Free Radicals - Michael Brooks [87]
Chandra arrived in England from India in 1930. He brought with him a startling new scientific insight that had come to him in a flash of inspiration while his boat crossed the Arabian Sea. Sat on a deckchair, it took Chandra little more than ten minutes to perform the calculation that confirmed his suspicion: at the end of their life, the heaviest stars would collapse in on themselves ad infinitum. They would create rips in the very fabric of space and time, rips that we now know as black holes. The discovery earned Chandra a Nobel Prize – but not until 1983. Arthur Eddington may have died in 1944, but his public excoriation of Chandra’s work had a long-lasting effect.
Hemingway describes the Spanish winter sky as ‘hard and sharp with stars’. To our eyes, the stars do appear sharp: pinpricks of heavenly light piercing the dark curtain of the sky. It is difficult to conceive of the stars as the astrophysicist sees them: huge balls of gas, mostly hydrogen, that have been burning for billions of years.
Even more difficult to grasp is the fact that the stars’ colossal energy output is derived ultimately from nothing more exotic than gravity. Once gravity has pulled enough hydrogen molecules together in one place, the ever-increasing proximity of the atoms to one another only increases gravity’s influence. In a sphere of hydrogen hundreds of thousands of kilometres across, the atoms at the centre feel the weight of thousands of trillions of tonnes pressing in on them. The result is that these atoms fuse together, releasing vast amounts of energy.
Nuclear fusion is an astonishing phenomenon: alchemy in its purest form. In the core, or nucleus, of a hydrogen atom sits a proton, a lone positive charge. Force two hydrogen atoms together, and their positively charged protons will repel each other. But overcome this repulsion with enough pressure, and one of the protons will convert itself into a neutron, and it and the proton join to form a hydrogen isotope called deuterium. Combination with another neutron creates tritium, and when two tritium isotopes combine they fuse to form a helium nucleus. Heavier elements are formed in a similar way.
Every fusion event releases energy – lots of energy. After ignition, the ball of burning gas around the star’s core will reach temperatures of millions of degrees. Stars can maintain this energy output for billions of years, but it can’t last for ever. While the star is burning, the energy release creates an outward pressure that counters the crushing effect of gravity and holds up its outer layers. When all the fuel runs out, though, gravity takes control again, and the star collapses. All the atoms in the dying star are pulled towards the centre, and the closer they get, the stronger the pull. When a star has a large mass, something like twice the mass of our Sun, that ever-increasing pull will carry on until the star vanishes to nothing. This was Chandra’s startling discovery: the star disappears from the universe. All that remains is the mysterious structure we know as a black hole.
Black holes have become a staple of science and of science fiction. Though we can’t see them directly – because nothing, not even light, escapes their gravitational pull – NASA’s telescopes have recorded their voracious nature in the X-rays emitted by particles spiralling inwards on their way to oblivion at the singularity, the heart of the black hole. The instrument that has done most to illuminate this process is the Chandra X-ray Observatory, a space-borne telescope named in Chandra’s honour and launched on the Columbia space shuttle in 1999, four years after his