Download File - Earth Science with Mr. Lanik

Our Star the Sun
http://sohowww.nascom.nasa.gov/classroom/classroom.html
Our solar system is composed of the
Sun and all things which orbit
around it: the Earth, the other eight
planets, asteroids, and comets. The
Sun is 150 million kilometers (93
million miles) away from the Earth
(this
distance
varies
slightly
throughout the year, because the
Earth's orbit is an ellipse and not a
perfect circle).
The Sun is an average star - there are
other stars which are much hotter or
much cooler, and intrinsically much
brighter or fainter. However, since it
is by far the closest star to the Earth,
it looks bigger and brighter in our
sky than any other star. With a
diameter of about 1.4 million
kilometers (860,000 miles) it would
take 110 Earths strung together to be
as long as the diameter of the Sun.
The Sun is mostly made up of
hydrogen (about 92.1% of the
number of atoms, 75% of the mass).
Helium can also be found in the Sun
(7.8% of the number of atoms and
25% of the mass). The other 0.1% is
made up of heavier elements, mainly
carbon, nitrogen, oxygen, neon,
magnesium, silicon and iron. The
Sun is neither a solid nor a gas but is
actually plasma. This plasma is
tenuous and gaseous near the
surface, but gets denser down
towards the Sun's fusion core.
The Sun, as shown by the illustration
to the left, can be divided into six
layers. From the center out, the
layers of the Sun are as follows: the
solar interior composed of the
core (which occupies the innermost
quarter or so of the Sun's radius),
the radiative zone, and the the
convective zone, then there is the
visible surface known as the
photosphere,
the chromosphere,
and finally the outermost layer,
the corona.
The
energy
produced
through fusion in the Sun's core
powers the Sun and produces all of
the heat and light that we receive
here on Earth. The process by which
energy escapes from the Sun is very
complex. Since we can't see inside
the Sun, most of what astronomers
know about this subject comes from
combining theoretical models of the
Sun's interior with observational
facts such as the Sun's mass, surface
temperature, and luminosity (total
amount of energy output from the
surface).
All of the energy that we detect as light and heat originates from nuclear reactions deep
inside the Sun's high-temperature "core." This core extends about one quarter of the way
from the center of Sun (where the temperature is around 15.7 million kelvin (K), or 28
million degrees Fahrenheit) to its surface, which is only 5778 K "cool".
Above this core, we can think of the
Sun's interior as being like two
nested spherical shells that surround
the core. In the innermost shell, right
above the core, energy is carried
outwards
by
radiation.
This
"radiative zone" extends about three
quarters of the way to the surface.
The radiation does not travel directly
outwards - in this part of the Sun's
interior, the plasma density is very
high, and the radiation gets bounced
around countless numbers of times,
following a zig-zag path outward.
Click on the image for an animation!
It takes several hundred thousand years for radiation to make its way from the core to
the top of the radiative zone! In the outermost of the two shells, where the temperature
drops below 2,000,000 K (3.5 million degrees F) the plasma in the Sun's interior is too
cool and opaque to allow radiation to pass. Instead, huge convection currents form and
large bubbles of hot plasma move up towards the surface (similar to a boiling pot of
water that is heated at the bottom by a stove). Compared to the amount of time it takes
to get through the radiative zone, energy is transported very quickly through the outer
convective zone.
The Sun's visible surface the photosphere is "only" about 5,800 K (10,000 degrees F).
Just above the photosphere is a thin layer called the chromosphere. The name
chromosphere is derived from the word chromos, the Greek word for color. It can be
detected in red hydrogen-alpha light meaning that it appears bright red. Above the
surface is a region of hot plasma called the corona. The corona is about 2 million K (3.6
million degrees F), much hotter than the visible surface, and it is even hotter in a flare.
Why the atmosphere gets so hot has been a mystery for decades; SOHO's observations
are helping to solve this mystery.
The Sun is not just a big bright ball.
It
has
a
complicated
and
changing magnetic field, which
forms
things
like sunspots and active
regions.
The magnetic field sometimes
changes explosively, spitting out
clouds of plasma and energetic
particles into space and sometimes
even towards Earth. The solar
magnetic field changes on an 11 year
cycle. Every solar cycle, the number
of
sunspots, flares,
and solar
storms increases to a peak, which is
known as the solar maximum.
Then, after a few years of high
activity, the Sun will ramp down to a
few years of low activity, known as
the solar minimum. This pattern is
called the "sunspot cycle", the "solar
cycle", or the "activity cycle".
Stars like the Sun shine for nine to
ten billion years. The Sun is about
4.5 billion years old, judging by the
age of moon rocks. Based on this
information, current astrophysical
theory predicts that the Sun will
become a red giant in about five
billion (5,000,000,000) years.