The Smallest, Lightest Neutron Star Ever Seen Might be a “Unusual Star”


The life of each star is a battle in opposition to gravity. Stars are so huge they danger collapsing beneath their very own weight, however that is balanced by the warmth and stress a star generates by means of nuclear fusion. Ultimately, that involves an finish. The outer layers of a star will likely be solid off, and the remaining core will change into a stellar remnant. Which sort of remnant is dependent upon the mass of the core.

If the core is lower than 1.4 photo voltaic lots, then it can collapse till the stress of electrons balances its weight, thus changing into a white dwarf. If the core is extra huge than that, as much as maybe 3 photo voltaic lots, it collapses till neutron stress resists, making a neutron star. Past that, the core will collapse right into a stellar-mass black gap.

At the very least that appears to be the case. Essentially the most huge white dwarf we’ve discovered is about 1.35 photo voltaic lots, and the smallest black gap we’ve noticed is about 2.6 photo voltaic lots. Essentially the most huge confirmed neutron star is about 2.14 photo voltaic lots. After all, the cut-off ranges aren’t absolute. A stellar core may be a bit beneath 1.4 photo voltaic lots however experiences an explosive collapse that pushes it to change into a neutron star. Neutron stars of 1.2 or 1.3 photo voltaic lots could be uncommon, however not unimaginable. However latest observations of the neutron star HESS J1731-347 appear to offer it a mass of 0.77 photo voltaic lots, which shouldn’t be doable.

Neutron stars are notoriously tough to mass. They’re solely about 20 kilometers in diameter and could be seen by the x-rays they offer off. To calculate the mass you usually both want the neutron star to be a companion of a star, so that you could decide mass by orbital dynamics, or it must be a pulsar so that you could use radio observations to get a mass estimate. HESS J1731-347 is neither of those, but it surely does have a remnant nebula surrounding it. That remnant is illuminated by a close-by common star, which we do know the space of. The common star was mapped by the Gaia spacecraft, so we all know its distance very well. From that information, the crew appeared on the mirrored gentle of the nebula to find out the space of HESS J1731-347. It’s about 8,000 light-years away, which is nearer than we thought.

Figuring out the space, the crew then checked out x-ray observations of the neutron star. Given their luminosity, the calculated mass comes out to be 0.6 – 0.9 photo voltaic lots, which is way under the white dwarf mass restrict. If this mass is correct, our understanding of neutron stars is off. One chance is that we don’t perceive how neutron stars type. The authors suggest one other chance, which is that it could possibly be a kind of quark star referred to as a wierd star.

The distinction between a neutron star and a quark star. Credit score: CXC/M. Weiss

The usual view of neutron stars is that they’re principally neutrons. Some theorists argue that throughout the core the boundary of neutrons would possibly break down, making a soup of up and down quarks, thus a quark star. A wierd star could be one the place colliding quarks within the core create unusual quarks, in order that the neutron star has a core of up, down, and unusual quarks. All of that is hypothetical, however since unusual quarks are twenty instances extra huge than up and down quarks, a wierd star would have a a lot higher density than a traditional neutron star. Thus, it might maintain itself collectively even with a mass a lot smaller than the Solar.

The third possibility is, in fact, that the mass result’s incorrect. This is just one outcome, and several other components might make the neutron star seem dimmer than it really is, thus skewing the mass outcome. It’s an fascinating outcome, however not conclusive. Even the authors acknowledge that extra observations are wanted to verify the outcome. It’s certainly a wierd sight, but it surely won’t be a wierd star.

Reference: Doroshenko, Victor, et al. “A surprisingly gentle neutron star inside a supernova remnant.” Nature Astronomy (2022): 1-8.

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