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A yellow dwarf star,
unique in that it is the star around which the Earth orbits, but
otherwise unremarkable. If it lay at a distance of 10 parsecs, it would hardly be visible to
the naked eye.
While the Sun may not be powerful in comparison with many other stars, it is still a remarkably
luminous body. The temperature at its surface is a little less the 6,000 Kelvin, and
its core reaches 15,000 Kelvin. Every second, it consumes several million tonnes of
hydrogen to produce 400,000,000,000,000,000,000,000,000 Watts of power. Though many distant
stars are much more luminous than this, the Sun is one of the most energetic stars in its
immediate stellar neighbourhood.
Because the Earth travels around the Sun in a year-long orbit, from the point of view
of an Earth-bound observer, the Sun appears to progress through the sky. This journey
always follows the same path, the Ecliptic, which passes through the twelve zodiacal
constellations, as well as Ophiuchus, the Serpent-holder. The Sun travels almost exactly
one degree across the sky each day, and it has been speculated that this is the reason
a circle is divided into 360 degrees.
A portrait of the Sun. The darker areas are 'sunspots', temporary cooler regions where the surface
temperature is no more than about 3,700 Kelvin, compared with nearly 6,000
Kelvin in the surrounding photosphere.
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The Life History of the Sun
About 5,000,000,000 years ago, our region of the Galaxy was very different from its
conditions today. It was dominated by a vast cloud that has come to be known as the Solar Nebula.
Gravitational effects within the cloud caused
a protostar to form, and over a period of tens of millions of years, this collapsed and heated
to the point where nuclear processes ignited in its core.
As the matter in the protostar coalesced, it formed into a broad disc shape around the
burning central core of the young Sun. The same gravitational effects that had caused the
protostar to collapse were at work within the disc, causing the gradual formation of
protoplanets, which over millions of years formed gradually into true
planets with their own systems of
moons and rings: our
Solar System.
Since its formation, the Sun has been steadily burning through its reserves of hydrogen fuel. It is
now a middle-aged star, and has about another 5,000,000,000 years of life before its fuel reserves
are exhausted. At this point, the outer layers of the Sun will expand to form a short-lived giant
red star. Eventually, this material will expand outwards into
space, creating a planetary nebula (such as the
Ring Nebula in Lyra). The core, meanwhile, will
collapse upon itself to form a white dwarf of about the same
diameter of the Earth, though rather more massive.
Eventually, this white dwarf will cool to form a Black Dwarf - a giant cinder in space,
perhaps accompanied by the dead remnants of its planetary system.
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