There are believed to be millions of neutron stars in the Milky Way.
The most massive known to date -a stellar corpse twice as massive as our
Sun, was discovered in 2010 by astronomers using the National Science Foundation's Green Bank Telescope
(GBT). With all their mass packed into a sphere the size of a small
city, neutron stars protons and electrons are crushed together into
neutrons. A neutron star can be several times more dense than an atomic
nucleus, and a thimbleful of neutron-star material would weigh more than
500 million tons. This tremendous density makes neutron stars an ideal
natural "laboratory" for studying the most dense and exotic states of
matter known to physics.
A pulsar is a rotating neutron star
that can produce radiation by spinning its powerful magnetic field
through space. A neutron star has used up most of its rotational energy
moving its magnetic field, and and so it gradually slows down. When it
slows down enough, it no longer radiates much energy, and so it is no
longer considered a pulsar. This usually happens within a few million
years. If a neutron star had only a weak magnetic field, it would also
not be a pulsar. The first pulsar, dubbed LGM-1 for "Little Green Men,"
was observed on November 28, 1967 by Jocelyn Bell and
Antony Hewish (who was awarded the Nobel Prize for the discovery
creating a hugely justified controvery), who were initially baffled as
to the seemingly unnatural regularity of its emissions.
At the time people had no idea what these radio signals were so the
unknownsource was dubbed " little green men" (LGM) as one possibility
could have been that the sounds came from some extraterrestrial form of
life. This link takes
you to an article originally presented as an after-dinner speech by
Bell - it's a very personal and entertaining account of the discovery:
"Little Green Men, White Dwarfs or Pulsars? during which Bell asked "Where these signals made by men, but men from another civilization."
The more recent discovery of the monster object was surprising
because that much mass means that several theoretical models for the
internal composition of neutron stars are ruled out," said Paul
Demorest, of the National Radio Astronomy Observatory (NRAO).
"This mass measurement also has implications for our understanding of
all matter at extremely high densities and many details of nuclear
physics," he added.
The scientists used an effect of Albert Einstein's theory of General Relativity to measure the mass of the neutron star and its orbiting companion, a white dwarf star.
The neutron star, a pulsar, called PSR J1614-2230, spins 317 times per
second, and the companion completes an orbit in just under nine days.
The pair, some 3,000 light-years distant, are in an orbit seen almost
exactly edge-on from Earth. That orientation was the key to making the
mass measurement.
As the orbit carries the white dwarf directly in front of the pulsar,
the radio waves from the pulsar that reach Earth must travel very close
to the white dwarf. This close passage causes them to be delayed in
their arrival by the distortion of spacetime produced by the white
dwarf's gravitation. This effect, called the Shapiro Delay, allowed the
scientists to precisely measure the masses of both stars.
"We got very lucky with this system. The rapidly-rotating pulsar
gives us a signal to follow throughout the orbit, and the orbit is
almost perfectly edge-on. In addition, the white dwarf is particularly
massive for a star of that type. This unique combination made the
Shapiro Delay much stronger and thus easier to measure," said Scott
Ransom, also of NRAO.
The astronomers used a newly-built digital instrument called the
Green Bank Ultimate Pulsar Processing Instrument (GUPPI), attached to
the GBT, to follow the binary stars through one complete orbit earlier
this year. Using GUPPI improved the astronomers' ability to time signals
from the pulsar severalfold.The researchers expected the neutron star
to have roughly one and a half times the mass of the Sun. Instead, their
observations revealed it to be twice as massive as the Sun. That much
mass, they say, changes their understanding of a neutron star's
composition.
Some theoretical models postulated that, in addition to neutrons,
such stars also would contain certain other exotic subatomic particles
called hyperons or condensates of kaons."Our results rule out those
ideas," Ransom said.
"This measurement tells us that if any quarks are present in a
neutron star core, they cannot be 'free,' but rather must be strongly
interacting with each other as they do in normal atomic nuclei," said
Feryal Ozel of the University of Arizona, lead author of the second
paper.
There remain several viable hypotheses for the internal composition
of neutron stars, but the new results put limits on those, as well as on
the maximum possible density of cold matter.The scientific impact of
the new GBT observations also extends to other fields beyond
characterizing matter at extreme densities.
A leading explanation for the cause of one type of gamma-ray burst --
the "short-duration" bursts -- is that they are caused by colliding
neutron stars. The fact that neutron stars can be as massive as PSR
J1614-2230 makes this a viable mechanism for these gamma-ray bursts.
Such neutron-star collisions also are expected to produce
gravitational waves that are the targets of a number of observatories
operating in the United States and Europe. These waves, the scientists say, will carry additional valuable information about the composition of neutron stars.
"Pulsars in general give us a great opportunity to study exotic
physics, and this system is a fantastic laboratory sitting out there,
giving us valuable information with wide-ranging implications," Ransom
explained. "It is amazing to me that one simple number -- the mass of
this neutron star -- can tell us so much about so many different aspects
of physics and astronomy," he added.
Source: The Daily Galaxy via the National Radio Astronomy Observatory
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