Black-hole is

Question

Black-hole is

UnlockIAS · Accepted Answer

Correct — D, a collapsed star. A star shines because the outward pressure of nuclear fusion in its core balances the inward pull of its own gravity. When the fuel runs out that balance fails and the core collapses. What it collapses into depends on the mass left behind: below about 1.44 times the mass of the Sun — the Chandrasekhar limit — electron degeneracy pressure halts the collapse and a white dwarf survives; above that, the electrons are crushed into the protons and neutron degeneracy pressure holds up a neutron star; and beyond roughly three solar masses no known force can stop the fall at all. The matter contracts past the point where the escape velocity equals the speed of light, and the object becomes a black hole — a region bounded by an event horizon of radius equal to the Schwarzschild radius, from which not even light can escape, which is why it is 'black'. The physicist John Wheeler popularised the name in 1967. The topic was live current affairs when this paper was set: on 10 April 2019, eight months before the exam, the Event Horizon Telescope collaboration released humanity's first image of a black hole — M87*, the supermassive black hole about 6.5 billion times the Sun's mass at the centre of the galaxy Messier 87, some 55 million light-years away.

Why the other options are wrong — (a) That is the black BOX — actually two devices, the flight data recorder and the cockpit voice recorder — and despite the nickname it is painted bright orange so that searchers can find it in wreckage or water. Pure word-association bait: 'black hole' and 'black box' share one word and nothing else.  (b) That is a sunspot: a region of the solar photosphere where an intense magnetic field suppresses convection, leaving it around 3,800 K against the surrounding 5,800 K, so it merely LOOKS dark by contrast. Sunspots wax and wane on the roughly eleven-year solar cycle and are a surface magnetic phenomenon, not a gravitational collapse.  (c) No such place exists. The phrase half-remembered here is the 'Black Hole of Calcutta' — the 1756 incident in the fort at Calcutta, in present-day Kolkata — which is Indian history, not Antarctic geography, and nothing to do with astrophysics either.

Concept: A black hole is the end state of gravitational collapse. Its defining boundary is the event horizon, the surface at which the escape velocity reaches the speed of light; inside it, every path leads inward, so no matter or radiation can leave. The horizon's radius is the Schwarzschild radius, which depends only on mass. Black holes come in classes — stellar-mass ones, of a few to a few tens of solar masses, formed from collapsing massive stars, and supermassive ones of millions to billions of solar masses, which sit at the centres of galaxies including our own. They are not detected directly but by their gravitational effect on nearby stars and gas, by the X-rays from superheated infalling material, and by the gravitational waves emitted when two of them merge.

State PCS 2019 Q103 — Science & Technology PYQ with Answer

State PCS 2019 Q103 — Science & Technology PYQ with Answer