Medium-sized starThe Sun is a medium-sized star measuring 1,392,000 km across.The Sun weighs 2000 trillion tonnes - about 300,000 times as much as the Earth - even though it is made almost entirely of hydrogen and helium, the lightest gases in the Universe. The Sun’s interior is heated by nuclear reactions to temperatures of 15 million °C.
PhotosphereThe visible surface layer of the Sun is called the photosphere. This sea of boiling gas sends out the light and heat we see and feel on Earth. Suns surface is a turbulent mass offlames and tongues of hot gases - very different from the even, yellowish ball we see from Earth.
ChromosphereAbove the photosphere is the chromosphere, a thin layer through which dart flames called spicules, making the chromosphere look like a flaming forest. Above the chromosphere is the Sun’s halo-like corona.
The heat from the Sun’s interior erupts on the surface in patches called granules, and gigantic, flame- like tongues of hot gases called solar prominences.The Sun gets hot because it is so big that the pressure in its core is tremendous - enough to force the nuclei of hydrogen atoms to fuse (join together) to make helium atoms.
This nuclear fusion reaction is like a gigantic atom bomb and it releases huge amounts of heat.
Halfway out from its centre to its surface, the Sun is about as dense as water. Two-thirds of the way out, it is as dense as air. prominence sunspot The nuclear fusion reactions in the Suns core send out billions of light photons every minute (see light) - but they take 10 million years to reach its surface.
The temperature of the Sun’s surface is 6000°C. Each centimetre burns with the brightness of 250,000 candles! The Sun is made mostly of hydrogen and helium, and has many layers. It has a core, where most heat is made, then a number of layers building to the flaming chromosphere on its surface. Sunspots Sunspots are dark spots on the Sun’s photosphere (surface), 2000°C cooler than the rest of the surface.
The dark centre of a sunspot is the umbra, the coolest bit of a sunspot. Around it is the lighter penumbra. Sunspots appear in groups which seem to move across the Sun over two weeks, as the Sun rotates. Individual sunspots last less than a day. A Infrared photographs reveal the dark sunspots that appear on the surface of the Sun.
The number of sunspots reaches a maximum every 11 years. This is called the solar or sunspot cycle. The next sunspot maximum will be in the year 2002. Earth’s weather may be warmer and stormier when sunspots are at their maximum.
Long-term sunspot cycles are 76 and 180 years, and are almost like the Sun breathing in and out. Observations of the Sun by satellites such as Nimbus-7 showed that less heat reaches the Earth from the Sun when sunspots are at a minimum.
Solar eruptionSolar eruptions takes place on the Sun’s surface. They flare up in just a few minutes, then take more than half an hour to die away again. Solar flares reach temperatures of 10 million °C and have the energy of a million atom bombs. Solar flares not only send out heat and radiation, but also streams of charged particles.
The solar wind is the stream of charged particles that shoots out from the Sun in all directions at speeds of over a million km/h. It reaches the Earth in 21 hours, but also blows far throughout the Solar System. Every second the solar wind carries away over a million tonnes of charged particles from the Sun. Earth is shielded from the lethal effects of the solar wind by its magnetic field . Solar prominences are gigantic, flame-like tongues of hot hydrogen that sometimes spout out from the Sun.Solar prominences reach temperatures of 10,000°C.
Coronal mass ejections are gigantic eruptions of charged particles from the Sun, creating gusts in the solar wind which set off magnetic storms on Earth. Magnetic storms are massive hails of charged particles that hit the Earth every few years or so, setting the atmosphere buzzing with electricity. Solar prominences can stretch for many thousands of kilometres.
Solar changesThe Sun is about five billion years old and halfway through its life - as a medium-sized star it will probably live for around ten billion years. Over the next few billion years the Sun will brighten and swell until it is twice as bright and 50 percent bigger. The Sun will end as a white dwarf .
The Sun seems to burn so steadily that we take for granted that it will be equally bright and warm all the time. In the short term, however, its brightness does seem to vary very slightly all the time, and over the next 5 billion years it will probably burn more and more ferociously. In five billion years, the Sun’s hydrogen fuel will have burned out, and its core will start to shrink.
As its core shrinks, the rest of the Sun will swell up with gases and its surface will become cooler and redder. It will be a red giant star. The Earth will have been burned to a cinder long before the Sun is big enough to swallow it up completely. The Sun’s brightness varies, but it was unusually dim and had no sunspots between 1645 and 1715 - this period is called the Maunder minimum.
The Earth suffered the Little Ice Age at this time. More of the chemical carbon-14 is made on Earth when the Sun is more active. The carbon-14 is taken into trees, which means scientists can work out changes in solar activity in the past by measuring carbon-14 in old wood.
EclipsesAn eclipse is when the light from a star such as the Sun is temporarily blocked off by another space object. A lunar eclipse is when the Moon travels behind the Earth, and into the Earths shadow Earth is between the Moon and the Sun. Lunar eclipses happen once or twice every year and last only a few hours. In a total lunar eclipse, the Moon turns rust-red. There will be a total lunar eclipse on'3 March 2007 and 28 August 2007.
A solar eclipse
A solar eclipse is when the Moon comes between the Sun and the Earth, casting a shadow a few kilometres wide on to the Earth’s surface. A During a total solar eclipse of the Sun, the Moon blocks out everything but the Sun s corona. In a total eclipse of the Sun, the Moon passes directly in front of the Sun, completely covering it so that only its corona can be seen .
There are one or two solar eclipses every year, but they are visible only from a narrow strip of the world. There will be a total solar eclipse on 29 March 2006 and 1 August 2008. Solar eclipses are possible because the Moon is 400 times smaller than the Sun, and is also 400 times closer to the Earth.
This means the Sun and the Moon appear to be the same size in the sky. Today people are advised never to look up at the Sun during an eclipse or they may damage their eyes permanently. This group is waiting on a beach in Hawaii to observe a total solar eclipse. They are wearing special glasses, but these really don’t make it any safer. Auroras Auroras are bright displays of shimmering light that appear at night over the North and South Poles.
The Aurora Borealis is the Northern Lights, the aurora that appears above the North Pole. The Aurora Australis is the Southern Lights, the aurora that appears above the South Pole. Auroras are caused by streams of charged particles from the Sun known as the solar wind crashing into the gases of the Earth’s atmosphere.
Oxygen gas glows yellow-green when it is hit low in the atmosphere, and orange higher up. Nitrogen gas glows bright red when hit normally, and bright blue when ionized. Auroras form a halo of light over the poles all the time, but they are usually too faint to see.
They flare up brightly when extra bursts of energy reach the Earth’s atmosphere from the Sun. Auroras appear at the poles and nowhere else in the world because there are deep cracks here in the Earth’s magnetic field . Auroras are more spectacular when the solar wind is blowing strongly. New York and Edinburgh get an average of ten aurora displays every year.
The Northern Lights above the Arctic Circle are among nature’s most beautiful sights. Shimmering, dancing curtains of colour - bright green rays flashing with red, and streamers of white - blaze into the darkness of the polar night