Neutroninės žvaigždės ir pulsarai

Neutron stars and pulsars

Dense, rapidly rotating remnants that form after certain supernova explosions, emitting beams of radiation

When massive stars reach the end of their lives, core collapse supernova, their nuclei can contract into extremely dense objects called neutron starsThese remnants have densities exceeding those of an atomic nucleus, packing the mass of the Sun into a sphere about the size of a city. Among these neutron stars, some are rapidly rotating and have powerful magnetic fields— pulsars, which emit sweeping beams of radiation that are visible from Earth. In this article, we will discuss how neutron stars and pulsars form, what makes them stand out in space, and how their energetic radiation allows us to study extreme physics at the limits of matter.

1. Formation after a supernova

1.1 Nuclear collapse and "neutronization"

High-mass stars (> 8–10 M⊙) eventually forms iron core, which can no longer support exothermic fusion. When the mass of the nucleus approaches or exceeds Chandrasekhar limit (~1.4 M)⊙), the electron degeneracy pressure no longer outweighs gravity, causing nuclear meltdownIn just a few milliseconds:

  1. The collapsing nucleus compresses the protons and electrons into neutrons (via reverse beta decay).
  2. Neutron degeneracy pressure stops further collapse if the core mass remains below ~2–3 M⊙.
  3. The resulting rebound, or neutrino-driven blast wave, ejects the outer layers of the star into space, causing core collapse supernova [1,2].

The center remains neutron star – an extremely dense object, typically ~10–12 km in radius, with 1–2 solar masses.

1.2 Mass and the equation of state

Accurate neutron star mass limit (the so-called "Tolman-Oppenheimer-Volkoff" threshold) is not precisely determined, usually reaching 2-2.3 M⊙. Beyond this limit, the nucleus continues to collapse into black holeThe structure of a neutron star depends on nuclear physics and ultradense matter equation of state – is an actively researched field that combines astrophysics with nuclear physics [3].

2. Structure and composition

2.1 Layers of a neutron star

Neutron stars have layered structure:

  • Outer crust: Consists of a lattice of nuclei and degenerate electrons, up to the so-called neutron drip density.
  • Inner crust: Matter enriched in neutrons, where "nuclear pasta" phases can exist.
  • Kernel: Mostly neutrons (and possibly exotic particles such as hyperons or quarks) in the supranuclear density.

Densities can exceed 1014 g cm-3 in the nucleus - as much or even greater than that of an atomic nucleus.

2.2 Extremely strong magnetic fields

Many neutron stars have magnetic fields much stronger than typical main sequence stars. As the star collapses, the magnetic flux compresses, increasing the field strength to 108–1015 G. The strongest fields are detected in magnetars, which can cause violent eruptions or "starquakes". Even "normal" neutron stars typically have 109–12 G fields [4,5].

2.3 Fast rotation

The law of conservation of torque accelerates the rotation of a neutron star during collapse.Therefore, many newly born neutron stars rotate with periods of milliseconds or seconds. In the long run magnetic braking force and flows can slow this rotation, but young neutron stars can start out as "millisecond pulsars", or to regenerate in binary systems by taking over mass.

3. Pulsars: Cosmic Beacons

3.1 Pulsar phenomenon

Pulsar – is a rotating neutron star, which magnetic axis and axis of rotation are not coincident. The strong magnetic field and rapid rotation generate radiation beams (radio, visible light, X-rays, or gamma rays) that radiate from the magnetic poles. As the star rotates, these beams sweep across the Earth like a beacon beam, creating pulses with each revolution [6].

3.2 Types of pulsars

  • Radio pulsars: They radiate mainly in the radio range, and are characterized by extremely constant rotation periods ranging from ~1.4 ms to several seconds.
  • X-ray pulsars: Often found in binary systems where a neutron star accretes material from a companion star, generating X-rays or pulses.
  • Millisecond pulsars: Rotating very rapidly (with periods of a few milliseconds), often "wound up" (recycled) by accretion from a binary companion. They are among the most accurate cosmic "clocks" known.

3.3 Pulsar Rotation Slowdown

Pulsars lose rotational energy through electromagnetic spin brakes (dipole radiation, winds) and gradually slow down. Their periods increase over millions of years, until eventually the radiation becomes too weak to detect, reaching the so-called "pulsar death thresholdSome pulsars remain active in a "pulsar wind nebula" phase, continuing to provide energy to the surrounding material.

4. Neutron star binaries and special phenomena

4.1 X-ray binaries

In X-ray binomials A neutron star accretes material from a nearby companion star. The falling material forms an accretion disk, which emits X-rays. Sometimes, intermittent bursts of light (transients) occur if the disk is unstable. By observing these bright X-ray sources, it is possible to determine the masses and rotation frequencies of neutron stars and to study the physics of accretion [7].

4.2 Pulsar and companion systems

Binary pulsars, whose second member is another neutron star or white dwarf, have provided essential general relativity tests, especially measuring orbital decay due to gravitational wave radiation. Binary neutron star system PSR B1913+16 (Hulse–Taylor pulsar) provided the first indirect evidence for the existence of gravitational waves. More recent discoveries, such asDouble pulsar" (PSR J0737−3039), further refines theories of gravity.

4.3 Mergers and gravitational waves

When two neutron stars approach each other in a spiral path, they can cause kilonova and radiate strong gravitational waves. Emerging detection GW170817 In 2017, a binary neutron star system was confirmed to have merged, consistent with a kilonova in multiwavelength observations. These mergers can also create the heaviest elements (such as gold or platinum) in the r-process nucleosynthesis, emphasizing neutron stars as cosmic "forges" [8,9].

5. Impact on galactic environments

5.1 Supernova remnants and pulsar wind nebulae

The birth of a neutron star core collapse supernova leaves supernova remnant – expanding shells of ejected material and a shock front. A rapidly rotating neutron star can create pulsar wind nebula (for example, Crab Nebula), in which relativistic particles from the pulsar provide energy to the surrounding gas, propagating in synchrotron radiation.

5.2 Dispersion of heavier elements

The formation of neutron stars in supernova explosions or neutron star mergers releases new isotopes of heavier elements (e.g., strontium, barium, and even heavier ones). This chemical enrichment enters the interstellar medium, later incorporating into future generations of stars and planetary bodies.

5.3 Energy and feedback

Active pulsars emit strong particle winds and magnetic fields that can inflate cosmic bubbles, accelerate cosmic rays, and ionize local gas. Magnetars with particularly extreme fields can produce giant flares, sometimes disrupting the nearby interstellar medium. Thus, neutron stars continue to shape their environment long after the initial supernova explosion.

6. Observable signs and research directions

6.1 Pulsar searches

Radio telescopes (e.g., Arecibo, Parkes, FAST) have historically scanned the sky for periodic radio pulses from pulsars. Modern telescope arrays and time-domain observations allow the discovery of millisecond pulsars, studying the Galactic population. X-ray and gamma-ray observatories (e.g., Chandra, Fermi) are discovering high-energy pulsars and magnetars.

6.2 NICER and time measurement arrays

Space missions such as NICE The Neutron Star Interior Composition Explorer (NSICOM), located on the International Space Station (ISS), measures X-ray pulsations of neutron stars, placing more precise constraints on their mass and radius, in order to elucidate their internal equation of state. Pulsar timing arrays (PTA) combines stable millisecond pulsars to detect low-frequency gravitational waves emanating from supermassive black hole binary systems on a large cosmic scale.

6.3 The significance of multiwavelength observations

Neutrinos and gravitational waves Detections in future supernovae or neutron star mergers could directly reveal the conditions for neutron star formation. Observations of kilonova events or supernova neutrino streams provide unique data on the properties of nuclear matter at extreme densities, bridging astrophysics with fundamental particle physics.

7. Conclusions and future prospects

Neutron stars and pulsars – are some of the extreme results of stellar evolution: after the collapse of massive stars, compact remnants are formed, with a diameter of only ~10 km, but a mass that often exceeds the mass of the Sun. These remnants have extremely strong magnetic fields and rapid rotation, manifested as pulsars, emitting radiation in a wide range of the electromagnetic spectrum. Their formation in supernova explosions enriches galaxies with new elements and energy, affecting star formation and the structure of the interstellar medium.

From the mergers of two neutron stars that generate gravitational waves to the flares of magnetars that can instantly outshine entire galaxies in the gamma-ray range, neutron stars remain at the forefront of astrophysics research.Advanced telescopes and time-of-flight arrays are increasingly revealing the fine details of pulsar radiation geometry, internal structure, and transient merger events—connecting cosmic extremes with fundamental physics. Through these spectacular remnants, we can see the final chapters of the lives of high-mass stars and how their deaths can trigger striking phenomena and shape the cosmic environment for eons.

Sources and further reading

  1. Baade, W., & Zwicky, F. (1934). “On Supernovae.” Proceedings of the National Academy of Sciences, 20, 254–259.
  2. Oppenheimer, JR, & Volkov, GM (1939). "On Massive Neutron Cores." Physical Review, 55, 374–381.
  3. Shapiro, SL, & Teukolsky, SA (1983). Black Holes, White Dwarfs, and Neutron Stars: The Physics of Compact Objects. Wiley-Interscience.
  4. Duncan, RC, & Thompson, C. (1992). "Formation of very strongly magnetized neutron stars: Implications for gamma-ray bursts." The Astrophysical Journal Letters, 392, L9–L13.
  5. Gold, T. (1968). "Rotating neutron stars as the origin of the pulsating radio sources." Nature, 218, 731–732.
  6. Manchester, R.N. (2004). "Pulsars and their place in astrophysics." Science, 304, 542–545.
  7. Lewin, WHG, van Paradijs, J., & van den Heuvel, EPJ (eds.). (1995). X-ray Binaries. Cambridge University Press.
  8. Abbott, BP, et al. (LIGO Scientific Collaboration and Virgo Collaboration) (2017). "G"W170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral.” Physical Review Letters, 119, 161101.
  9. Drout, M. R., et al. (2017). "Light curves of the neutron star merger GW170817/SSS17a.” Science, 358, 1570–1574.
  10. Demorest, P.B., et al. (2010). "A two-solar-mass neutron star measured using Shapiro delay." Nature, 467, 1081–1083.
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