In the universe, what are pulsars?

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Correct — B, Rotating neutron stars. A pulsar is a rapidly spinning, intensely magnetised neutron star, and each part of that description does real work. When a star of roughly eight to twenty-five solar masses exhausts its nuclear fuel, its iron core collapses in a supernova until neutron degeneracy pressure halts the fall; what survives is a neutron star of about 1.4 solar masses compressed into a sphere some 20 km across, at a density of the order of 10^17 kg per cubic metre — a teaspoonful would weigh around a billion tonnes at the Earth's surface. Two conservation laws then produce the pulsing. Angular momentum is conserved as the core shrinks from a stellar radius to ten kilometres, so a rotation that took days speeds up to seconds or even milliseconds; magnetic flux is conserved as well, so the surface field is amplified to something of the order of 10^8 to 10^12 gauss. Because the magnetic axis is generally tilted away from the rotation axis, charged particles accelerated along the magnetic poles radiate in two narrow beams that swing round with the star, and each time a beam crosses the Earth we record a pulse. That is the standard 'lighthouse model', and it explains why the emission is periodic without the star itself changing at all. The pulse train is extraordinarily regular — known periods run from about 1.4 milliseconds for the fastest millisecond pulsars up to several seconds, and the steadiest of them keep time to a precision comparable with an atomic clock. The first was found in November 1967 by Jocelyn Bell Burnell, then a research student at Cambridge, in chart-recorder traces from Antony Hewish's Interplanetary Scintillation Array; the 1.337-second signal was half-jokingly labelled LGM-1, for 'Little Green Men', before its natural origin was established, and it is catalogued today as PSR B1919+21 — PSR standing for Pulsating Source of Radio. A pulsar is therefore an object, a physical star with a surface, and not a light show, a burst, or a beam.

Why the other options are wrong — (a) That describes a star cluster or a constellation, not a pulsar. An open cluster such as the Pleiades holds a few hundred young stars loosely bound together; a globular cluster such as Omega Centauri holds hundreds of thousands of old ones in a dense sphere. There is a real connection worth knowing — globular clusters are the richest hunting grounds for millisecond pulsars, because their crowded cores let a neutron star capture a companion and be spun up by accretion — but the cluster is the group and the pulsar is a single star within it.  (c) That is a nova or a supernova, and the confusion is a reasonable one, because a pulsar is exactly what a core-collapse supernova leaves behind. The Crab Pulsar sits at the centre of the Crab Nebula, the expanding debris of the supernova of AD 1054 that Chinese and Japanese observers recorded as a 'guest star'. But the explosion is a single event that fades over weeks and months, while the pulsar is the compact remnant that keeps spinning for millions of years afterwards.  (d) The most tempting wrong answer, because pulsars were discovered in the radio band and radio telescopes are still the main instruments used to find them. It nevertheless confuses the signal with its source: the pulses are what we observe, and the pulsar is the star producing them. The point is sharpened by the fact that pulsars also shine in X-rays and gamma rays — the Fermi space telescope has catalogued hundreds of gamma-ray pulsars — while a few, such as Geminga, are essentially radio-quiet and were found in gamma rays alone.

Concept: A star's end state is decided by the mass left in its core once fusion stops. Below about 1.4 solar masses — the Chandrasekhar limit, worked out by S. Chandrasekhar in 1930 — electron degeneracy pressure can hold the core up and a white dwarf results. Above that, electrons and protons are crushed together into neutrons and the collapse is arrested by neutron degeneracy pressure, giving a neutron star of some 10 km radius. Above roughly two to three solar masses even that fails and a black hole forms. Neutron stars therefore occupy the middle rung of stellar remnants, and a pulsar is simply a neutron star whose emission beam happens to sweep across the Earth; most neutron stars are not observed as pulsars because their beams point elsewhere. Sub-classes matter for prelims: millisecond pulsars are old neutron stars spun back up by matter accreted from a binary companion, and magnetars are neutron stars with fields up to about 10^15 gauss, the strongest magnets known in nature. Because their spin is so stable, pulsars are used as natural clocks — for testing general relativity, for mapping interstellar plasma, and even as a proposed navigation beacon system for deep-space craft.

State PCS 2023 Q9 — Science & Technology PYQ with Answer

State PCS 2023 Q9 — Science & Technology PYQ with Answer