Like a dried-up lemon from the again of the fridge, neutron stars are much less squeezable than anticipated, physicists report.

New measurements of essentially the largest recognized neutron star discover that it has a surprisingly giant diameter, suggesting that the matter inside isn’t as squishy as some theories predicted, physicists with the Neutron star Interior Composition Explorer, or NICER, reported April 17 at a digital assembly of the American Physical Society.

When a dying star explodes, it may well go away behind a memento: a remnant filled with neutrons. These neutron stars are terribly dense — like compressing Mount Everest right into a teaspoon, mentioned NICER astrophysicist Zaven Arzoumanian of NASA’s Goddard Space Flight Center in Greenbelt, Md. “We don’t know what happens to matter when it’s crushed to this extreme point.”

The larger the neutron star, the more excessive the situations in its core. Jammed collectively at great densities, particles might type uncommon states of matter. For instance, particles generally known as quarks — often contained inside protons and neutrons — might roam freely in a neutron star’s heart.

The core’s composition determines its squeezability. For instance, if quarks are free brokers inside essentially the largest neutron stars, the immense strain will compress the neutron star’s core more than if quarks stay inside neutrons. Because of that compressibility, for neutron stars, more mass doesn’t essentially translate to a bigger diameter. If neutron star matter is squishy, the objects might counterintuitively shrink as they become more massive (SN: 8/12/20).

To perceive how neutron stars innards reply to being put via the cosmic wringer, scientists used the X-ray telescope NICER aboard the International Space Station to estimate the diameters of quickly spinning neutron stars known as pulsars. In 2020, NICER sized up a pulsar with a mass of about 1.4 instances of the solar’s: It was about 26 kilometers extensive (SN: 1/3/20).

Researchers have now gauged the girth of the heftiest confirmed neutron star, with about 2.1 instances the mass of the solar. But the beefy neutron star’s radius is about identical to its more light-weight compatriots, in response to two impartial groups inside the NICER collaboration. Combining NICER information with measurements from the European Space Agency’s XMM-Newton satellite tv for pc, one workforce discovered a diameter of around 25 kilometers whereas the opposite estimated 27 kilometers, physicists reported in an information convention and in two talks on the assembly.

Many theories predict that the more large neutron star ought to have a radius that’s smaller. “That it is not tells us that, in some sense, the matter inside neutron stars is not as squeezable as many people had predicted,” mentioned astrophysicist Cole Miller of the University of Maryland in College Park, who introduced the second end result.

“This is a bit puzzling,” mentioned astrophysicist Sanjay Reddy of the University of Washington in Seattle, who was not concerned within the analysis. The discovery means that inside a neutron star, quarks usually are not confined inside neutrons, however, they nonetheless work together with each other strongly, relatively than being free to roam about unencumbered, Reddy mentioned.

The measurements reveal one other neutron star enigma. Pulsars emit beams of X-rays from two scorching spots related to the magnetic poles of the pulsar. According to the textbook image, these beams ought to be emitted from reverse sides. But for each of the neutron stars measured by NICER, the recent spots had been in the identical hemisphere.

“It implies that we have a somewhat complex magnetic field,” mentioned NICER astrophysicist Anna Watts of the University of Amsterdam, who introduced the primary workforce’s end result. “Your beautiful cartoon of a pulsar … is for these two stars completely wrong. And that’s brilliant.”

two beams of light stream out from the bottom of a bright orb in the center of the picture
Beams of radiation are emitted from the magnetic poles of spinning neutron stars known as pulsars. Scientists usually envision pulsars with two beams on reverse sides, like a lighthouse. But the beams of a newly measured pulsar (illustrated) come from the identical hemisphere.NASA’s Goddard Space Flight Center