Because of its proximity to the earth, and because it is such a typical star, the sun is a unique resource for the study of stellar phenomena. No other star can be studied in such detail. The star closest to the sun is 4.3 light-years (4 × 1013 km/2.5 × 1013 mi) away. To observe features on its surface comparable to those that can be seen routinely on the sun would require a telescope almost 30 km (18.6 mi) in diameter. Such a telescope, moreover, would have to be put into space to avoid distortions caused by the earth's atmosphere.

    The Sun is an ordinary G2 star, one of more than 100 billion stars in our galaxy.

   The Sun is personified in many mythologies: the Greeks called it Helios and the Romans called it Sol.

   The Sun is, at present, about 75% hydrogen and 25% helium by mass (92.1% hydrogen and 7.8% helium by number of atoms); everything else ("metals") amounts to only 0.1%. This changes slowly over time as the Sun converts hydrogen to helium in its core.

   The outer layers of the Sun exhibit differential rotation: at the equator the surface rotates once every 25.4 days; near the poles it's as much as 36 days. This odd behaviour is due to the fact that the Sun is not a solid body like the Earth. Similar effects are seen in the gas planets. The differential rotation extends considerably down into the interior of the Sun but core of the Sun rotates as a solid body.

   Conditions at the Sun's core (approximately the inner 25% of its radius) are extreme. The temperature is 15.6 million Kelvin and the pressure is 250 billion atmospheres. At the centre of the core the Sun's density is more than 150 times that of water.

   The Sun's energy output (3.86e33 ergs/second or 386 billion billion megawatts) is produced by nuclear fusion reactions. Each second about 700,000,000 tons of hydrogen are converted to about 695,000,000 tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the surface, it is primarily visible light. For the last 20% of the way to the surface the energy is carried more by convection than by radiation.

   The surface of the Sun, called the photosphere, is at a temperature of about 5800 K. Sunspots are "cool" regions, only 3800 K (they look dark only by comparison with the surrounding regions). Sunspots can be very large, as much as 50,000 km in diameter. Sunspots are caused by complicated and not very well understood interactions with the Sun's magnetic field.

   A small region known as the chromosphere lies above the photosphere.

   The highly rarefied region above the chromosphere, called the corona, extends millions of kilometers into space but is visible only during eclipses (left). Temperatures in the corona are over 1,000,000 K.

   The Sun's magnetic field is very strong (by terrestrial standards) and very complicated. Its magnetosphere (also known as the heliosphere) extends well beyond Pluto.

   In addition to heat and light, the Sun also emits a low density stream of charged particles (mostly electrons and protons) known as the solar wind which propagates throughout the solar system at about 450 km/sec. The solar wind and the much higher energy particles ejected by solar flares can have dramatic effects on the Earth ranging from power line surges to radio interference to the beautiful aurora borealis. 

   The solar wind has large effects on the tails of comets and even has measurable effects on the trajectories of spacecraft.

   Spectacular loops and prominences are often visible on the Sun's limb (left).

   The Sun's output is not entirely constant. Nor is the amount of sunspot activity. There was a period of very low sunspot activity in the latter half of the 17th century called the Maunder Minimum. It coincides with an abnormally cold period in northern Europe sometimes known as the Little Ice Age. Since the formation of the solar system the Sun's output has increased by about 40%.

   The Sun is about 4.5 billion years old. Since its birth it has used up about half of the hydrogen in its core. It will continue to radiate "peacefully" for another 5 billion years or so (although its luminosity will approximately double in that time). But eventually it will run out of hydrogen fuel. It will then be forced into radical changes which, though commonplace by stellar standards, will result in the total destruction of the Earth. 

The Sun's satellites

Earth Will Not Be Swallowed by the Sun

The astronomy textbooks will have to be rewritten, say astrophysicists at the University of Sussex who have re-examined standard calculations about solar evolution and the distant future of the Earth. The textbooks tell us that one day the Sun will burn up its nuclear fuel and expand to an enormous size, finally engulfing its inner planets including Earth.

However, using the latest data based on real stars, the University of Sussex researchers suggest a (slightly) less catastrophic future for our planet.

As their hydrogen fuel runs out at the end of their 'lives', stars like the Sun expand to become a red supergiant of several hundred times their initial diameter. Most astronomers expect the solar red supergiant to swallow Mercury, Venus and then Earth in about 7.5 billion years' time, when it has expanded beyond the orbit of our planet.

But Earth may survive after all, say the Sussex astronomers, if an important extra detail is considered: the ongoing loss of mass and weakening gravity while a star is a red supergiant.

Dr Robert Smith, Reader in Astronomy, explains the significance of this effect: "Taking this into account, the orbit of the Earth would increase beyond the Sun's outer atmosphere by a small but crucial margin at all phases of the Sun's evolution - allowing our planet to continue."

The new calculations are published in the current issue of Astronomy & Geophysics. They were made by Dr Smith together with Dr Klaus-Peter Schröder from the University's Astronomy Centre and Kevin Apps, the famous student stargazer who co-discovered 10 planets while still an undergraduate at Sussex.

Although the Earth may survive, long before then its surface will have become too hot to sustain human life. But the good news from the team of researchers is that it will be 5.7 billion years before our planet becomes a no-go zone for life - about 200 million years later than previously thought.

So, ask the Sussex astronomers, is there anywhere in the solar system that would be safe, or does our survival depend on finding another star system? Is it possible to hop outwards from one planet or satellite to the next, always keeping ahead of the Sun? There are periods, they calculate, when we could in principle survive on one of the outer planets such as Mars, but there will be long gaps when none of them is habitable.

Dr Smith concludes: "We had better get used to the idea that we shall need to build our own survival capsules - the planets are simply too far apart for planet-hopping to be a viable solution. Perhaps this is the ultimate justification for developing an International Space Station."

Source: Sussex University Press Release