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Black holes & revelations

Tiny but massive neutron star changes the rules.

Photo courtesy of NASACXM.Weiss

by Millie Jones & Jake Roslin

Looking out into the ether on a cloudless autumn night, away from exasperating urban light pollution, the fraction of the universe visible gives an awe-inspiring sense of permanence. Indeed, the movements of our own solar system aside, our night sky is practically identical to that gazed upon by the first inhabitants of planet Earth that had the capacity to wonder. They must have been even more perplexed as to what exactly all these tiny dots of light are than we are—we, who still only understand a tiny fraction of what there might be to know.


Yet so vast is the universe that despite their longevity, somewhere or other a star reaches the end of its lifespan no less than once every second, on average. Exactly what happens next depends on what the star is made of, and its mass. Astrophysicists thought they had it all sussed out, but it turns out not.


The snappily named J0740+6620, 4,200 light-years from Earth, was recently spotted by Puerto Rican neutron star searchers as part of a routine cataloguing exercise. However, unlike your common or garden white dwarf, this star's mass was huge. Indeed, it's changed our assumptions about the fate of larger stars as a pulsar with this mass—around 2.14 times that of the sun—and with its dimensions—under 20km across—would until now have been thought impossible. This star should have become a black hole.


The snappily named J0740+6620, 4,200 light years from Earth, was recently spotted by Puerto Rican neutron star searchers as part of a routine cataloguing exercise.

We all love the mystique of black holes, especially in 2019, the year in which one was finally imaged. Or, to be more precise, the light bending around it was—black holes by their nature are not very photogenic. While not exactly the cosmic vacuum cleaners of myth, their gravitational pull, a product of their extreme density, is so great even light can't escape. What goes on beyond the event horizon, the kind of outer ring road of your neighbourhood black hole, is still, and may always be, hazy. Does time slow and stop? Is information sucked in somehow “lost”? Or do black holes act as a portal to another, perhaps inverse, universe, where matter dragged in reappears through a “white hole”, to a place where everything is just the opposite of what it is here, even the direction time runs in?


J0740+6620 somehow avoided this black hole fate, and collapsed instead into a dizzily spinning pulsar of improbable heaviness, with more matter than our own solar system doubled, but a surface area you couldn't even fit Scotland into. A teaspoonful of matter from the star would weigh about 90 million tonnes—or to put it another way, you’d need a new teaspoon.


One thing astrophysicists are quite certain about, however, is that the nearby yellow dwarf Sol,  better known as our own Sun, is too small to ever go supernova, to end up as a neutron star, or to become a black hole. Its own fate, in several billion years’ time, is to expand into a red giant. Once there is no more hydrogen to burn, it will grow, albeit temporarily, to a size which will engulf Mercury, Venus and possibly the Earth too. Yet, it's largely academic whether the latter cataclysm will happen since long before that, around 3.5 billion years from now, its expansion will evaporate the last drop of water on our planet—and we better have packed up and left by then! Subsequently, over trillions of years, the sun will cool into a white dwarf and eventually end up a black, burnt-out cosmic cinder. It will never, however, have the experience of the real giants of the universe like J0740+6620, for which more uncertain and potentially rather more exotic fates await.

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