The Primordial Era ends when the universe is about a million (10⁶) years old and generates its first stars. We then enter into the current era, called the Stelliferous Era, which extends from cosmological decade six to about fourteen (10⁶-10¹⁴), ending when the universe is about 100 trillion years old.

	The Cosmological Decade
	As we move into vast expanses of future time, I'm going to introduce a logarithmic time unit, called the cosmological decade. So if I write the time in years as 10 to some power (e.g. 105), which is 100,000 years, the exponent is called the cosmological decade. Keep in mind that the universe is now 10 billion (1010) years old, so we're now living in the tenth cosmological decade. When the universe is 10 times older than it is now, the universe will be 1011, and we'll be in the eleventh cosmological decade. We'll talk about the history of the universe up to a hundred cosmological decades. Remember that this kind of "decade" does not last a set amount of time; the tenth cosmological decade takes 10 times as long as the ninth cosmological decade took, and the eleventh cosmological decade will take 10 times longer than the tenth, or 100 times longer than the ninth.

Right now, stars are the most important objects in the universe, in a sense, because the stars are the source of most of the energy that is generated in our universe today. The sun is a star, and the number of stars in the universe is about the same as the number of sand grains in a big sand dune, about 10²³ if you want to put a number on it. Most energy in the Stelliferous Era is generated through the process of nuclear fusion--the fusion of hydrogen into helium--which releases energy. This process powers the sun now and will continue to do so for about 7 billion years. At that time, the sun will turn into a red giant, with its outer surface expanding from its current small position to about the radius of the Earth's orbit. Now, you don't have to worry about that event 7 billion years from now, because long before, in about 3.5 billion years, life on Earth will already be gone. As the sun gets older, it gets brighter, heating the Earth, creating a catastrophic, runaway greenhouse effect that will make global warming seem like a walk in the park. It will boil the oceans and completely scald the entire biosphere.

You might have heard that our sun is an ordinary star. Well, you've been lied to. If you look at the 50 nearest stars, the sun is actually the fourth largest. The typical star has a mass about a quarter of that of our sun. If you look at the population of stars in the galaxy as a whole, most stars are actually smaller-mass stars. These red dwarfs live much longer than our sun, typically trillions of years. The smallest star that can burn hydrogen is about 8 percent of the sun's mass and about a thousand times dimmer. When it dies, a star like the sun becomes a red giant, growing about a hundred thousand times brighter than its current luminosity. But these little stars never become red giants; they just stay at about the same small size, then turn around and become white dwarfs when they die.

Fred Adams and Greg Laughlin

In this Hertzsprung-Russell diagram, the power output of the stars increases toward the top of the diagram, and the surface temperature of the stars increases toward the left of the diagram. The 50 nearest known stars are shown here, with the physical sizes of the stars indicated by the size of their spheres. Notice that most of the sun's nearest neighbors are small stars (called red dwarfs) and hence the sun is rather bright compared to most of its neighbors.

Let's take inventory of the stellar population of the universe. About half of the stellar bodies are brown dwarfs, which are failed stars. They are objects with a mass of less than about 8 percent of the sun and are too small to sustain hydrogen burning. Brown dwarfs sit around for trillions of years and do essentially nothing. That's very important in the future because all of the accessible unburned hydrogen in the universe will be wrapped up in these brown dwarfs. Half the stars that exist really are stars in the sense that they burn their hydrogen into helium. The vast majority of these are red dwarfs, stars much smaller than the sun. There are a small number of sun-like stars, and an even smaller number of massive stars that burn themselves out more rapidly.

We can determine how long stars continue to burn to burn their hydrogen into helium. Most of the stars that are hydrogen-burning stars (every star from about 8 percent of the solar mass all the way up to eight solar masses, which is 997 out of 1000 stars) become white dwarfs when they die. Our sun will do this after its red giant phase. The sun will shed about half of its mass, and the core at its middle will shrink to about the size of our Earth. This future sun will have a density about a million times denser than the current sun, and it will be a degenerately supported object called the white dwarf. A red dwarf star also becomes a white dwarf but preserves most of its mass. Becoming a white dwarf is the fate of the vast majority of all hydrogen-burning stars.

About three out of a thousand true stars have a more dramatic end in store for them. At the end of their lifetimes, they blow up in a supernova explosion. When a super nova explodes, two possible things are left behind--a neutron star and a black hole. A neutron star is what you get when you take something the mass of the sun and compress it down to about the size of Ann Arbor, about 10 kilometers in radius. It's almost one big atomic nucleus, and that object is supported by the degeneracy pressure of its neutrons. If you then take that object and compress it another three or four times smaller in terms of radius, it will become a black hole.

When you take all of the relevant processes into account, the longest that a galaxy like our Milky Way can sustain star formation is about 10 trillion years, close to the lifetime of the longest-lived star. This tells you that the universe will undergo a fairly sharp transition between a universe with stars and a universe without stars. During the thirteenth cosmological decade (10¹³ years), when the universe is 10 trillion years old, the stars will still be shining brightly. Because the stars get brighter as they get older, the galaxy won't be much dimmer than it is today even though most of the stars will be small stars. But when the universe is 10 times older, in the fourteenth cosmological decade (10¹⁴ years), all of the stars will have burned out, or exhausted their hydrogen. The galaxy will have run out of gas to make new stars, so the process of star formation will also shut down.