The Sun is the ultimate source of energy for life on Earth, and it sustains nearly all aspects of human existence. The Sun is an ongoing hydrogen fusion reaction, producing immense quantities of energy in the form of light. At the Earth's surface the Sun's energy evaporates water and thus drives the hydrologic cycle. It powers photosynthesis and it produces the raw material for the creation of fossil fuels. Via its interaction with the Earth, the Sun also drives the seasons, weather and climate, and currents in the oceans. Variations in the Sun's radiation output as well as longer term periodicity of Earth-Sun geometrics are known to strongly affect the Earth's climate over time. The Sun also directly and indirectly supports key environmental services such as the provision of clean water, a stable climate, the creation of fertile soil, nutrient cycles, and biological diversity. Given these vital roles, it is no surprise that the Sun is a dominant historical theme in art, literature, religion, and in the evolution of culture itself.
Not only does the Sun power the natural world, but it is a potential sustainable source of electricity and heat for part of human society's energy needs. Currently, solar power contributes less than one percent of humankind's electrical energy at present. This amount may increase to as much as 5% of society's energy needs by the year 2030. The efficiency of solar panels is also expected to increase significantly in the next decade. The downside of this rapid progress towards more sustainable energy production is the fact that most solar systems presently being manufactured will be economically obsolete in five to seven years.
The temperature (15,000,000°C; 27,000,000°F) and pressure (340 billion times Earth's air pressure at sea level) in the Sun's core is sufficient to cause four protons of hydrogen nuclei to fuse together to form one alpha particle or helium nucleus. The basic fusion reaction is:
41H → 4He + 2e+ 2νe + energy
The alpha particle is about 0.7 percent less massive than the four protons. The difference in mass is expelled as energy and is carried to the surface of the Sun, through a process known as convection, where it is released as light and heat. Energy generated in the Sun's core takes a million years to reach its surface. Every second about 700 million tons of hydrogen are converted into helium. In the process five million tons of pure energy is released to space; therefore, as time goes on the Sun is gradiually becoming lighter.
Luminosity is the amount of energy radiated into space per second by a star. The Sun's luminosity is about is 4 x 1026 watts. To put this into perceptive, the world's largest power generation facility is the Three Gorges Dam in China, which when fully completed will only have a capacity of about 22,500 megawatts (MW). Thus, the Sun's luminosity is about 1.8 x 1016 greater than the Three Gorges Dam.
All of the planets orbit the Sun because of its enormous gravity. The Sun contains more than 99 percent of the entire mass in the solar system (most of the rest is in Jupiter). Its mass is about 1.9 x 1030 kilograms, which is about 333,000 times the Earth's mass and over 1000 times as massive as Jupiter. It has so much mass that it is able to produce its own light. This feature is what distinguishes stars from planets. Its diameter is about 1,392,000 kilometers. This is equal to 109 Earth diameters and almost 10 times the size of the largest planet, Jupiter.
Distance from Earth
The Sun is at an average distance of 93,026,724 miles (149,680,000 km or 1 Astronomical Unit) from the Earth, a distance which sunlight travels in eight minutes, whereas the distance to the moon is just 1.3 light-seconds.
The Sun rotates once on its axis in about 27 days. This rotation axis is tilted by about 7.25 degrees from the Earth's orbital plane (or ecliptic plane). The Sun rotates in the counterclockwise direction (when viewed from the north), the same direction that our solar system's planets rotate (and orbit). The Sun does not rotate rigidly like a solid planets because it is made of gas. The Sun's equatorial regions rotate faster (one rotation in 25.6 days) than its polar regions (36 days).
Composition of the Sun
The Sun is mostly made up of hydrogen (about 92.1% of the number of atoms, 75% of the mass) and (7.8% of the number of atoms and 25% of the mass). The other 0.1% is made up of heavier elements, mainly carbon, nitrogen, oxygen, neon, magnesium, silicon and iron.
The Interior of the Sun
The solar interior is separated into four regions by the different processes that occur there. Energy is generated in the core, the innermost 25% of the interior. Energy diffuses outward by radiation (mostly in the form of gamma-rays and x-rays) through the radiative zone, and by convective fluid flows (boiling motion) through the convection zone, the outermost 30%. The thin interface layer (the "tachocline") between the radiative zone and the convection zone is where the Sun's magnetic field is generated.
The innermost layer of the Sun is the core. The temperature at the very center of the core is about 15,000,000°C (27,000,000°F) and the density is about 150 g/cm3 (about ten times the density of gold or lead). Both the temperature and the density decrease as one moves outward from the center of the Sun. The nuclear fusion process ceases for the most part beyond the outer edge of the core (about 25% of the distance to the surface or 175,000 km from the center). At that point the temperature is only half its central value and the density drops to about 20 g/cm3.
The Radiative Zone
The next layer out from the core is the zone that emits radiation that diffuses outwards. Photons travel in this zone at the speed of light, they bounce so many times through this dense material that an individual photon takes about a million years to finally reach the interface layer. The density drops from 20 g/cm3 (about the density of gold) down to only 0.2 g/cm3 (less than the density of water) from the bottom to the top of the radiative zone. The temperature falls from 7,000,000°C to about 2,000,000°C over the same distance.
The Interface Layer (Tachocline)
This thin layer between the radiative and convective zones generates the Sun's magnetic field. Here the relatively calm motion in the radiative zone transitions to the fluid motions of the convective zone.
The Convective Zone
The convection zone is the outer-most layer of the solar interior. Here the photons continue to make their way outwards via convection (towards lower temperature and pressure). The convective zone extends from a depth of about 200,000 km up to the visible surface. At the base of the convection zone the temperature is about 2,000,000°C. This is "cool" enough for the heavier ions (such as carbon, nitrogen, oxygen, calcium, and iron) to retain electrons. This traps heat that ultimately makes the fluid unstable and it starts to convect.
Hotter gas coming from the radiative zone expands and rises through the convective zone. It does so because the convective zone is cooler than the radiative zone, and is therefore less dense. As the gas rises, it cools and begins to sink again. As it falls down to the top of the radiative zone, it heats up and starts to rise. This process repeats, creating convection currents and the visual effect of "boiling" on the Sun's surface. At the visible surface the temperature has dropped to 5,700°K and the density is only 0.0000002 gm/cm3 (about 1/10,000th the density of air at sea level).
Most of the visible (white) light comes from the photosphere, the part of the Sun we actually see. The photosphere is one of the coolest regions of the Sun (6000 K), so only a small fraction (0.1%) of the gas is ionized (in the plasma state). The photosphere is about 100 km thick, and thus is a very, very, thin slice of the Sun that has a radius of about 700,000 km. The photosphere looks like a disk with some dark spots. These "sunspots" are the site of strong magnetic fields.
The chromosphere is an irregular layer above the photosphere that is about 2500 kilometers thick and exhibits a temperature increase from 6000°C to about 20,000°C.
Just prior to and just after the peak of a total solar eclipse, the chromosphere appears as a thin reddish ring. The conspicuous color of the chromosphere (compared to the mostly white corona) led to its name (meaning "color sphere"). The higher temperatures of the chromosphere causes hydrogen to emit light that gives off a reddish color (H-alpha emission).
The corona is the outermost layer of the Sun that is visible during eclipses. It has a low density cloud of plasma with higher transparency than the inner layers. The white corona is a million times less bright than the inner layers of the Sun, but is many times larger. Its average temperature is one million degrees K (two million degrees F) but in some places it can reach three million degrees K (5 million degrees F). The source of the corona's heat remains a puzzle. It is almost certain that its energy comes from the Sun's internal furnace, which also supplies the rest of the Sun's heat. However, as a rule, temperatures are expected to drop the further one gets from the furnace, whereas the million-degree corona lies outside the surface layer where sunlight originates, whose temperature is less that 6000°C.
Life Cycle of the Sun
Birth and Adolescence
About 4.6 billion years ago, an interstellar cloud began to collapse, forming many dense cores of dust and gas hidden under a thick haze. The cores, perhaps a light year across, continued their slow gravitational implosion. As they spun faster and faster, their shapes changed from roundish globules to flattened pancakes and disks, with most of their mass falling into a central dense ball of contracting gas. Within a million years, the central core temperatures climbed above 10 million degrees and hydrogen atoms began to fuse into deuterium and helium at an accelerated pace. The enormous outward pressure provided by thermonuclear fusion quickly halted the further contraction of the young Sun, and our Sun became a full-fledged star for the first time.
Within 50,000 to 100,000 years the first planets to form were the gas giants, followed millions of years later by the smaller rocky planets in the inner solar system. Once formed, the giant planets felt the friction of the surrounding gaseous disk and they slowly began to fall closer and closer to the Sun. This inward migration stopped once their speed came into equilibrium with the Sun's gravitational attraction.
As the nuclear reactions became more efficient, the infant Sun began to expand very slowly. At first the Sun only shone with 70% of its modern brightness. But as it continued to evolve over eons of time, its brightness grew by 7% every billion years. When trilobites first crawled on shallow ocean bottoms 500 million years ago, the Sun was much fainter in the sky than it is today. Earth would have been in a deep-freeze had it not been for the warming actions of an atmosphere laced with greenhouse gases like water and carbon dioxide.
In the eons to come, the Sun will continue to expand and shine more brightly for the next 5 billion years. Then a major physical change will start to happen with unprecedented speed. The inner core has become heavily laden with the helium atoms of over 9 billion years of fusion. Collapsing steadily under its own weight, it has increased the temperature of the Sun's core, making the Sun expand to find a new equilibrium. When the core temperature reaches around 100 million K, helium will begin to fuse to form carbon. This unleashes a massive increase in energy and pressure, and the Sun's outer layers will be propelled outwards; first beyond the orbit of Mercury, then Venus, and then Earth. The Sun has ended its middle age as a red giant star.
Over the course of the last 3 billion years of its life, the Sun will become a white dwarf and then finally a black dwarf star. During the white dwarf stage, the solar core begins to shrink because it is unable to produce radiation and heat by fusion. At this stage, gravity will begin to pull the entire mass of the Sun inward to form a body the size of the Earth. This white dwarf then rapidly begins to cool. This body will radiate yellow light and then red light, as it uses up its remaining thermal energy. The carbon-oxygen-rich contents of the Sun will then be packed in a space as small as physically possible until no further collapse can occur. At this point in its life, the Sun will continue to cool until it is as cold as interstellar space. At this black dwarf stage, the Sun will no longer emit any form of light.
The Solar Wind
The solar wind contains roughly equal number of electrons and protons, along with a few heavier ions, and blows continuously from the surface of the Sun at an average velocity of about 400 km/second, or about one million miles per hour. This wind leads to a mass loss of more than one million tons of material per second, which may seem like a large number, but is insignificant relative to the total mass of the Sun. The source of the solar wind is the Sun's corona. The temperature of the corona is so high that the Sun's gravity cannot contain it. Scientists do not fully understand how and where the coronal gases are accelerated to these high velocities.
Sunspots and Their Cycle
In 1610, shortly after viewing the Sun with his new telescope, Galileo Galilei made the first European observations of sunspots. Sunspots are cooler regions on the photosphere that are formed where denser bundles of magnetic field lines from the solar interior break through the surface. Since they are 1000-1500 K cooler than the rest of the photosphere, they do not emit as much light, and appear darker. They can last a few days to a few months. Their sizes vary over a wide range, with a few having been measured to be 50,000 km in diameter.
The "sunspot number" is calculated by first counting the number of sunspot groups and then the number of individual sunspots. A German amateur astronomer, Heinrich Schwabe, published a paper in 1843 that stated that the number of sunspots visible on average varied with a period of about ten years. This conclusion has been substantiated by observations over the 140 years since. The period of repetition on average is 11.1 years, but has been as short as eight years and as long as 16 years.
Cyclical nature of sunspots has been under intense study for the last several decades, since this phenomenon has a pronounced effect upon the Earth's climate trends. There are clear correlations between periods of intense sunspot activity and elevated surface temperatures on the Earth. The contribution to global warming trends is not fully understood at present.
- Kenneth J.H. Phillips. 1995. Guide to the Sun. Cambridge University Press, Cambridge United Kingdom.
- P. Riley, J.A. Linker and Z. Miki?, Z. 2002. Modeling the heliospheric current sheet: Solar cycle variations. Journal of Geophysical Research 107 (A7): SSH 8–1.
- K.-P. Schröder and R.C. Smith. 2008. Distant future of the Sun and Earth revisited. Monthly Notices of the Royal Astronomical Society 386 (1): 155.
- M. Woolfson. 2000. The origin and evolution of the solar system. Astronomy & Geophysics 41: 1.12.
- Jack B. Zirker. 2002. Journey from the Center of the Sun. Princeton University Press, Princeton, USA.
Editor's note: A similar version of this article has been published in The Energy Library under authorship by Cutler Cleveland.