The Present Universe
The Primordial Era ends when the universe is about 100-200 million years old and the first generation of stars start to form. At this point, we enter the present era, sometimes called the `Stelliferous Era`, which extends from 108-1014 years, i.e. it will extend into the future until finally ending when the universe is about 100 trillion years old.
Stars are assumed to be the source of most of the energy that is generated in our universe today and, as such, our sun is the source of energy that supports virtually all life on planet Earth. Most energy in the Stelliferous Era is generated through the process of nuclear fusion, i.e. a process that converts hydrogen into helium and releases radiated energy as a by-product. This process, which powers our sun, can continue for some 5-10 billion years depending on the size and composition of the star. When a star, like the sun, comes to the end of its life, it becomes a red giant, growing to about a hundred thousand times brighter than its current luminosity. In the case of our own sun, this expansion will extend to a radius that will probably consume the Earth. However, a typical star has a mass about one quarter of our sun, while the smallest star that can burn hydrogen is about 8% of the sun's mass and about a thousand times dimmer. This type of star is called a red dwarfs and, due to its low fusion rate; it will live much longer than a larger star, typically in the order of trillions of years. However, about half of the stellar bodies are not stars, but brown dwarfs, which are essentially stars that are just too small to ignite fusion. As such, brown dwarfs can basically exist for hundreds of trillions of years and therefore may have a key role to play in the life of the universe, long after all the bright fusion stars have died.
Over 99% of stars are hydrogen-burning stars; this includes stars in the range 8-800% of our sun. However, our sun is destined to become a white dwarf after its red giant phase, where it will shed about half of its mass and its core will shrink to about the size of our Earth and have a density about a million times denser than the current sun. A red dwarf star can also become a white dwarf, but unlike its giant cousin, preserves most of its mass during the transition process. Therefore, becoming a white dwarf is the fate of the vast majority of all hydrogen-burning stars.
About 3 in every 1000 stars come to a far more dramatic end, marked by an enormous explosion called a supernova. However, this seemingly destructive process is critical to the evolution of complex structure, e.g. life, which requires heavier elements above iron in the periodic table - see inset right 'Nuclear Synthesis'. After the supernova, two possible objects can result, a neutron star or a black hole. A neutron star results when the mass of our sun is compressed to about 10 kilometres in radius, which has a density analogous to a giant atomic nucleus. However, dependent on the initial mass, the supernova may also collapse into a black hole, which has an event horizon, and is some 3-4 times smaller than a neutron star. A black hole has a value of gravity [g] that exceeds the escape velocity of light, i.e. photons, and so (almost) nothing can escape its gravitational pulls.
Collectively, taking all these processes into account, the longest that a galaxy, like our own Milky Way, can sustain star formation is about 10 trillion years. At which point, the universe will undergo another phase transition from a universe with stars to a universe without stars. Ultimately, by the laws of thermodynamics and entropy, all of the galaxies will exhaust their supply of hydrogen and the process of star formation will shut down forever. By this stage, the universe will be over 100 trillion year old.
As a somewhat tangential side-note, if the stelliferous age extends to 1014 years, then within this timeframe, our universe at 13.7 billion years is only 0.01% into this era. The fact that life has already evolved to sentient intelligence here on earth may therefore be both remarkable and unique at what amounts to a very early stage in the timeline of the universe. Therefore, maybe humanity should consider the possibility that it might be one of the very first lifeforms ever to reach this stage. In the great scheme of things, something has to be first and it is interesting to speculate whether it might be humanity. Given that higher lifeforms require heavier elements that only formed within supernova, it is possible that the first star systems capable of supporting life of any description may have required, at least, 1 billion years to stabilise, which is about 10% of the current age of the universe, but only 0.001% of the projected stelliferous age. If we then consider the number of remarkable conditions that have existed for the last 5 billion years it has taken for planet Earth to evolve sentient intelligence, then the fact that another intelligent life has not been found to-date is possibly not so remarkable, even if we are not the very first.