By human standards, the Sun is eternal. It rises in
the east every morning, sets in the west every evening, and shines
brightly as it crosses the sky. Like all stars, though, the Sun
undergoes constant change. Some of the changes take place over days
or even minutes, others require decades, and still others require
millions or billions of years.
The Sun was born about 4.6 billion years ago from the
gravitational collapse of a vast cloud of gas and dust. Material in
the center of the cloud was squeezed so tightly that it became hot
enough to ignite nuclear fusion.
Today, the Sun continues to fuse hydrogen atoms to make helium in
its core. It fuses about 600 million tons of hydrogen every second,
yielding 596 million tons of helium. The remaining four million tons
of hydrogen are converted to energy, which makes the Sun shine. Most
of this energy is in the form of gamma-rays and X-rays. As the
energy works its way to the surface -- a process that takes
centuries -- it is absorbed by other atoms, then re-radiated at
other wavelengths. When it reaches the surface, where it can escape
into space, most of the energy is in the form of visible light.
The motions of the hot gas below the Sun's surface create a
powerful magnetic field. The field encircles the Sun with lines of
magnetic force. These lines become entangled, forming relatively
cool, dark magnetic storms on the Sun's surface known as sunspots.
Occasionally, the entangled lines "snap," triggering enormous
explosions of energy known as solar flares. Magnetic effects also
pull out big streamers of hot gas from the Sun's surface, and they
heat the Sun's thin outer atmosphere to more than one million
|Anatomy of the Sun
- Core. The Sun's nuclear
"furnace," where fusion reactions initially combine hydrogen
atoms to produce helium, yielding energy in the process.
- Radiative Zone. Energy
moves through a surrounding envelope of gas toward the Sun's
- Convection Zone. Big
"bubbles" of hot gas transport energy to the surface.
- Photosphere. The Sun's
visible surface. Because of its high temperature, it glows
- Sunspot. A magnetic
"storm" on the Sun's surface.
- Prominence. An eruption of
hot gas that can extend thousands of miles into space.
- Corona. The Sun's outer
atmosphere, which is heated by the magnetic field to
millions of degrees.
The number of sunspots and flares peaks every 11 years, when the
Sun's magnetic field flips over. It takes two "flips" to complete a
The Sun will continue to burn its hydrogen for several billion
years more. As it depletes the supply of hydrogen, its core will
shrink and temperatures will climb high enough for it to burn helium
instead. The Sun's surface will puff up like a balloon, growing
cooler, brighter, and redder, forming a red giant.
Eventually, as the Sun burns helium to form heavier elements, it
will reach a critical point where fusion cannot release enough
energy to form new elements, so fusion will end.
After that, the Sun will shed its outer layers, surrounding
itself with a colorful bubble of gas called a planetary nebula. As
the nebula dissipates, distributing carbon, oxygen, and other
elements into the galaxy, only the Sun's collapsed core will remain
-- a dense ball no bigger than Earth, containing about 60 percent of
the Sun's original mass. This dead remnant is called a white dwarf.
Over many billions of years, the white-dwarf Sun will cool and fade
from sight, leaving behind a dark cosmic ember.
and Solar Flares