Imagine the Sun dying, and Earth being swallowed up by it. Astronomers predict this in 7.6 billion years. It was believed that as the Sun exhausted its fuel and expanded into a ‘red giant’, a scorched Earth would still escape ultimate destruction. But new research says that the Sun’s outer atmosphere will cause enough drag on Earth to force it to drift inwards — to be eventually captured and vaporised.
The longevity of a star depends on its mass. The bigger a star, the quicker it dies, as more mass means more heat, and the hotter it gets, the faster it uses up its fuel. Temperatures of millions of degrees and high densities deep inside it force four hydrogen atoms to fuse into one helium atom. In the process, 0.7 per cent of the mass is converted into energy. (Einstein famously summed this up in his equation that energy is equal to mass times the speed of light squared). In the Sun, for instance, 600 million tonnes of hydrogen are converted to helium every second.
In four billion years, the Sun will run out of hydrogen and its core will collapse under its own gravity. Its atmosphere will start expanding, turning it into a red giant that engulfs closer planets like Mercury, Venus, and Earth. The core ignites its helium atoms, which fuse to form carbon. Once the helium runs out, the centre collapses again, puffing out the atmosphere still more. Without enough mass to re-ignite its core for a third time, the Sun goes on expanding and ‘shedding’ its atmosphere, till the dying core forms a white dwarf: a hard, Earth-sized diamond made of carbon and oxygen. Afterwards, the Sun becomes dimmer and dimmer, until its light is finally snuffed out.
Stars larger than the Sun explode as supernovae, with flashes as bright as whole galaxies, leaving behind spinning neutron stars. Those 20 times bigger than the Sun collapse past the neutron star stage, their implosion bending the fabric of space and time to form black holes. But life on Earth may disappear long before 7.6 billion years. Earthquakes, asteroid strikes, epidemics, climate catastrophes, or man-made disasters — anything could switch off Earth’s lights in the next few hundred million years. By then humans will either be extinct, or evolved and spread across the galaxy.