, are white dwarfs; above 2 to 3 solar masses (the Tolman-Oppenheimer-Volkoff limit), a Quark star might be created, however this is uncertain. Gravitational collapse will always occur on any star over 5 solar masses, inevitably producing a black hole. As the core of a massive star is compressed during a supernova, and collapses into a neutron star, it retains most of its angular momentum. Since it has only a tiny fraction of its parent's radius (and therefore its moment of inertia is sharply reduced), a neutron star is formed with very high rotation speed, and then gradually slows down. Neutron stars are known to have rotation periods between about 1.4ms to thirty seconds.

About Arecibo Observatory
The Arecibo Observatory is a very sensitive radio telescope located approximately 9 miles (14 km) south-southwest from the town of Arecibo in Puerto Rico. It is operated by Cornell University under cooperative agreement with the National Science Foundation. The observatory works as the National Astronomy and Ionosphere Center (NAIC) although both names are officially used to refer to it. NAIC more properly refers to the organization that runs both the observatory and associated offices at Cornell University.
The observatory's 305 m radio telescope is the largest single-aperture telescope (cf. multiple aperture telescope) ever constructed. It carries out three major areas of research: radio astronomy, aeronomy (using both the 305 m telescope and the observatory's lidar facility), and radar astronomy observations of solar system objects. Usage of the telescope is gained by submitting proposals to the observatory, which are evaluated by an independent board of referees.
The telescope is visually distinctive and has been used in the filming of two notable motion pictures: as the villain's antenna in the James Bond movie GoldenEye and as itself in the film Contact. The telescope received additional international recognition in 1999 when it began to collect data for the SETI@home project.

Note for Globular Cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is derived from the Latin globulus—a small sphere. A globular cluster is sometimes known more simply as a globular.
Globular clusters, which are found in the halo of a galaxy, contain considerably more stars and are much older than the less dense galactic, or open clusters, which are found in the disk. Globular clusters are fairly common; there are about 150 currently known globular clusters in the Milky Way, with perhaps 10–20 more undiscovered. Large galaxies can have more: Andromeda, for instance, may have as many as 500. Some giant elliptical galaxies, such as M87, may have as many as 10,000 globular clusters. These globular clusters orbit the galaxy out to large radii, 40 kiloparsecs (approximately 131 thousand light years) or more.

Astronomers also working on this research are: Maureen van den Berg, Northwestern University, Evanston, Ill.; Jason W. T. Hessels, Astronomical Institute “Anton Pannekoek” of the University of Amsterdam in the Netherlands; and Alex Wolszczan, Pennsylvania State University, State College, Pa.