By Brian von Konsky

HET 603 (S1/99)

The Sun is an average star, but it produces vital energy required by life on Earth. Not only does the Sun heat the Earth and prevent it from freezing, it also provides the energy used in many biochemical processes.

Plants and animals have evolved to take full advantage of the Sun's energy and other resources available at the Earth's surface. Photosynthesis enables plants to turn the Sun's energy and carbon dioxide into oxygen and food.

Life as we know it on Earth cannot exist without liquid water. Thanks to the Sun's energy output, its distance from the Earth, and the insulating atmosphere blanketing the planet, atmospheric pressure and temperature are just right to enable liquid water to exist here.

Nuclear reactions in the core of the Sun have reliably produced the energy required by life on Earth for some 5 billion years. But this will not always be so. As with all stars, the Sun will eventually age and die as its nuclear fuel is exhausted, making life as we know it impossible on Earth. How long will it be until this occurs? When the time arrives, will our distant descendants be ready? What will they experience? What are their alternatives? To answer these questions requires understanding what a star is, how it evolves, and how it generates energy.

The Sun was formed in a dense interstellar cloud of gas and dust, similar to the Orion, Horsehead, and Eagle Nebulas. The gas was primarily hydrogen, the most common element in the universe. This was mixed with other elements, produced by a previous generation of stars and expelled into the interstellar medium as they died long ago.

Over time, dust and gas in the cloud were drawn together by gravity. As the cloud compressed, the protosun began to glow as gravitational energy was converted to thermal energy.

Gravity also drew dust from the solar cocoon into a disc revolving around the protosun. Matter in the disc accreted onto the protosun, contributing to its mass. Dust coalesced into clumps to form planetesimal bodies. These would later combine to form the planets. Recent Hubble Space Telescope images have shown several similar discs around many young stars.

Meanwhile, charged particles in the core of the protosun repelled like charges, preventing the onset of nuclear fusion. Eventually, thermal energy resulting from gravitational contraction overcame this tendency, as particles moving faster and faster were packed closer and closer together, enabling nuclear fusion to commence in the core. The sun was born, now a full-fledged star.

A main sequence star with the mass of the Sun remains in a state of relative equilibrium for 10 billion years. During this time, the inward force of gravity is balanced by outward gas pressure, regulating the rate of nuclear fusion and temperature of the star.

The Sun converts hydrogen to helium and energy through nuclear fusion in its core, consuming a staggering 6 hundred thousand million kilograms of hydrogen each second (Universe, 1999). But the Sun cannot consume hydrogen at this rate forever. Some 5 billion years from now, all of the hydrogen in the core will be exhausted. It is then that the people of Earth will need to consider their options, as the life giving Sun begins to die.

When hydrogen in the core of the sun is exhausted, the core will cool slightly so its pressure will drop. Reduced pressure will enable the core to contract. As it contracts, the helium core will get hotter, raising the temperature of the surrounding hydrogen shell. When the shell becomes hot enough, fusion will commence in the shell, so nuclear reactions will begin converting shell hydrogen to helium. Fusion in the shell will produce considerable energy, increasing the pressure in outer layers. This will cause the Sun to swell to 200 times its main-sequence size (Universe, 1999). A much larger mature Sun will become red in colour as the outer layers expand and cool. Due to its great size and relatively cool surface temperature, an aged star in this state is said to be a Red Giant.

Meanwhile, helium produced from shell hydrogen burning will be incorporated into the core. The core will contract further and become hotter. Eventually, the core will become hot enough for helium fusion to commence, producing carbon, oxygen and energy.

When all of the helium in the core is consumed, gravity will again compresses the oxygen and carbon core as it cools and the pressure decreases. As the core contracts, its temperature will rise. Consequently, fusion will commence in a shell of helium surrounding the core and the process will begin anew.

As this process repeats itself, the Sun will expel its outer layers into space due to cyclic changes in luminosity in what others have described as a "shudder" of seismic energy (STSCI, 1999b). Up to 40% of the outer layers will be returned to the interstellar medium. Eventually enough matter will be expelled to expose the core, which will be white due to its extreme temperature. A star in this state is said to be a white dwarf.

The hot white dwarf will ionise the expelled gas, causing it to glow. This will create an eerie system of luminescent shells called a planetary nebula, a visually compelling but highly inaccurate term coined by William Herschel in the nineteenth century.

At first, the shells of the Solar Planetary Nebula will glow in the red colour characteristic of H II emission. Eventually, green oxygen emission and blue helium emission may be seen, as the expelled interior shells become excited by the hot white dwarf at their centre (STSCI, 1999b).

Life on Earth is often called "carbon-based" because it contains organic molecules based on the carbon atom. Indeed, human bodies contain elements like carbon that can only have been produced by nuclear reactions deep insides stars! The Sun will not begin producing carbon until helium burning commences late in the Red Giant phase of its evolutionary development. This will not occur for over 5 billion years. Where, then, did the carbon in our bodies come from? There is only one answer! Carbon in the organic molecules in our bodies can only have been produced in nuclear reaction deep within old stars that shed their matter at the end of their existence!

Where did the iron in the core of the Earth come from? Where did the iron in human blood come from? Not the sun! Iron is produced in nuclear reactions in very massive stars. The iron which comprises the core of the Earth and which surges through our veins in our blood was created in nuclear reactions in very massive stars that exploded in ancient times in a catastrophic event called a supernova.

Our solar system is an example of the universe recycling itself. Old star systems die, expelling matter into the interstellar medium to be used as the raw material for a new generation of stars and their planets.

On Earth, the long process of biological evolution has led to the living creatures that are seen today, using matter that was minted within very ancient stars, far away in time and space. We are made from the ancient matter of the universe; matter which has organised itself and become self-aware.

As the Sun ages, it will become a Red Giant. Although its surface will be relatively cool, its luminosity will increase as its surface area grows and its energy output increases. The inner planets will be engulfed as the sun slowly expands to 200 times its current size.

Mercury and Venus will be the first to be consumed. Initially, the earth will be spared, but its oceans and its atmosphere will boil away as the temperature rises to 2000 degrees Celsius. A swollen red Sun will fill half the sky at noon (STSCI, 1999a). Earth will be no place to live; a living Hell.

Eventually the Earth will be engulfed as the radius of the Sun grows to one Astronomical Unit (Universe, 1999). Well before this occurs, humankind will need to consider evacuating to a safer home.

Titan is a moon in orbit about the planet Saturn. Perhaps Titan might provide a reasonable location for colonisation (STSCI, 1999a). Titan has a relatively dense atmosphere. Although its atmosphere is poisonous to humans, it is dense enough to produce a greenhouse effect, insulating Titan and providing some degree of protection and comfort to its potential residents.

Initially, Titan is far enough away to buy humanity time as the Sun advances into old age. Eventually, the outer planets and their satellites will also become untenable. If humanity is to survive, there will be no choice but to flee the cradle of our species, and find a home amongst the distant stars.

We cannot prevent the inevitable. One day the Sun will die and what is left of the solar system will cease to be a suitable home for humankind. If it is of any consolation, our Sun and its planets will one day become the raw material used to build new stars; perhaps even contributing to the birth of a new race.

After a long journey spanning many generations, Starship Earth Arc will one day arrive at a new world, in orbit around a living star. Only then will the children of Earth disembark from their metallic cocoon to chart a new destiny for humankind. Perhaps they will find a world populated by their distant cousins; beings evolved from matter ejected into space by the same supernovae and planetary nebulae that long ago gave rise to the young Sun, its system of planets, and life on an ancient Earth and Mars.

Balick, B (1999) A Guide to Hubble Space Telescope ("HST") Images of Planetary Nebulae taken by Bruck Balick and his collaborators, http://www.astro.washington.edu/balick/WFPC2/.

Kaufmann, WJ, and Freedman, RA (1999) Universe, 5th ed., WH Freeman & Co, New York.

Nemiroff, R and Bonnel, J, eds. (1999) Astronomy Picture of the Day, http://antwrp.gsfc.nasa.gov/apod/astropix.html.

STSCI (1999a) Life on the Edge, The Association of Universities for Research in Astronomy, http://oposite.stsci.edu/pubinfo/pr/97/38/astrofile2.html.

STSCI (1999b) The Glorious End of Stellar Life, The Association of Universities for Research in Astronomy, http://oposite.stsci.edu/pubinfo/pr/97/38/astrofile1.html.