Slide2What does death mean, for the sun? It means our sun will run out of fuel in its interior. It’ll cease the internal thermonuclear reactions that enable stars to shine. It’ll swell into a red giant, whose outer layers will engulf Mercury and Venus and likely reach the Earth. Life on Earth will end. If the sun were more massive estimates vary, but at least several times more massive it would explode as a supernova. So no supernova. But what? What happens next? An international team of astronomers recently used a new stellar data-model that predicts the life cycle of stars to answer this question. Their research is published in the peer-reviewed journal Nature Astronomy. It suggests that the sun is almost exactly the lowest mass star that at the end of its life produces a visible, though faint, planetary nebula. WHAT WILL HAPPEN OUR SUN DIES?
Slide3Their research is published in the peer-reviewed journal Nature Astronomy. It suggests that the sun is almost exactly the lowest mass star that at the end of its life produces a visible, though faint, planetary nebula. WHAT WILL HAPPEN OUR SUN DIES?
Slide5Stars are born, they live, and they die. The sun is no different, and when it goes, the Earth goes with it. But our planet won't go quietly into the night. Rather, when the sun expands into a red giant during the throes of death, it will vaporize the Earth. WHAT WILL HAPPEN OUR SUN DIES?
Slide6Perhaps not the story you were hoping for, but there's no need to start buying star- death insurance yet. The time scale is long 7 billion or 8 billion years from now, at least. Humans have been around only about 40-thousandth that amount of time; if the age of the Earth were compressed into a 24-hour day, humans would occupy only the last second, at most. If contemplating stellar lifetimes does nothing else, it should underscore the existential insignificance of our lives. WHAT WILL HAPPEN OUR SUN DIES?
Slide7So what happens when the sun goes out? The answer has to do with how the sun shines. Stars begin their lives as big agglomerations of gas, mostly hydrogen with a dash of helium and other elements. Gas has mass, so if you put a lot of it in one place, it collapses in on itself under its own weight. That creates pressure on the interior of the proto-star, which heats up the gas until it gets so hot that the electrons get stripped off the atoms and the gas becomes charged, or ionized (a state called a plasma). The hydrogen atoms, each containing a single proton, fuse with other hydrogen atoms to become helium, which has two protons and two neutrons. The fusion releases energy in the form of light and heat, which creates outward pressure, and stops the gas from collapsing any further. A star is born (with apologies to Barbra Streisand). WHAT WILL HAPPEN OUR SUN DIES?
Slide8There's enough hydrogen to keep this process going for billions of years. But eventually, almost all of the hydrogen in the sun's core will have fused into helium. At that point, the sun won't be able to generate as much energy, and will start to collapse under its own weight. That weight can't generate enough pressure to fuse the helium as it did with the hydrogen at the beginning of the star's life. But what hydrogen is left on the core's surface wil fuse, generating a little additional energy and allowing the sun to keep shining. WHAT WILL HAPPEN OUR SUN DIES?
Slide10That helium core, though, will start to collapse in on itself. When it does, it releases energy, though not through fusion. Instead it just heats up because of increased pressure (compressing any gas increases its temperature). That release of energy results in more light and heat, making the sun even brighter. On a darker note, however, the energy also causes the sun to bloat into a red giant. Red giants are red because their surface temperatures are lower than stars like the sun. Even so, they are much bigger than their hotter counterparts. WHAT WILL HAPPEN OUR SUN DIES?
Slide11A 2008 study by astronomers Klaus-Peter Schröder and Robert Connon Smith estimated that the sun will get so large that its outermost surface layers will reach about 108 million miles (about 170 million kilometers) out, absorbing the planets Mercury, Venus and Earth. The whole process of turning into a red giant will take about 5 million years, a relative blip in the sun's lifetime. WHAT WILL HAPPEN OUR SUN DIES?
Slide12On the bright side, the sun's luminosity is increasing by a factor of about 10 percent every billion years. The habitable zone, where liquid water can exist on a planet's surface, right now is between about 0.95 and 1.37 times the radius of the Earth's orbit (otherwise known as astronomical units, or AU). That zone will continue to move outward. By the time the sun gets ready to become a red giant, Mars will have been inside the zone for quite some time. Meanwhile, Earth will be baking and turning into a steam bath of a planet, with its oceans evaporating and breaking down into hydrogen and oxygen. WHAT WILL HAPPEN OUR SUN DIES?
Slide13As the water gets broken down, the hydrogen will escape to space and the oxygen will react with surface rocks. Nitrogen and carbon dioxide will probably become the major components of the atmosphere rather like Venus is today, though it's far from clear whether the Earth's atmosphere will ever get so thick. Some of that answer depends on how much volcanism is still going on and how fast plate tectonics winds down. Our descendants will, one hopes, have opted to go to Mars by then or even farther out in the solar system. WHAT WILL HAPPEN OUR SUN DIES?
Slide14But even Mars won't last as a habitable planet. Once the sun becomes a giant, the habitable zone will move out to between 49 and 70 astronomical units. Neptune in its current orbit would probably become too hot for life; the place to live would be Pluto and the other dwarf planets, comets and ice-rich asteroids in the Kuiper Belt. WHAT WILL HAPPEN OUR SUN DIES?
Slide15One effect Schröder and Smith note is that stars like the sun lose mass over time, primarily via the solar wind. Planets' orbits around the sun will slowly expand. It won't happen fast enough to save the Earth, but if Neptune edges far enough out it could become a home for humans, with some terraforming. WHAT WILL HAPPEN OUR SUN DIES?
Slide17Eventually, though, the hydrogen in the sun's outer core will get depleted, and the sun will start to collapse once again, triggering another cycle of fusion. For about 2 billion years the sun will fuse helium into carbon and some oxygen, but there's less energy in those reactions. Once the last bits of helium turn into heavier elements, there's no more radiant energy to keep the sun puffed up against it's own weight. The core will shrink into a white dwarf. The distended sun's outer layers are only weakly bound to the core because they are so far away from it, so when the core collapses it will leave the outer layers of its atmosphere behind. The result is a planetary nebula. WHAT WILL HAPPEN OUR SUN DIES?
Slide18Since white dwarfs are heated by compression rather than fusion, initially they are quite hot surface temperatures can reach 50,000 degrees Fahrenheit (nearly 28,000 degrees Celsius) and they illuminate the slowly expanding gas in the nebula. So any alien astronomers billions of years in the future might see something like the Ring Nebula in Lyra where the sun once shone. WHAT WILL HAPPEN OUR SUN DIES?
Slide20WHAT HAPPENS WHEN ALL THE STARS DIE? When did the universe start, how come it has all the elements it has and what happens when it’s all over? The force of gravity drives the evolution of stars from birth to death. During this process, many of the elements we know in the periodic table are built out of hydrogen and helium. These elements are returned to interstellar space either gently through stellar winds and planetary nebulae or, more dramatically through the huge explosions we call supernovae.
Slide21THREE MINUTES TO MAKE A UNIVERSE There was a short time, between one and three minutes after the creation of the universe, when the temperature was similar to the interior of a star and hydrogen was converted into helium. After three minutes the rapidly expanding universe was too cold and the reactions stopped, leaving 98 atoms of helium for every 1000 of hydrogen and a very small amount of lithium. Perhaps a thousand million years later the first, now cool, clouds of gas collapsed to form the first generation of stars. Some of these stars would have become supernovae within a few million years and contaminated the pristine gas clouds with the first smattering of heavy elements.
Slide22MIDDLE-AGED UNIVERSE Thereafter the situation was very much as we find it today, with the clouds of gas and dust collapsing to form stars with a range of masses. The heavy stars and some of the double stars become supernovae, while the lighter stars become planetary nebulae. Each steadily increases the proportion of heavy elements in the clouds, out of which new stars will be born. This process had been running for about 8000 million years before our Sun and solar system were born 4560 million years ago. If our Sun had been born much earlier then there might not have been enough heavy elements around to form our planet and ourselves.
Slide23COSMIC DUMP Not everything is recycled, what isn’t ends up in the 'cosmic ash heap'. Into this go the neutron stars and black holes from the heart of Type II supernovae. Also included are the brown dwarfs, stars so small that they are destined to glow faintly and then slowly fade away. The white dwarfs, which first appear in the centres of planetary nebulae, at temperatures of 100,000K will also slowly fade away. Some planets are consigned to the cosmic ash heap but those too close to their parent stars will be returned to interstellar space when their stars become red giants.
Slide24WHAT THE UNIVERSE IS MADE OF Although the process of star birth and death has been running for almost the age of the universe, about 13,000 million years, only 5% of all stars ever born have evolved past the stage of converting hydrogen into helium. The process of element production is also quite slow. Today, for every million atoms of hydrogen and 98,000 of helium, there are 850 of oxygen, 400 of carbon, 120 of neon, 100 of nitrogen, 47 of iron and 2 of sodium and fewer than 100 of all the other elements put together.
Slide25GRAVITY WINS, EVERYTHING IS OVER Eventually the cycle of star birth and death will come to an end. Gravity will have won, a victory delayed by the ability of stars to call on the resources of nuclear fusion. But ultimately, gravity will reduce all stars to a super-dense state as black holes, neutron stars or cold white dwarfs. We already see galaxies in which the stars all appear quite old and there no longer seems to be any interstellar gas out of which to build new stars. Long before our galaxy gets to this stage it will collide with our nearest neighbour, the Andromeda galaxy. In this collision, forecast to occur in 6000 million years time, the two galaxies will combine bringing fresh dust and gas and stirring up many new bursts of star formation, like we currently see in Orion.