Super Star Smashes into the Record Books.


The discovery of a super massive neutron star has thrown our understanding of stellar evolution into turmoil. The new star, called PSR J1614-2230 contains twice the mass of the Sun but compressed down into a star that is smaller than the Earth (you could fit over a million Earth’s inside the Sun by comparison). It is estimated a thimbleful of material from the star could weigh more than 500 million tons — that equates to about a million airliners. The study has cast serious doubt over how matter reacts under extreme densities.

The study by a team of astronomers using the National Radio Astronomy Observatory in New Mexico focussed its attention on the star which is about 3,000 light years away (the distance light can travel in 3,000 years at a speed of 300,000 km per second). The stellar corpse whose life ended long ago is now rotating at an incredible speed, completing 317 rotations every second. Its emitting an intense beam of energy from its polar regions which just happens to point in the direction of us here on Earth. We can detect this radiation beam as it flashes on and off like a celestial lighthouse. This type of neutron star is classed a pulsar.

Artist impression of Pulsar
Artist impression of Pulsar

Rather fortuitously, the star is part of a binary star system and is orbited by a white dwarf star which completes one orbit in just nine days. Its through the measurements of the interaction of the two which gave astronomers the clue as to the pulsar’s mass. The orbit of the white dwarf takes it between the beam of radiation and us here on Earth so that the energy from the beam has to pass close by the companion star. By measuring the delay in the beam’s arrival caused by distortion of space-time in the proximity of the white dwarf, scientists can determine the mass of both objects. Its an effect called the Shapiro Delay and its simply luck that the orientation of the stars to the Earth allows the effect to be measured.

Dave Finley, Public Information Officer from NRAO told Universe Today ‘Pulsars are neutron stars, whose radiation beams emerge from the poles and sweep across the Earth.  The orientation of the poles (and thus of the beams) is a matter of chance. We just got very lucky with this system.’

The discovery which was made possible by the new ‘Green Bank Ultimate Pulsar Processing Instrument (GUPPI) was able to measure the pulses from the pulsar with incredible accuracy and thus come to the conclusion that the star weighed in at a hefty two times the mass of the Sun. Current theories suggested a mass of around one and a half solar masses were possible but this new discovery changes the understanding of the composition of such stars, even to the subatomic level.

Neutron stars or pulsars are extreme objects at the very edges of the conditions that matter can exist. They really test our knowledge of the physical Universe and slowly but surely, through dedicated work of teams of astronomers, we are not only learning more about the stars above our heads but more and more about matter in the Universe in which we live.

Mark Thompson is a writer and the astronomy presenter on the BBC One Show. See his website, The People’s Astronomer, and you can follow him on Twitter, @PeoplesAstro

Source: NRAO

24 Replies to “Super Star Smashes into the Record Books.”

  1. Have I missed something? According to the information I have neutron stars can have masses up to 4.7 solar masses (over that limit they would be black holes). So what is the fuzz about?

  2. Isn´t that limit only valid for white dwarfs as it is the limit for the pressure that electrons in atoms can hold back? If the pressure is increased the electrons are pushed into the atoms annihilating the protons leaving almost only neutrons left as is the case with a neutron star. Then there is only the neutron degeneration holding back further compaction. I have been under the impression that it (theoretically) should be able to hold back further gravitational collapse all the way to 4.7 solar masses.

    I can´t find any link to that now but I will see what I can find an post later.

  3. Yeah, I also wonder what this fuzz is about.

    1) A neutron star has a size of roughly 20km. So, saying that it is smaller than the earth is a little misleading — it is WAAAAAY smaller than the earth. 😉
    2) The theoretical limit is, as far as I know, at least 3,8 solar masses. So a neutron star with 2 solar masses is nothing but special.

    So, what’s the point here?

  4. The limit for a neutron star is called Tolman-Oppenheimer-Volkoff limit, and afaik it is not well known because it delves deep into particle physics with many uncertainties, but its definitely larger than Chandrasekhar limit, i have seen around 3.0 mentioned at some time. More recent values might very well be 4.7, i cant say for sure.

    However, in the few cases neutron stars have had their masses measured, they have had masses around 1.5 Msun or lower – that makes a 2.0 Msun neutron star a whopper.

  5. It’s around 3.332 Sol mass, I think. At 3.34, the radius of the neutron star equals to the even horizon of a black hole.

  6. Not an astronomer, but I believe the Chandrasekhar limit just tell us a mass region where neutron stars are born:

    “the basic idea is that when the central part of the star fuses its way to iron, it can’t go any farther because at low pressures iron 56 has the highest binding energy per nucleon of any element, so fusion or fission of iron 56 requires an energy input. Thus, the iron core just accumulates until it gets to about 1.4 solar masses (the “Chandrasekhar mass”), at which point the electron degeneracy pressure that had been supporting it against gravity gives up the ghost and collapses inward.”

    That wouldn’t tell us exactly how massive an accretional neutron star is.

    What Voisey points to is a press release with more meat on the bones:

    “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. […]

    Their result has further implications, outlined in a companion paper, scheduled for publication in the Astrophysical Journal Letters. “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.”

    So basically, if I get this correct, what goes out the window is new and thus exciting physics to be “replaced with” mundane quark physics as we already know it. (Of neutron star cores, the outer “neutron drip” layer of my link above is safe AFAIU.) Which is an exciting development on its own, yet another test of the standard model, in its derived physics, I assume.

    I would like to read the Astrophysical Journal Letters companion paper.

  7. I agree with Dr Flimmer about the size of neutron stars.
    I think the author has made a confusion between white dwarf size (typically Earth-sized) and neutron stars.
    Anything Earth-sized rotating 317 times/second would have its equator moving at 40 times the speed of light.

  8. Considering the system, a white dwarf companion, i suspect the neutron star has accumulated some of the leftovers and that it used to be cataclysmic x-ray nova (when the white dwarf was still a star). It could also have been that there was an unusual amount of fallback on top of the neutron star during the initial supernovae explosion.

  9. The Chandrasekhar limit for a white dwarf is 1.3 or 1.4 solar masses. A star below that mass can maintain a hydrostatic equilibrium by degenerate electron pressure. This pressure is due to the Fermi-Dirac statistics of spin 1/2 particles, or the Pauli exclusion principle which prevents any two such particles to occupy the same state. The electrons of the atoms are pressed into a different state, existing on a Fermi surface for the high pressure state. If the mass is larger than this it is a neutron star. This is a phase change where the electrons are forced to combine with protons into neutrons. Neutrons are also spin 1/2 particles and obey the same statistics as electrons. Due to their different mass the analogue of the Chandrasekhar limit is the Tolman-Oppenheimer-Volkoff limit, which is about 3.1 solar masses. As star with a greater mass becomes a quark star, and for even higher mass it becomes a string star. A string star is basically a black hole.

    What is surprising about this case is that the apparent white dwarf exceeds the Chandrasekhar limit, and rather largely so. However, one must remember that a white dwarf is generally not a star with a single state of matter. It is more realistically an onion layered system, with a neutron star-like state in its core, a degenerate electron state for most of the way out to the surface, and then a crust made up of dense but still more or less ordinary matter, A neutron star may also have a quark-gluon plasma in its center, a neutron gas-liquid phase in what might analogous to the mantle, and then a degenerates electron state of material made of largely iron at its surface or crust, So with the case of the white dwarf there is lots to “model space” to play with. Some other physics is going on which nobody has taken into account.

    If I had to hazard a guess I would say that probably this white dwarf has a fusion process going on within the outer layers of the star. This star may then be a ~ 1.0 solar mass degenerate electron state core with a comparable mass of what would be called the “mantle” made of material which is held up against in hydrostatic equilibrium by fusion. So this star is a new beast of sorts which we have not encountered before. Of course the data will have to flesh this out, or any alternative model.


  10. Lawrence, you may want to reread the article. It is the neutron star that is measured at 2.0 Msun, not the white dwarf.

  11. @ Torbjorn Larsson OM

    Thanks, that clarifies a lot and shows why this finding is indeed significant. I didn’t know that neutron stars had been found only slightly more massive than the Chandrasekhar limit until now.

    The mass a neutron star gets depends on the last burning region (core) of the progenitor star. If that region is quite large and thus much iron is formed (this is not a continuous process, since it happens very rapidly in only a few days at most) the neutron star is more massive than as if the inner core would be small. This depends on the mass of the progenitor, of course.

  12. The mass is measured by the gravitational influence of the pulsar EM signals. I quote from the article here:

    … the star weighed in at a hefty two times the mass of the Sun. Current theories suggested a mass of around one and a half solar masses were possible but this new discovery changes the understanding of the composition of such stars, even to the subatomic level.

    This appears to be referencing the Chandrasekhar limit of white dwarf stars and that the mass measured was of the white dwarf. A neutron star of 2 solar masses would not be at all surprising, but a white dwarf of that mass is.


  13. The only thing I can think of is that the author of this article made a mistake and is confusing white dwarfs and neutron stars. I suspect that the authors of the paper were originally talking about white dwarfs, since they seem to mention the Chandrasekhar limit.

  14. Hmmm. According to Wikipedia, neutron stars mass between 1.44 solar masses and 2.1 solar masses, with anything greater than 2 solar masses possibly becoming a quark star, and anything greater than 3 and a bit becoming a black hole.

    I always just lumped compact stars into 3 groups:

    xx3 = black hole

    I didn’t realize that there were finer subdivisions.

    Regardless, this article (and others in the media) were unclear. IIUC, the news here is that this is a) the heaviest neutron star accurately measured, and b) this star cannot be a quark star, due to the precision of the measurements. Therefore we know that quark stars do not form unless a neutron star is at least 2+ solar masses.

    Also of note is that the Tolman–Oppenheimer–Volkoff limit (the point at which a neutron star collapses into a black hole) isn’t well known. Models predict a value somewhere between 1.5 and 3.0 solar masses, which is an enormous range. The fact the pulsar in this article had its mass measured to an exceptional degree of accuracy the helps narrow that range. Of course it was *suspected* that the limit was closer to 3 solar masses than 1.5, but this observation confirms that the limit has to be greater than 1.93 solar masses.

  15. … I forgot that this isn’t a BBCode forum, and used angle brackets in the middle of that post as greaterthan/lessthan signs. Oops. It wasn’t anything important though. I was just mentioning that I thought that anything between 1.44 solar masses and 3 solar masses was a neutron star.

  16. I guess I was wrong about which body has a 2 solar masses, or 1.94 solar masses. The Shapiro or radar delay should reflect the mass of the intervening gravity field. Since the two bodies oribt in a plane which approximately contains the solar system one can use the gravity field of either to determine the mass of the other.

    I never thought a neutron star of 2 solar masses as that exceptional,


  17. Please stop making every third word a link to your encyclopedia entry. It makes finding links to -real- content very difficult. For example, the word “discovery” should link to the NRAO press release about the discovery of the neutron star, not the space shuttle. Thats how the web works, folks.

    Last I checked, thimbles and airliners aren’t units. For future reference, neither are football fields and library of congresses.

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