Astronomy Benalla General Meeting Presentations - Wednesday 17th November 2010
Reports - 2010
A Star - Our Sun. Presenter: Rupe Cheetham
Depicted in various incarnations, the Sun has been worshiped over the past 5000 years in many countries.
However, in the 1600s with the development of the telescope factual information changed our outlook.
Today we know that the Sun is one of 400 billion stars in our Galaxy. It takes 250 million years for the Sun
to orbit our Galaxy centre, a black hole 27,000 light years away.
The Sun formed about 4.5 billion years ago and is about half way through its life. It provides our source of
heat from a distance of approximately 150,000,000 km and is the centre of the solar system of 7 other
planets and millions of asteroids.
The Sun has a diameter of 1.4 million km and comprises of, by number of atoms, about 91 % hydrogen
and 8 % helium, plus minor amounts of other elements. The Sun rotates at it’s equator every 25.67 earth
days. The Sun’s core is about 25% of its diameter, has a temperature of 15,000,000 degrees K and a
density of 150 grams per cubic centimeter. This core is surrounded by a Radiative Zone, a Convection
Zone and the surface we see called the Photosphere where the temperature has dropped to about 5800
degrees K.
The Sun generates its energy by the conversion, through the nuclear proton-proton process, of
approximately 600 million tonnes of hydrogen into helium per second with 5 million tonnes becoming
gamma rays in it’s core. These extreme photons take up to 170,000 years to reach the surface where the
released energy then takes only 8.3 minutes to reach the Earth.
Today we can see & record such things as gigantic coronal holes from which emanate the coronial mass
ejections containing billions of tonnes of plasma, magnetic solar winds which can cause serious damage to
our GPS satellite electronics, and knock out our power grids. Also visible are granules, prominences,
filaments, spicules and sun-spots.
Many countries now have research observatories both on Earth and in space to find out more about the
Sun and how it effects our weather patterns. SOHO launched in 1995 and still in operation has 12
instruments and beside monitoring the sun has found 1600 comets. It is supported by TRACE for ultraviolet
imaging. RHESSI high energy imager. Ace, with sufficient fuel to last to 2024, has 10 instruments which
sample low energy particles and gives us about one hour advanced warning of potentially dangerous solar
activity. Hinode a Japanese spacecraft directly watches the sun 24/7 with 3 instruments providing imaging
of extreme ultraviolet radiation and powerful X-ray flares. Stereo A and B, launched in 2006, orbit the Sun
from in-front and behind the Earth and are solar storm watchers, detecting the plasma clouds hurtling
towards Earth at 1000km/sec.
The latest solar observatory is the SDO (Solar Dynamic Observatory) launched in February 2010 with 3
instruments having 100 times better resolution than previous instruments. Photos returned are truly
outstanding.
Planed for the future is the Solar Orbiter which lacks finance at present, but The Solar Probe Plus contains
4 instruments and will plunge into the Sun’s corona and provide “touch, taste, and smell” for the first time. It
is programmed for 2018 and will approach the Sun to within 7,000,000 km with a heat shield made of 15
cm carbon-composite designed to withstand 2000 K temperature.
