Pulsars
A pulsar is a rotating, strongly magnetised neutron star whose electromagnetic radiation is emitted in narrow beams along its magnetic axis; because this axis is misaligned with the rotation axis, the beams sweep through space like a lighthouse, and a distant observer sees a periodic pulse. Observed periods range from about 1.4 milliseconds to several seconds, and they lengthen very slowly as the star loses rotational energy.
Curious
In 1967 a Cambridge graduate student, Jocelyn Bell, noticed a strange, perfectly regular pulse of radio waves repeating every 1.337 seconds in her telescope's chart recordings. The signal was so regular that the team jokingly labelled it “LGM” (Little Green Men) before ruling out an artificial origin. Within months more pulsars turned up, and the explanation was a rotating neutron star: a collapsed stellar core, about the size of a city but heavier than the Sun, spinning many times per second.
The pulsar in the Crab Nebula, the remnant of the supernova seen in 1054, spins about thirty times per second. Pulsars are born in supernovae, and their stable pulses make them useful tools for testing gravity.
Exploring
| Type | Typical period | Notes |
|---|---|---|
| Normal radio pulsar | 0.1–several s | Field ~1011–1013 G |
| Millisecond pulsar | 1.4–30 ms | Spun up by a companion; very stable |
| Magnetar | 2–12 s | Field ~1014–1015 G (inferred) |
Deep Dive
Spin-down. A rotating magnetic dipole radiates energy at a rate proportional to B²R⁶Ω⁴sin²α/c³, so the rotational energy loss (Ė = −IΩΩ̇) gives B ∝ √(PṖ). With a moment of inertia of about 1038 kg m² the Crab loses roughly 4.5×1031 W, enough to power the surrounding nebula. The two relations above assume a pure vacuum dipole, a simplification; real magnetospheres contain plasma and braking indices differ from 3 (the Crab's measured value is about 2.5).
Tests of gravity. In 1974 Russell Hulse and Joseph Taylor discovered the first binary pulsar, PSR B1913+16. Its orbit shrinks at exactly the rate general relativity predicts for energy lost to gravitational waves, indirect evidence for gravitational radiation that earned the 1993 Nobel Prize in Physics (see Gravitational Waves). Arrays of millisecond pulsars are now used as detectors of nanohertz gravitational waves, and in 2023 pulsar timing collaborations reported evidence for a background of such waves, whose source (probably supermassive black hole binaries) is still being established.
Open questions. The radio emission mechanism is not fully understood. The same is true of the dense-matter equation of state inside neutron stars: the most massive confirmed pulsars, near 2 solar masses, rule out many soft models. A nearby neutron star can also be probed by NICER X-ray timing for radius measurements, with results that remain model dependent.
Concept Relationship Map
Requires: neutron stars, supernovae
Enables: binary-pulsar tests of gravity, pulsar timing arrays
Commonly confused with: pulsating variable stars (which really change brightness)
Sources
- Hewish, A., et al. (1968). Nature, 217, 709–713.
- Hulse, R.A., Taylor, J.H. (1975). “Discovery of a pulsar in a binary system.” The Astrophysical Journal, 195, L51–L53.
- Lorimer, D.R., Kramer, M. (2005). Handbook of Pulsar Astronomy. Cambridge University Press.
- ATNF Pulsar Catalogue (Manchester et al., 2005), Australia Telescope National Facility.