One of the most extreme stars in the Milky Way just got even crazier.
Scientists measured the mass of a neutron star called PSR J0952-0607 and found that it is the most massive neutron star discovered so far, clocking in at 2.35 times the mass of the Sun.
If true, this is very close to the theorized upper mass limit of about 2.3 solar masses by neutron starswhich represents an excellent laboratory to study these ultra-dense stars in what we believe to be on the verge of collapse, in the hope of better understanding the strange quantum state of matter they are made of.
“We know more or less how matter behaves at nuclear densities, like in the nucleus of a uranium atom.” said astrophysicist Alex Filippenko from the University of California, Berkeley.
“A neutron star is like a giant core, but when you have one and a half solar masses of these things, which is about 500,000 Earth masses of cores attached to each other, it’s not entirely clear how they will behave.”
Neutron stars are the collapsed cores of massive stars that were between 8 and 30 times the mass of the Sun, before going supernova and expelling most of their mass into space.
These cores, which tend to be around 1.5 times the mass of the Sun, are among the densest objects in the Universe; the only thing denser is a black hole.
Its mass is packed into a sphere just 20 kilometers (12 miles) wide; at that density, protons and electrons can combine into neutrons. The only thing keeping this ball of neutrons from collapsing into a black hole is the force they would need to occupy the same quantum states, which is described as degeneracy pressure.
In a way, this means that neutron stars behave like massive atomic nuclei. But what happens at this tipping point, where neutrons form exotic structures or mix into a soup of smaller particles, is hard to say.
PSR J0952-0607 was already one of the most interesting neutron stars in the Milky Way. It is what is known as a pulsar, a neutron star that rotates very fast, with jets of radiation emitted from the poles. As the star rotates, these poles pass in front of the observer (us) like a cosmic lighthouse, so that the star appears to pulsate.
These stars can be incredibly fast, their speed of rotation on scales of milliseconds. PSR J0952-0607 is the second fastest pulsar in the Milky Way, spinning at a mind-boggling 707 times per second. (The fastest is only slightly faster, with a rotation speed of 716 times per second.)
It is also what is known as a “black widow” pulsar. The star is in a close orbit with a binary companion, so close that its immense gravitational field pulls material from the companion star. This material forms a spinning accretion disk that feeds the neutron star, a bit like water swirling around a drain. Angular momentum from the accretion disk is transferred to the star, causing it to spin faster.
A team led by astrophysicist Roger Romani of Stanford University wanted to better understand how PSR J0952-0607 fits into the timeline of this process. The binary companion star is small, less than 10 percent of the mass of the Sun. The research team made careful studies of the system and its orbit and used that information to obtain a new precise measurement of the pulsar.
His calculations gave a result of 2.35 times the mass of the Sun, plus or minus 0.17 solar masses. Assuming a standard neutron star starts out with a mass around 1.4 times the mass of the Sun, that means PSR J0952-0607 has absorbed the matter of an entire Sun from its binary companion. This, the team says, is really important information about neutron stars.
“This provides some of the strongest constraints on the property of matter at several times the density observed in atomic nuclei. In fact, this result rules out many popular physics models of dense matter.” roma explained.
“A high maximum mass for neutron stars suggests that it is a mixture of nuclei and their dissolved up and down quarks down to the core. This rules out many proposed states of matterespecially those with an exotic interior composition”.
The binary also shows a mechanism by which isolated pulsars, without binary partners, can have spin speeds of milliseconds. J0952-0607’s companion is almost gone; once it’s devoured completely, the pulsar (if it doesn’t tip over the upper mass limit and collapse further into a black hole) will retain its incredibly fast rotational speed for quite some time.
And it will be alone, like all other isolated millisecond pulsars.
“As the companion star evolves and starts to become a red giant, material spills onto the neutron star and that spins it. As it spins, it now becomes incredibly energized and a wind of particles starts to come out of the star.” neutron star. That wind then hits the donor star and starts shedding material, and over time, the donor star’s mass decreases to that of a planet, and if even more time passes, it disappears entirely.” Philippenko said.
“So this is how lone millisecond pulsars could form. They weren’t alone to begin with, they had to be in a binary pair, but gradually they vaporized their companions, and now they’re lone.”
The research has been published in astrophysical journal letters.