The Sun has been consuming itself in this fashion for the past five billion years, burning hydrogen to produce the radiation pressure needed to counteract the relentless inward pull of its own gravity. Despite this enormous rate of fuel consumption, astronomers believe the Sun has enough hydrogen left in its core to remain a relatively stable main sequence star for another seven billion years.
But as the Sun's core loses mass, its temperature and density must increase to maintain hydrogen equilibrium. When the Sun was an infant it was slightly smaller, cooler, and dimmer than it is today. As its rate of nuclear fusion slowly increases in the eons ahead, it will continue to expand and more energy will radiate from it. In other words, our Sun will get brighter. That’s not an immediate problem for a star. But it's a death sentence for planet Earth.
Over the next several hundred million years, the Sun's increasing temperature will accelerate the evaporation of Earth's oceans, driving up the opacity of our atmosphere and eventually triggering a runaway greenhouse effect. Ultraviolet radiation will break down the water molecules in our atmosphere and the component hydrogen in H2O will escape into space. Earth will begin to resemble Venus.
Within a little more than one billion years, Earth's once teeming oceans will disappear as surface temperatures soar above the boiling point of water, turning a once-verdant world into a lifeless, burned-out cinder. Still, our slowly brightening Sun will remain on the main sequence for some six billion years beyond Earth's hellish demise, before finally exhausting the hydrogen in its core. At that point, the core's proton-proton chain of fusion reactions will stop, the radiation pressure that supports the star's overlying layers will suddenly drop, and the core will collapse due to the force of gravity. Hydrogen in a shell around the core will continue burning, but the core's collapse will heat this shell and cause it to expand. As the radius of the Sun grows, its surface temperature will consequently drop off. But the Sun's luminosity, the rate at which energy radiates from a star, will increase dramatically and the Sun will become a bloated red giant.
The effect as viewed from an orbiting planet - could any one survive to see it - would be awesome. Reddish and bloated, it will appear 50 degrees across in the Earth's sky, quadruple the angular size of Orion and (if we ignore the inevitable slowing of terrestrial rotation) will take over three hours to rise and set. The Sun's core - now only planet size - has a temperature of nearly 90 million degrees Fahrenheit. It resists further compression not by fusion but by a quantum mechanical effect known as degenerate gas pressure, in which electrons with identical properties cannot be crammed closer together. Eventually, gravitational contraction pushes temperatures high enough to trigger the fusion of helium nuclei into carbon.
Because the core is degenerate gas, this temperature increase does not immediately cause the core to expand. Instead, it causes the rate at which helium is converted into carbon to accelerate in a burst called a helium flash. Eventually, core temperatures reach more than 600 million degrees, the electrons become "non-degenerate" and the core expands and cools off a bit as helium transforms into carbon. Hydrogen burning continues in a shell around the helium-burning core.
But the helium burns relatively quickly. Once it's gone, fusion stops and the electrons in the Sun's carbon core becomes degenerate. Once again, the star expands, this time to truly gargantuan proportions. Just how big is open to question, but recent calculations indicate its outer layers may reach or even exceed Earth's present orbit. If it overtakes our now-molten planet, the Earth will actually orbit inside the Sun's low-density envelope. Friction with the gases will cause it to lose orbital energy and spiral inward, eventually to be utterly destroyed along with Venus and Mercury. Mars likely will be spared, and the heat may be enough to render conditions springlike on the outer planets.
During this second period of expansion, nuclear energy will be supplied by shells of hydrogen and helium, which will shut down and re-ignite in a self reinforcing feedback loop. As the end approaches explosive helium flashes will come closer and cloer together and the Sun's luminosity will rise and fall up to 50 percent over periods as short as a few decades. These explosions will spawn a super wind that will blow away the Sun's outermost layers, creating a spectacular planetary nebula.
In the Sun's interior, nuclear reactions will finally grind to a halt. The naked core that is left behind will have about six-tenths of the Sun's original mass jammed into a hot, ultra-dense sphere about the size of Earth. This is not massive enough to generate the gravitational energy needed to fuse carbon into heavier elements. And so, less than 100,000 years after our Sun's second period of expansion begins, its core will become a white dwarf, a stellar ember that slowly dims as its pent-up heat radiates away over billions of years.
Such is the fate of the vast majority of stars in the universe, those between 0.1 and 8 times as massive as our Sun. Smaller stars never reach the main sequence at all, becoming degenerate before fusion reactions can begin. These are brown dwarfs. Stars between 0.1 and about 0.4 solar masses fuse hydrogen into helium but never get hot enough to fuse helium into carbon; they end up as helium-rich white dwarfs. Stars like our Sun evolve into carbon-rich white dwarfs, while slightly heavier ones develop the heat to fuse carbon into oxygen; they become oxygen-rich white dwarfs.
The very rare stars with masses between 8 and 30 times that of our Sun face a different fate. Because they burn so hot and fast, they can fuse heavier and heavier elements as they struggle to maintain their hydrostatic equilibrium. They end up with compact cores of solid iron, surrounded by shells of lighter elements. Eventually these massive stars undergo a supernova explosion - stay tuned for part two!
Image Description: A white dwarf lurks within the shroud like remains of the planetary nebula NGC 2440. Planetary nebulae have nothing to do with planets but are the clouds that result from the death of a star the size of our Sun. This dwarf is so hot - perhaps 360,000 degrees Fahrenheit - that it illuminates its surrounding nebula.
Image Credit Image credit: NASA/R. Ciardullo (PSU)/H. Bond (STScI)