The Sun
The Sun 101 was the title of Darren Bellingham’s presentation. Some fascinating statistics about our nearest star were brought to light. While we all know that the temperature at the centre of the sun is about 15 million degrees, such heat suddenly becomes more than an abstract sum when given the information that a single grain of sand that hot would cook a person at a distance of 150km!
The Sun was considered by Greek astronomers to be one of the seven planets (Greek planetes "wanderer"), after which the seven days of the week are still named in some languages. One of the first people in the Western world to offer a scientific explanation for the sun was the Greek philosopher Anaxagoras, who reasoned that it was a giant flaming ball of metal even larger than the Peloponnesus, and not the chariot of Helios. For teaching this heresy he was imprisoned by the authorities and sentenced to death (though later released through the intervention of Pericles).
The Sun is the star at the centre of our Solar system. It is a ball of plasma with a mass of about 2.0×1030 kg, just above that of an average star. The mass of the Sun is so much greater than that of the planets that the centre of mass of the solar system is within the bounds of the Sun itself. About 74% of its mass is hydrogen, with 25% helium and the rest made up of trace quantities of heavier elements. It is thought that the Sun is about 4.6 billion years old, and is about halfway through its main sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. In about 5 billion years time the Sun will evolve into a red giant and then a white dwarf. Not being a solid body the Sun rotates faster at its equator (about 25 days) than it does at higher latitudes (about 35 days near its poles). The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field loops to erupt from the Sun's surface and trigger the formation of the Sun's dramatic sunspots and solar prominences that can even affect the “space weather” and create the eery auroras on Earth . The magnetic field of the sun reverses each 11-year sunspot cycle. Its influence on the plasma in the interplanetary medium creates the largest structure in the Solar System, the heliospheric current sheet. The plasma in the interplanetary medium is also responsible for the strength of the Sun's magnetic field, which at the orbit of the Earth is now known to be over 100 times greater than originally anticipated. Not being a solid body also means the Sun does not have a definite boundary as rocky planets do. The density of its gases drops off at an exponential relationship with distance from the centre of the Sun. Most of the Sun's mass lies within about 0.7 radii of the centre. The Sun's radius is measured from centre to the edges of the photosphere, below which the gases are thick enough to be opaque. The corona is the extended outer atmosphere and is much larger in volume than the Sun itself and merges smoothly with the solar wind that fills the solar system.
Although it is the nearest star to Earth and has been intensively studied by scientists for centuries, many questions about the Sun remain unanswered, such as why its outer atmosphere (the chromosphere) has a temperature of over 106 K whereas its visible surface (the photosphere) is just 6,000 K.
At the end of the talk many questions were asked of Darren, who was ably supported by Barry Adcock.