The Sun

Our nearest star, the Sun, is a yellow Main Sequence star of intermediate mass, the only star close enough to us to be studied in detail. The Sun is a second generation star, formed about 4.6 billion years ago from an enriched solar nebula within the Orion arm of the Milky Way galaxy. The Suns composition is very much dominated by hydrogen (74%) and helium (25%) with heavier elements making up the rest (1%). Like other Main Sequence stars, the Sun burns hydrogen in its core. As a result, temperature within the core of the Sun is about 15 million BC. In contrast, the surface temperature of the Sun is a mere 6000C. The Sun is about 1.4 million kilometres in diameter, over 100 times the size of the Earth. The Sun exhibits what is known as differential rotation. Studies of sunspots and other solar features have revealed that the Sun rotates once every 25 days at its equator but once every 35 days at its poles. The rotation of the Sun and a phenomenon known as limb darkening provide simple evidence that the Sun is almost perfectly spherical in shape. As the Sun is a star, it does not have a solid surface, instead it has layers of gas of different density. The outermost (or atmospheric) layers are of particular interest to astronomers and these are described as follows:

The photosphere

The photosphere (about 300-400km thick) is the name given to the layer which produces the Suns visible disk, the part we normally see during sunrise and sunset (never look directly at the Sun for any period of time during the day and certainly never with binoculars or a telescope). Close observation reveals the photosphere to be granulated. This is due to the convection of gases from deep within the solar interior. Cooler areas of the photosphere known as sunspots appear black against its normally bright appearance. Sunspots, typically a few tens of thousands of kilometres across and with temperatures of about 4000C, are associated with variations in the Suns magnetic field.


The chromosphere

The chromosphere (about 2,000km thick) is a relatively cool, dim layer lying between the photosphere and the corona. The chromosphere can only be seen during a solar eclipse. Close observation reveals spicules or spikes of gas rising to over 10,000km within the chromosphere.


The corona The corona is the hot, outermost layer of the Sun which extends for millions of kilometres into space where it gradually becomes the solar wind. The corona can only be seen during a solar eclipse. Prominences, jets of glowing gas up to 100,000km high, rise from the limb of the Sun into the corona. Outbursts or flares of high energy radiation and atomic particles from the photosphere pass through the corona and can affect radio and telecommunications on Earth.


The Universe
The Milky Way Galaxy
Constellations, Surveying the Solar System
Planets and their moons
Asteroids, comets and meterorites, Exploring the Earth-Sun-Moon System
Day and night
Seasonal change
The Moon's phases and eclipses
Further Reading
Self assessment(1)
Self assessment(2)
Self assessment(3)