The sky is the ultimate art gallery just above us.
— Ralph Waldo Emerson
The Art of the Universe
Even after all this time, the Sun never says to the Earth, "You owe Me." Look what happens with a love like that, it lights the whole sky.
– Hafiz
Radius: 695,800 km
Mass: 1.99x10^30 kg
Luminosity: 3.83x10^26 J/s
Surface temperature: 5778K (5504.85ËšC/9940.73ËšF)
The Sun is the center of the Solar System. It holds around 99.8% of the Solar System's mass and provides light and warmth to all the bodies orbiting it. Light from the Sun takes 8 minutes to reach Earth. Most of what we know about the Sun is based off of mathematical calculations which are themselves based off our observations about how the Sun acts, as well as observations of other stars. We use mathematical calculations because we can't actually visit it or send probes there without being fried to a crisp. The information that follows is only some of what we have discovered based on those calculations.
Nuclear Fusion
Nuclear fusion is fuled by the proton-proton cycle. This is the process by which the Sun shines and produces energy. Nuclear fusion occurs in the Sun's core, where it is densest and hottest. There are three steps to the proton-proton cycle.
Step 1
Two protons rush towards each other repeatedly, nearly colliding each time but being unable to due to a force
called the electric force. Each time they rush toward each other they are repelled with greater and greater force
until eventually they gain enough energy to tunnel past the electric force and fuse to make a heavy hydrogen
nucleus with one proton and one neutron. A positron (positive electron) and a neutrino are also produced.
Step 2
Heavy hydrogen and a proton go through the same process, nearly colliding but not quite until they gain energy
to tunnel past the electric force. When they tunnel past the barrier, they fuse to form light helium, with two
protons and one neutron. A gamma radiation particle is also produced by this step.
Step 3
Two light heliums repeat the process once more, eventually fusing to make normal helium, with two protons and
two neutrons, and two extra protons. The two extra protons then go off elsewhere in the core and begin the
process over.
Life Cycle
The Sun most likely began as giant molecular gas cloud. Eventually something disturbed the cloud and it collapsed on itself,
creating pockets of dense gas, each of which became its own star. As the pocket that created our Sun collapsed more and
more, it got hotter and hotter and began to spin faster and faster. Soon the core of the pocket had enough heat to produce its
own light. However, the falling gas still reached the center, and it became a proto-star. The spinning gas became a plasma,
and a magnetic field was created that moved away from the core, creating a bi-polar jet (so called because there were two:
one at either pole). Anything that didn't fall towards the core or the accretion disk was flung away along these jets. When the
center of the gas cloud reached 1,000,000K (999726.85ËšC/1799540.33ËšF) nuclear fusion began, and any gas and dust
trying to fall towards the center was swept away by radiation and heat.
Fast forward 10 billion years, long after humanity has come into existence (and probably passed out of existence), and the
Sun begins to lose its stability. The helium produced by the proton-proton cycle has gotten in the way of nuclear fusion, and
protons can no longer fuse to continue the cycle. This causes the core to collapse, because the outward pressure created by
the proton-proton cycle is no longer enough to hold up the weight of the material being pulled inward by gravity. The collapse
of the core causes the atmosphere to expand with an increase of temperature, and the Sun has now become a red giant.
Around the helium core, a shell of hydrogen begins to fuse to helium and the core gets increasingly hotter.
Once the core reaches 100,000,000K (99999726.85˚C/179999540.33˚F) – about 1 billion years later – the helium begins to
fuse to heavier elements and the Sun flares to become a yellow giant. However, this stage is only stable for about 100 million
more years, as soon the carbon, nitrogen, and oxygen "ash" interferes with helium fusion and the core begins collapsing
again. It becomes an unstable pulsating red giant (also known as a Mira variable star). As the core continues to collapse, the
temperature increases so much that the outer atmosphere is blown away, but not enough so that even heavier elements can
be created. The mass pressing down on the core causes the core to collapse even more until the only thing preventing holding
up the Sun are the electrons in the core, which, thanks to laws of physics, cannot occupy the same space as each other
(this is called Fermi degenerate pressure).
The Sun has now become a white dwarf, which isn't actually a star because it doesn't go through nuclear fusion. It is about
the size of the Earth, and is so dense that one teaspoon would weigh a ton on Earth. As the atmosphere is blown away it
may form a planetary nebula around the dwarf.
Structure
The Sun has a similar structure to the planets, sort of. It is similar in that there is a core, which is around
15000000K (14999726.85ËšC/26999540.33ËšF), and that around the core are other layers. Nuclear fusion
occurs in the core. Surrounding the core is the radiation zone. Energy from the core takes about 170,000
years to pass through the radiation zone. After the radiation zone is the convection zone. The temperature
at the bottom of the convective zone is around 200273.15K (2000000ËšC/3600032ËšF), allowing some
heavier ions to keep some of their electrons. The convective zone extends to the visible surface.
Around the Sun is the atmosphere, which is comprised of the photosphere, chromosphere, and corona.
The photosphere is about 500 kilometers thick, and is where the energy from the core finally manifests as
sunlight. The chromosphere is beyond the photosphere, and is where super-heated hydrogen burns off.
This layer of the atmosphere is only visible during a total solar eclipse. The outer layer of the Sun's
atmosphere is the corona. It is made up of ionized gas streaming off into space. Temperatures in the corona
can reach around 2000000K. Like the chromosphere, it can only be seen during a total solar eclipse.
