Neutron star matter and its equation of state

6 problems

• How do hyperons fit inside two-solar-mass neutron stars?

  Contested

  At high density it becomes energetically favorable to turn some neutrons into hyperons (baryons containing a strange quark), which softens the matter so much that most models cannot then support stars of two solar masses. Yet such stars are observed.

• Maximum mass of a nonrotating neutron star

  Open

  Above a certain mass a neutron star must collapse into a black hole. The heaviest known neutron stars weigh a little over two Suns, but the true limit is not known.

• Pressure of neutron-star matter between two and eight times nuclear density

  Partially resolved

  How hard dense matter pushes back sets how large a neutron star of a given mass is. The goal is to pin down this pressure from star observations combined with theory.

• Do the heaviest neutron stars have quark-matter cores?

  Contested

  At the centre of the heaviest neutron stars, protons and neutrons may dissolve into free quarks. Whether this happens is not known.

• Does the sound speed exceed the conformal bound inside neutron stars?

  Partially resolved

  In matter made of free quarks at very high density, sound travels at the speed of light divided by the square root of 3. Neutron-star data suggest sound travels faster than this somewhere inside, but the evidence depends on modeling assumptions.

• Location and origin of the sound-speed maximum in dense matter

  Open

  If sound in neutron-star matter first speeds up beyond the free-quark value and then slows down again, there must be a peak. Where the peak lies and what causes it, for example a gradual change from nucleons to quarks, is unknown.