Red Dwarf Stars: The Embers of Creation

The cosmos holds many different kinds of stars. There are huge giant stars, hundreds of times
larger than our Sun that burn bright and fast - for the most part, alive only a few hundred
million years.
There are stars like our sun, relatively smallish stars peppered throughout the galaxies. They
shine at a more moderate temperature compared to the giants and last for around 10 billion
years. Our Sun has been around about half that long and will die some five billion years
from now.
But the vast majority, around 85 percent of all stars in the sky are cooler, dimmer ones.
They are known as red dwarfs.
As their name implies a red dwarf is small, no bigger than half the size of the sun. They
are also very dim, the brightest known red dwarf has only ten percent of the luminosity
of the Sun. And they generate as little as 1/10000 th the energy.
The smallest among them are barely able to shine at all. These smaller red dwarf stars,
those that are a mere 7.5 percent the size of the Sun, are the smallest objects in the
universe able to shine by the process of nuclear fusion.
But they're small size and dim luminosity has one big advantage: red dwarf stars live
the longest. Relative to all other stars in the sky, they are immortal. Their lifetimes
extend into the trillions of years. They will live roughly a thousand times longer than
the Sun and one hundred thousand times longer than most giants.
Red dwarfs live so long because they burn their hydrogen fuel slower than brighter stars.
They are of such small size, and generate so little energy, they can fuse their hydrogen
more efficiently.
To understand why, we need to look at what's underneath the surface of a star. All stars,
regardless of their size have layers underneath. The details of what and how many layers depend
on it's mass.
The Sun, for example, has a layer known as the radiative zone. It lies between the core
and another region, called the convective zone. When a photon of energy is generated
in the Sun's core, once it hits the radiative zone, it hits a kind of wall, energy can only
pass through this zone by collision with another atom and re-emission.
Giant stars have a much smaller radiative zone and a larger convection zone. Radiation
by fusion is a more efficient process here.
This process of collision and re-emission takes a lot of time. It can take a photon
passing through the radiative zone nearly three million years just to get to the surface.
These photons from the Sun had to go through a lot, as well as millions of years, to reach
our eyes,
Light from the Sun is a lot older than you might think.
Red dwarf stars on the other hand, do not have this radiative zone. Photons emitted
from the core go straight to the convective zone, where the hydrogen gas is more efficently
mixed with the core, allowing more of it to be used. Red dwarfs stars are able to convert
virtually all of it's hydrogen fuel to helium - to a degree other stars with large radiative
zones cannot.
So how will red dwarfs die? Long after all the other stars have perished, red dwarfs
will still be shining. Glowing dimly and feebly, these smoldering embers of creation with be
the main source of radiation in our universe. Dim reminders of a once bright and vibrant
universe.
Long after all other stars are gone, after every ounce of their hydrogen has been spent,
red dwarfs will slowly fade into oblivion, their embers cooling, and for the first time
since the beginning of the universe, the sky will be dark.