Download The Sun`s Exterior

Our Star, the Sun
What processes control the Sun’s appearance?
What is the temperature of the Sun’s atmosphere?
What is the solar cycle?
Galileo and Sunspots
Galileo Galilei was the first to recognize sunspots
as features in the Sun’s atmosphere.
He used the motion of sunspots to measure the
rotation of the Sun: approximately 1 month.
The Sun’s Surface
The Sun has no solid surface. The features we
see on the Sun are located where the Sun goes
from opaque to transparent.
This layer is known as the “photosphere”.
The layers below are dense enough that light
must scatter many, many times to flow outward.
The Sun’s interior is opaque.
The photosphere and the layers above are much
lower density and light can escape from the
Sun’s inner layers. This gas is about 5800 Kelvin.
The Solar Atmosphere
The photosphere is also the bottom layer of the
Sun’s atmosphere. The layers above are hotter
than the photosphere.
The layer above is called the chromosphere. This
is the layer that contains the bright red storms
visible during a solar eclipse.
The temperature in
the chromosphere
ranges from 6,000
to 50,000 Kelvin.
The Solar Atmosphere
Above the chromosphere is the corona, the faint
halo visible around the Sun during an eclipse.
The corona is extremely
hot: 1 million Kelvin.
But it is also very low
density.
It does not produce
much light of its own,
but scatters sunlight
very strongly.
(Image credit: Fred Espenak)
The Sun’s Magnetic Field
It is now believed that the intense magnetic field
of the Sun transports heat from the Sun’s interior
out through the photosphere to the corona. The
process is not yet fully understood, but there is a
wealth of data to study.
This image shows tubes
of hot gas and magnetic
field lying close to the
photosphere.
This hot gas glows in
ultraviolet and x-rays.
Sunspots and Magnetic Fields
Dark regions in the photosphere (left) are strongly
magnetic (right: bright x-rays). Sunspots are
regions of intense magnetic activity!
(below): visible light image
(right): x-ray image
Sunspots
Sunspots can appear as individuals or in a group.
Typical sunspots are about the size of Earth.
They appear dark because they are cooler: 5000 K.
Sunspot Cycle
The number of sunspots varies on an 11-year cycle.
The amount of sunlight and magnetic activity on
the Sun increases and decreases every 11 years.
Coronal Mass Ejections
Portions of the corona can be ejected in what is
called a coronal mass ejection or CME.
If this gas strikes Earth’s magnetic field, it can
trigger auroras all around the world.
The Solar Wind
The gas in the Sun’s corona is so hot it can escape
the Sun’s gravity far from the photosphere.
This stream of hot gas from the corona is known
as the solar wind. It is often “gusty” as it is hit
with shock waves from solar flares and coronal
mass ejections.
At 1 AU from the Sun, the solar wind is typically
moving between 300-800 kilometers per second.
It contains 1-100 protons and electrons per cubic
centimeter. A fast, low-density plasma!
Blowing Bubbles
The solar wind blows a bubble into the gas that
fills the space between the stars. Pushing out on
this interstellar gas makes the solar wind slow
and eventually stop.
Into Interstellar Space
The 4 most distant spacecraft are close to the edge:
Pioneer 10: 102.2 AU from Earth
Pioneer 11: 82.2 AU
(Both Pioneers dead.)
Voyager 1: 115.5 AU
Voyager 2: 93.9 AU
(Both Voyagers alive.)
(14 light-hours!)