The sun is our nearest star,
and our closest friend.
It is the source of our light and heat.
It drives our weather and grows our crops. We fly hundreds of miles
just to lie in its rays and it has been worshiped as a god by many
civilisations for thousands of years. And yet, scientifically, it
is still a bit of a mystery.
We must know
something…
We know a fair number of the sun's
vital statistics. It is about 1.4 million Km in diameter or about
109 Earths across. It's about 93 million miles distant (8 light
minutes) and has an effective surface temperature of around 5,700C.
The outer atmosphere or Corona is much hotter at 5 million degrees
Celsius (no one is quite sure why) and the core is probably at
about 15 million degrees Celsius. But that again is really just an
educated guess. The sun doesn't have a 'surface' like a rocky
planet does, but its density does drop very quickly the further
away from the centre you get. It's made of 92 per cent Hydrogen,
about 7 per cent Helium and a tiny bit of everything else.
There must be something more
than that
What we humans are mostly interested in
are the bits that affect us: the outer bits. So what can you see if
you look closely?
It was the Chinese that first noticed
that the sun wasn't always a featureless disc, they wrote about
seeing sunspots 2000 years ago, and naked eye sunspots have been
seen by keen sun watchers many times since. If you're
lucky you may see one when the sun is setting through thin cloud
and the intensity has been reduced. You must never look directly at
the sun when it is high in the sky and definitely not through a
telescope or binoculars. You will be blinded, instantly and
permanently.
Spinning sun
The first person to make a proper study
of sunspots was Galileo, the 16th century Italian scientist. He
used the newly-invented telescope to project an image of the sun.
Galileo noticed that sun spots appeared to move across the surface.
He realised that this was just an 'apparent' movement and that the
sun was actually spinning.
In time it was discovered that the sun
spins on its axis about once a month, the rotational period is very
different for the equator and the poles. It spins much quicker
around its middle than the top and bottom. Sunspots also come and
go in a regular 11-year cycle. We are just coming out of a deep
'solar minimum' when there were very few sunspots and the sun was
generally very inactive. Over the next few years sunspots will
become more and more common again. So keep an eye out during
sunsets!
Sunny stuff
The sun throws out lots of
electromagnetic radiation at nearly all wavelengths. From radio
waves through to X- rays the sun makes them all. My favourite part
is the ultra violet bit, preferably studied from a beach in Spain.
The second most important thing that the sun produces is a stream
of charged particles called the solar wind. They are mostly
electrons and protons and move very fast, which is why they can
escape from the gravitational pull of the sun and head out into
space. They are so energetic that the Japanese Space Agency (JAXA)
is launching a 'sun jammer'.
A sun jammer is a solar wind-powered
spacecraft. It is called IKAROS. It will have a solar sail that
will use the momentum of these particles to push the space craft
along. It is hoped that one day spacecraft with giant solar sails
will be accelerated up to near light speeds to set sail across the
oceans of space between the stars.
Aurora borealis
When the solar wind comes into the
vicinity of the Earth the charged particles are channelled down the
Earth's magnetic field lines striking the top of the atmosphere
creating Auroras, or the Northern and Southern lights. The Northern
and Southern lights are one of the most awe inspiring sights in the
whole of nature. The atmosphere is lit up like the inside of a
gigantic TV tube. It causes great sheets of colour to dance and
sway.
If you have a granny that lives in
Canada go and visit her and the Aurora borealis! Very occasionally
it can be seen from the UK but a good display is very rare this far
from the poles.
Breezes that will blow you
away!
The downside of the solar wind is that
sometimes instead of a gentle breeze you can end up with a force
nine gale. This can be very destructive. In 1988 the entire
Canadian national grid was knocked by a solar gale.
Their proper name is Coronal Mass
Ejections (CME). A CME happens when a huge blast of charged
particles explodes from the surface of the sun. CMEs happen pretty
regularly, the problems only start if we happen to have one heading
directly in our direction. Back in 1988 very little damage was done
to anything apart from to the poor Canadians.
Today, the Earth is surrounded by a
cloud of thousands of satellites. If a big CME struck today most of
the world's communications would be under threat. This would be a
very big deal. NASA has a satellite stationed immediately between
the Earth and the sun to pick up just such an event and give us
some warning. It has a very good view of the sun but doesn't see it
in the finest detail. This is where the SDO comes in.
What's an SDO?
The Solar Dynamics Observatory (SDO)
sees the sun in incredible detail. The sun has massive magnetic
fields associated with it. If you look at the images you can see
magnetic field lines all over the place, just think magnets and
iron filings. It seems that sunspots are low temperature areas,
about 2000C cooler than the surrounding regions that are connected
together by colossal magnetic loops. CMEs originate in these areas.
They are flung into space when giant magnetic field arcs suddenly
snap releasing huge amounts of energy.
SDO can see this happening in real time
and record movies showing how each event unfolds, step by step,
showing every incremental change. It can also see the sun in a
range of different wavelengths. By looking at the sun in very
specific colours what is happening at different depths in the solar
atmosphere can be seen.
By putting all this information
together scientists will be able to unlock many more of the sun's
most guarded secrets. This will help the Earth and its fragile
inhabitants survive a little longer sitting as we do, right in the
middle of the solar shooting gallery.