The Sun is the ultimate source for life on Earth, and it sustains nearly all aspects of human existence. The Sun evaporates water and thus drives the hydrologic cycle, it powers photosynthesis, and it created the raw material for the generation of fossil fuels.Via its interaction with the earth, the Sun also drives the seasons, currents in the oceans, weather and climate. 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.

The Sun. Credit: NASA

Energy Generation

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 or hydrogen nuclei to fuse together to form one alpha particle or helium nucleus. The basic fusion reaction is:

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 ashes. In the process 5 million tons of pure energy is released; therefore, as time goes on the Sun is becoming lighter.

Energy Output

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 in perceptive, the world's largest power generation facility is the Three Gorges Dam in China which, when full completed, will 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.

Size

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 1,000 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 93,026,724 miles (149,680,000 km or 1 Astronomical Unit) from the Earth, a distance which sunlight covers in 8 minutes, whereas the distance to the moon is just 1.3 light-seconds.

Rotation

The Sun rotates on its axis once in about 27 days. Its rotation axis is tilted by about 7.25 degrees from the axis of the Earth's orbit so we see more of the Sun's north pole in September of each year and more of its south pole in March. The sun does rotate rigidly like solid planets and moons do because it's a ball of gas. The Sun's equatorial regions rotate faster (one rotation in 24 days) than its polar regions (30 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.

Solar Structure

The Interior of the Sun

Sun structure. Schematic figure constructed by Jouni Jussila using material courtesy of SOHO/LASCO consortium.

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 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 Core

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 10 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 burning 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. Although the photons travel 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 generates the sun's magnetic field. Here the relative calm motion in the radiative zone transition 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 "boil" or 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 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).

The Photosphere

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

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

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 1 million K (2 million degrees F) but in some places it can reach 3 million °K (5 million °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 fetal 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. Once the Sun's winds had completed their work, the frictional decay of orbits ceased and the giant planets took up their present orbital positions.

Middle Age

As the nuclear fires 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 trace gases like water and carbon dioxide-rich.

In the eons to come, the sun will continue to expand and shine more brightly for the next 6 billion years. Then a major physical change will start to happen with unprecedented speed. The inner core has become heavily laden with the helium 'ash' of over 11 billion years of fusion. Collapsing steadily under its own weight, it has increased the temperature of the sun's core making the fusion reactions burn more fiercely, and making the sun expand to find a new equilibrium. But suddenly a tipping point is reached and the inert helium ash begins to fuse to form carbon. This unleashes a massive increase in energy and pressure and the sun's outer layers are 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.

Death

Over the course of a few 100 million years, the sun continues to shed much of its mass into space as a red giant, and later forms a spectacular 'planetary nebula' as its last gesture. There are numerous examples known to astronomers of what happens to stars like our sun when they reach their last few millennia of life. Spanning nearly a light year or more, the illuminated veil of gases from the dying sun expand out into space until they invisibly mix with the other gases in interstellar space. Deep within the nebula, a brilliant white dwarf remains; the last vestige of the sun seen as a hydrogen, oxygen and carbon-rich ember. At first it glows brilliant white at a temperature of 100,000 degrees, but with no nuclear fusion to sustain it, it is destined to cool to a blackened hulk after another trillion years.

The Solar Wind

Solar Wind Credit: NOAA

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 1 million miles per hour. This wind leads to a mass loss of more than 1 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

Sunspots. Credit: NASA

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 10 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 8 years and as long as 16 years.

Sources

• Hamilton, Calvin J., Views of the Solar System, Accessed 21 August 2008.
• National Aeronautics and Space Administration, Solar Structure - A Closer Look, Accessed 21 August 2008.
• National Aeronautics and Space Administration, Living in the Atmosphere of the Sun, Accessed 21 August 2008.
• National Aeronautics and Space Administration, The solar interior, Accessed 21 August 2008.
• Department of Physics and Astronomy, University of Tennessee, Lecture Notes for Astronomy 162, Accessed 21 August 2007.