This is taken largely from Future of an expanding universe and Formation and evolution of the Solar System, and in some cases I've added info from other sources or my own comments. Some of this is conjecture on my part.
Timeline
editStelliferous Era
editFrom 106 (1 million) years to 1014 (100 trillion) years after the Big Bang
The universe is currently 13.7×109 (13.7 billion) years old. This time is in the Stelliferous Era. About 155 million years after the Big Bang, the first star formed. Since then, stars have formed by the collapse of small, dense core regions in large, cold molecular clouds of hydrogen gas. At first, this produces a protostar, which is hot and bright because of energy generated by gravitational contraction. After the protostar contracts for a while, its center will become hot enough to fuse hydrogen and its lifetime as a star will properly begin.
Stars whose mass is very low will eventually exhaust all their fusible hydrogen and then become helium white dwarfs. Stars of low to medium mass will expel some of their mass as a planetary nebula and eventually become a white dwarf; more massive stars will explode in a core-collapse supernova, leaving behind neutron stars or black holes.
Astronomers estimate that the Solar System as we know it today will not change drastically until the Sun has fused all the hydrogen fuel in its core into helium, beginning its evolution off of the main sequence of the Hertzsprung-Russell diagram and into its red giant phase. Even so, the Solar System will continue to evolve until then. Phobos's orbit will eventually take it within Mars' Roche limit within 30 to 50 million years, tearing it apart and possibly forming a ring system. Such a fate also awaits Triton of Neptune in 3.6 billion years. Hopefully we would be able to use material from these moons to build space colonies before this happens.
The Sun and planetary environments
editIn the long term, the greatest changes in the Solar System will come from changes in the Sun itself as it ages. In one billion years' time, as the Sun's radiation output increases, its circumstellar habitable zone will move outwards, making the Earth's surface hot enough that liquid water can no longer exist there naturally. At this point, all life on land would become extinct. Evaporation of water, a potent greenhouse gas, from the oceans' surface could accelerate temperature increase, potentially ending all life on Earth even sooner. During this time it is possible that as Mars's surface temperature gradually rises, carbon dioxide and water currently frozen under the surface soil will be liberated into the atmosphere, creating a greenhouse effect which will heat up the planet until it achieves conditions parallel to those on Earth today, providing a potential future abode for life. By 3.5 billion years from now, Earth's surface conditions will be similar to those of Venus today.
Around 5.4 billion years from now, all of the hydrogen in the core of the Sun will have fused into helium. The core will no longer be supported against gravitational collapse and will begin to contract, heating a shell around the core until hydrogen begins to fuse within it. This will cause the outer layers of the star to expand greatly, and the star will enter a phase of its life in which it is called a red giant. Within 7.5 billion years, the Sun will have expanded to a radius of 1.2 AU—256 times its current size. As a result of the vastly increased surface area, the Sun's surface will be much cooler (about 2600 K) than now and its luminosity much higher—up to 2,700 current solar luminosities. During these times, it is possible that Saturn's moon Titan could achieve surface temperatures necessary to support life.
As the Sun expands, it will swallow the planets Mercury and, most likely, Venus. Gradually, the hydrogen burning in the shell around the solar core will increase the mass of the core until it reaches about 45% of the present solar mass. At this point the density and temperature will become so high that the fusion of helium into carbon will begin, leading to a helium flash; the Sun will shrink from around 250 to 11 times its present (main sequence) radius. Consequently, its luminosity will decrease from around 3,000 to 54 times its current level, and its surface temperature will increase to about 4770 K. The helium-fusing stage will last only 100 million years. Eventually, it will have to again resort to the reserves of hydrogen and helium in its outer layers and will expand a second time. Here the luminosity of the Sun will increase again, reaching about 2,090 present luminosities, and it will cool to about 3500 K. If the Earth survived the sun's first giant phase it probably won't survive this one, and Mars may be swallowed up as well. The gas giants will most likely have their atmospheres driven off; they will become dark, frigid hulks, completely devoid of any form of life. They will continue to orbit their star, their speed slowed due to their increased distance from the Sun and the Sun's reduced gravity. Hopefully we'll have been able to mine their atmospheres for He3 for thermonuclear energy for spacecraft propulsion. This phase lasts about 30 million years, after which, over the course of a further 100,000 years, the Sun's remaining outer layers will fall away, ejecting a vast stream of matter into space and forming a halo known (misleadingly) as a planetary nebula. Colonization of Trans-Neptunian Objects may enable us to ride out the sun's red giant and planetary nebula phases, and after the sun becomes a white dwarf we may be able to use what's left of the giant planets to create Dyson sphere. Initially, this white dwarf may be 100 times as luminous as the Sun is now. It will consist entirely of degenerate carbon and oxygen, but will never reach temperatures hot enough to fuse these elements. Thus the white dwarf Sun will gradually cool, growing dimmer and dimmer.
Two billion years later, when the Sun has cooled to the 6000–8000K range, the carbon and oxygen in the Sun's core will freeze, with over 90% of its remaining mass assuming a crystalline structure. Eventually, after billions more years, the Sun will finally cease to shine altogether, becoming a black dwarf. By this time we will have needed to travel to the red dwarf stars for energy. Actually, we'll need to travel to the red dwarf stars before this, in case an upcoming collision between the Milky Way and Andromeda galaxies throws the solar system into intergalactic space.
Galactic collision and planetary disruption
editMilky Way Galaxy and the Andromeda Galaxy merge into one
edit3 billion years from now (17 billion years after the Big Bang) Main article: Andromeda-Milky Way collision
One thing that will influence whether these stellar collisions take place will be the possibility of galaxies colliding. Although the vast majority of galaxies in the Universe are moving away from the our galaxy, the Local Group of galaxies is being drawn together by gravitational attraction, and it is believed the Milky Way and Andromeda may collide in some three billion years from now.
There's very little likelihood of direct collisions between stars in this kind of encounter. What's more likely is that some stars and their planetary systems may be thrown out of the galaxy as a result of the changed gravitational interaction. Also, gas clouds in the galaxies will collide, and if there is still enough hydrogen there may be a short period of intensive star formation called a starburst. Eventually, in roughly 7 billion years, the Milky Way and Andromeda will complete their merger into a giant elliptical galaxy. Stars that haven't been thrown out will be more likely to collide over the eons as a result, because their orbits have been changed (Eventually, most of these will be thrown out as well by a process known as dynamical relaxation, but they will have exhausted their fuels and become black dwarfs long before this happens.). Because it is not known precisely how fast the Andromeda Galaxy is moving transverse to us, it is not certain that the collision will happen.
Coalescence of Local Group
edit1011 (100 billion) to 1012 (1 trillion) years
The galaxies in the Local Group, the cluster of galaxies which includes the Milky Way and the Andromeda Galaxy, are gravitationally bound to each other. It is expected that between 1011 (100 billion) and 1012 (1 trillion) years from now, their orbits will decay and the entire Local Group will merge into one large galaxy. In addition, the local group is being drawn toward the Virgo Cluster, while the entire Virgo Supercluster is being drawn to the Great Attractor. Assuming that dark energy continues to make the universe expand at an accelerating rate, 2×1012 (2 trillion) years from now, all galaxies outside the Local Supercluster will be red-shifted to such an extent that even gamma rays they emit will have wavelengths longer than the size of the observable universe of the time. Therefore, these galaxies will no longer be detectable in any way. However, this author personally wonders if the attraction among galaxies in the local group might not be influencing readings of the more distant galaxies.
Stellar mergers
edit| “ | The most efficient use of stellar material would be for stars to burn hydrogen as red dwarfs, then merge them to form helium stars. | ” |
Red dwarfs are the most numerous and longest-living stars in the universe, with a lifetime of the order of 1013 (10 trillion) years for those with a mass of about 0.08 solar masses, and because they are fully convective they burn most of their hydrogen unlike their more massive cousins, which begin to die after the core hydrogen is used up. However, they do not go through helium or carbon burning stages. Once they exhaust their hydrogen fuel, nuclear fusion will cease and they become helium white dwarfs. One way to extend the useful lifetime of these stars would be if they merged into larger stars, just as a red dwarf star may be produced if brown dwarfs with mass less than 0.08 solar masses merge. RAMBOs may be a source of such mergers. Brown dwarfs may also be a source of lithium.
Red dwarfs could in theory be merged so that their mass is high enough to trigger the triple-alpha process. The resulting stars may have a lifetime of a few hundred million years. If we can survive the red giant and planetary nebula stages of these stars we should be able to construct a Dyson-type habitat around them in their white dwarf stage.[1] See also Astroengineering, stellar engineering
The galactic voids
editThe galactic voids may contain a few galaxies, and with less possiblity of galactic collisons they may remain spiral galaxies longer, and continue manufacturing stars after the more densely populated regions of the universe have exhausted their supply of hydrogen, at a slower pace. Overall star production will have tapered off, and it is estimated that star formation will cease in 100 trillion years.