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Neutron stars are the final state of massive stars that have exhausted their energy and exploded as supernovae. Their mass is limited because a fairly massive star will not become a neutron star; it's going to become a black hole. But finding the upper limit of mass, or tipping point, between a star that becomes a black hole and another that becomes a neutron star, is a subject on which astronomers are still working.
A new discovery of astronomers using the National Bank's (NSF) Green Bank Telescope (GBT) has uncovered the most massive neutron star ever created, putting in place reliable data on the point tipping.
Neutron stars are made of ultra-dense material. They are tightly compressed and are the densest objects in the Universe, made up of normal matter. (The black holes are actually denser, but they are not normal.) In fact, they are so dense that a sugar-cube neutron star would weigh about one hundred million tons on Earth. And there are many: astronomers think they are about 100 million in the Milky Way alone.
"These city-sized objects are essentially ginormous atomic nuclei."
Grateful Cromartie, Senior Author, Graduate Student at the University of Virginia, Member of the National Radio Astronomy Observatory in Charlottesville, Virginia.
The researchers at the origin of this discovery are members of the NANOGrav Physics Frontiers Center. The star they discovered is a fast-moving pulsar, the most massive ever measured. It's called J0740 + 6620 and is 2.17 times more massive than our Sun. And all this mass is stuck in a tiny sphere of about 30 km in diameter.
According to our understanding of these types of stars, this neutron star is about as massive and compact as a star can be before it collapses into a black hole. According to LIGO (gravitational wave laser interferometer observatory) and the gravitational waves observed during the fusion of neutron stars, 2.17 times the mass of the Sun could constitute the upper limit.
"Neutron stars are as mysterious as they are fascinating," said Thankful Cromartie, a graduate student at the University of Virginia and Grote Reber's pre-doctoral fellow at the Charlottesville National Radio Astronomy Observatory, in New York. Virginia. "These city-sized objects are essentially ginormous atomic nuclei. They are so massive that their interiors take on strange properties. Finding the maximum mass allowed by physics and nature can teach us a lot about this otherwise unreachable field of astrophysics. "
The discovered neutron star is a binary pair and it is also a pulsar. These two facts allowed the team to measure its mass.
Pulsars emit beams of electromagnetic radiation from their magnetic poles. These radio waves sweep the space when the star is spinning. Some of them can spin very fast, hundreds of times per second. Because of their speed and predictive regularity, they can be used as atomic clocks. Astronomers can use them to measure the masses of objects in space.
The other star of this binary pair is a white dwarf. The two stars are almost one beside the other, as can be seen from the Earth, which has created a kind of natural laboratory for measuring the mass of J0740 + 6620.
They first measure the mass of the white dwarf companion star. The white dwarf and the pulsar revolve around a common center of gravity. When the pulsar emits its radio waves and the white dwarf moves between the Earth and the pulsar, the gravitational attraction of the white dwarf exerts a tiny gravitational force on the radio waves, forcing them to move a little further. This is what is known as the Shapiro delay.
By measuring this delay, they can find the mass of the white dwarf companion. Astronomers can measure the total mass of the binary pair. Subtraction of the mate's mass from the white dwarf therefore gives the mass of the neutron star.
"The focus of this binary star system has created a fantastic cosmic lab," said Scott Ransom, NRAO astronomer and co-author of the newspaper. "Neutron stars have this tipping point where their inner densities become so extreme that the force of gravity exceeds even the neutrons' ability to resist a new collapse. Each "most massive" neutron star we find brings us closer to identifying this tipping point and helps us understand the physics of matter at these breathtaking densities. "
The tipping point, or maximum mass that a neutron star may have, is only one of the unknown neutron stars. Astrophysicists also have questions about the exact nature of matter in these stars. Scientists believe that when a neutron star is compressed, electrons and protons are ground together to form neutrons and neutrinos. Neutrinos travel in space, leaving only neutrons.
Due to the extreme gravity, there are probably structured layers in a neutron star. These layers probably have different compositions and densities. Scientists believe that the most solid material in the universe, what they call "nuclear pulps," lies deep within the crust of neutron stars. But for the moment, in any case, there is really no way to know. All the scientists can do is destroy the neutron stars and see where the evidence leads them.
These results appear in a new article published in the scientific journal Nature Astronomy. The title of the article is "Relativistic Shapiro delay measurements of an extremely massive millisecond pulsar".
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