The Sun! Has no surface - just a bubbling mass of gas.
Sunlight emanates from 400km layer called the photosphere. Below this
layer is the solar interior. T decreases as come out through
photosphere, edges appear darker - limb darkening. Cools from T=5800K
to 4400K. Absorption lines as a cooler gas round the hot black body.
See granulation on the suns 'surface'. These have delta T = 300K. They
are 1000km across (size of Texas plus Oklahoma).
Caused by convection - heating from a lower heat
source. Spectroscopy shows blue and red shift. Last for 4 mins. 4
million at a time.
Close up of convection.
The next layer is the chromosphere. This is very low density - 10^{-8}
less than our atmosphere. Normally invisible unless an
eclipse. Emission line spectrum if seen directly - hot gas. Dominated
by Halpha line at 656.3nm. Temperature increases in this layer from
4400K to 25000K.
See spicules - verticle filaments - that rise at 20 km/s to
10,000km. Fade after 15 mins. 300,000 at any time covering 1% of the
surface. Found on boundaries of super granules.
Supergranules are 30,000km across - many 100's of granules.
The final layer is the solar corona. Extends to several million km and
the becomes the solar wind - a stream of protons and electrons
that escapes from the sun at high speeds. It is 1 millionth as bright
as the photosphere. Can see with a coronagraph or during an
eclipse. Not spherical. Varies on time scales of days/weeks. Emission
line spectrum T>2x10^6 K - strange iron emission. Not 'hot' - little
thermal energy 10^11 atoms per m^3 (10^25 in the atmosphere). Feel
heat from photosphere though. Like an oven.
Transition region - not understood. T increases rapidly across it.
Wonderful weird northern lights.
At the north and south pole there is a fairly constant mass ejection
rate of 80 km/s. Occasionally get coronal mass ejections where 10^12
kg blasted off at 100's km/s. Arrives at Earth a few days later -
causes interference. Occur every month, not always pointed towards earth.
Seen best in X-ray
Sun spots are easily seen with the right filter. Occur alone of in
groups of 1-10. 1000 km across. Dark core called umbra, brighter
border called prenumbra (nothing to do with eclipse just
appearance). Appear dark as T=4400K, less than the photosphere.
Solar prominances seem to emanate from the Sunspots.
Galileo first studied them. They last for approx 2 months. Say them
rotate around the sun. Takes 27 1/2 days at the equator and 35 days at
the pole. Differential rotation
Sunspots vary on an 11 year cycle. 2000 was a sunspot maximum year. In
11 years there will be another sunspot maximum.
Noticed the Zeeman effect - lines are split due to the presence of a
strong magnetic field.
Location of Sunspots changes. They start at 30 degrees north and south
and then migrate.
Sunspots have polarity - a +'ve and -'ve pole.
The differential rotation, the magnetic fields and the sunspots seem to
suggest that it is the magnetic fields that cause the sunspots. The
polarity of the sun gets changed on a 22 year cycle.
How is the sun fueled? Normal burning and gravitational collapse
cannot explain the sun. Nuclear reactions, the proton-proton chain is
the answer.
We cannot see into the sun but we know it is not undergoing any major
changes. Can construct models of how we think it works.
Picture of the solar interior
The only way we can probe under the surface is by listening - helioseismology.
Neutrinos also come from the sun.
