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Sun Microsystems, Inc. Business Information, Profile, and History

901 San Antonio Road
Palo Alto, California 94303
U.S.A.

Company Perspectives:

Sun was founded with one driving vision. A vision of computers that talk to each other no matter who built them. A vision in which technology works for you, not the other way around. While others protected proprietary, stand-alone architectures, we focused on taking companies into the network age. As a result, we've become the dot in .com, providing systems and software with the scalability and reliability needed to drive the electronic marketplace.

History of Sun Microsystems, Inc.

Archrival of Microsoft Corporation, Sun Microsystems, Inc. focuses on network computing rather than desktop mainframes, designing and manufacturing its own software and hardware. A 1980s start-up company, Sun generated in excess of $10 billion in sales during the late 1990s, recording astounding success with its own computer chip, SPARC, and own operating system, Solaris. Sun pioneered the use of shared software and hardware components among competing workstation manufacturers in order to create industry standards. After making a reputation for itself as a designer of high-powered workstation computers and servers, Sun expanded its talents, positioning itself as an Internet and electronic-commerce specialist during the latter half of the 1990s. The company's seminal achievement was the introduction of Java technology, the first universal software platform that enabled developers to write applications once to run on any computer. The company maintained offices in 150 countries, selling its products and services to the telecommunications, manufacturing, education, financial, and government markets.

Founding

Sun began as a computer project designed by Andreas Bechtolsheim while he was a graduate student at Stanford. His computer was a modification of a relatively new kind of computer, the workstation, which, like the PC (personal computer), can be utilized by single users. The workstation, however, provides users with more power. Workstations are designed for network integration and equipped with high-resolution graphics, and are fast enough to handle demanding engineering and graphics tasks. Unlike the first workstations, which had been introduced to the market only the previous year by Apollo Computer, Bechtolsheim's workstation used off-the-shelf parts, thus making it more affordable.

Bechtolsheim not only shunned custom-made hardware, but also broke with the industry tradition of adhering to proprietary operating system software. Instead, he hoped to enable different workstation brands running on a common operating system to share data. AT&T's UNIX operating system was the obvious choice; it could operate on a wide variety of computers and was already very popular among scientists and engineers because it enabled users to perform several tasks on screen at once. He began selling licenses for his computer, called the Sun (which stood for Stanford University Network) at $10,000 each in 1981.

Within a year Bechtolsheim's project attracted the interest of Stanford M.B.A. graduates Vinod Khosla and Scott McNealy, each of whom had some experience in the computer business. They were named president and director of manufacturing, respectively, of Sun Microsystems, Inc., upon its founding in February 1982. Bechtolsheim, who was the brains behind the hardware, became vice-president of technology. One of the first people the founders hired was Bill Joy, a Berkeley Ph.D. well known for his design of a popular version of the UNIX operating system. His task was to design the company's software.

Sun's use of standard hardware components and standard operating system software produced short-term payoffs for the fledgling company. Sun's workstations, unlike those of industry pioneer Apollo, operated on UNIX and from the outset networked easily with the hardware and software already on the market. In addition, although Sun's design could easily be copied, the strategy of using existing technologies allowed Sun to enter the market quickly with a low-priced machine. Sales grew rapidly as a result. Within six months of incorporation the company became profitable.

Sun's first workstations, the Sun-1 and Sun-2, were instant successes, achieving $8 million in sales the first year, 80 percent of which came from sales to the university market. Sun's founders, however, had their eyes on the mainstream technical market, dominated at that time by the major computer companies. Sun's first big success in this area was the contract it signed in its second year with ComputerVision, a major CAD (computer-aided design) systems supplier that had decided to drop its proprietary hardware in favor of a new platform for its software products. ComputerVision had decided to sign a contract with Apollo, but, aggressively courted by Sun executives, the company reversed its decision and accepted a counteroffer made by Sun. Thus, Sun established its reputation as a serious player in the computer business and simultaneously earned the envious wrath of its competitors.

Growth in the Late 1980s

Expanding rapidly, Sun moved out of its original location in Santa Clara to a larger building in Mountain View, which became its new headquarters. In January 1984 Sun opened its first European sales office. In that same year Sun established a subsidiary, Sun Federal, to serve the government market. By 1991 Sun Federal was shipping more than half the workstations ordered by local, state, and federal government. Sun's informal corporate culture attracted engineers from the top universities. At the same time Sun hired additional managers who had experience working at other leading computer companies. Also in 1984 McNealy took over as president, as Khosla realized his dream of being able to retire as a millionaire before the age of 30.

During this period Sun continued to promote open systems. In 1984 it began broadly licensing Joy's design of a distributed file system software, called NFS (Network File System), that allowed data to be shared among many users in a network regardless of processor type, operating system, or communications system. NFS soon became an industry standard. Sun was so successful with this strategy that in 1984 Apollo was forced to abandon its exclusive design and instead produce a system that operated with standard software.

Between 1985 and 1989 Sun was the fastest-growing company in the United States, according to Forbes magazine, with a compound annual growth rate of 145 percent. It had become a public company with its successful initial public offering in 1986. The following year Sun surpassed Apollo in sales, and by the close of that year it had become the leader in workstation sales. Only six years after incorporation Sun achieved $1 billion in annual sales. Part of the reason for Sun's stupendous early success was the fact that the product in which it chose to specialize, the workstation, was becoming popular just at the time Sun entered the market. Furthermore, because it was a workstation industry pioneer, it established strong relations with the most sought-after clients and the most important software developers. Sun's corporate strategy also enabled it to offer its new customers the latest technology, while its competitors had to support established clients reluctant to scrap their outdated computer systems. Industrywide, sales of workstations rapidly displaced those of minicomputers, and the large computer companies that sold these had to compensate by offering workstations as well.

Debut of SPARC: Late 1980s

In the increasingly competitive market for workstations, where the speed of the computer is an important factor, Sun developed an even faster workstation in the late 1980s. Based on a different kind of microprocessor, this new product utilized RISC (reduced instruction set computing) architecture. RISC was simpler yet quicker than the then prevailing CISC (complex instruction set computing) architecture. As had been the case with the workstation itself, Sun was not the first company to design a RISC-based computer (IBM had introduced a model in 1986). Sun made improvements on it, however, and designed its own RISC architecture called SPARC (scalable performance architecture); it soon dominated the market of RISC-based workstations. In April 1989 Sun introduced its SPARCstation 1, a small, low-cost desktop computer with expanded capabilities. SPARCstation 1 employed new levels of integration and miniaturized the essential electronic components. By the end of the year Sun could claim to be the world's largest supplier of RISC-based computers, with the SPARCstation the most popular workstation on the market.

As Sun was not a manufacturer of its own processors or computer chips, in 1987 it licensed Bechtolsheim's SPARC design to a few silicon chip manufacturers, which then began to produce them for Sun's needs. Then, in keeping with its tradition of the "open system," in July 1988 Sun announced that it would offer its RISC design for license to other computer makers in recognition that for RISC to succeed it needed to become a pervasive presence in the marketplace. By licensing SPARC it stimulated low-cost, high-volume production of SPARC systems and thus increased the number of third party applications available. In 1989 licensing of SPARC was turned over to a new coalition of computer companies called SPARC International, an independent testing organization founded in nearby Menlo Park, California. McNealy hoped SPARC would produce the same kind of phenomenal growth for workstations that IBM brought to PCs a decade earlier when it permitted others to copy its standard PC hardware and software designs. In April 1991, however, Sun told its dealers it would prefer that they not sell SPARC clones. Sun claimed that small dealers would have difficulty succeeding against Sun in selling "clones" and were thus encouraging the smaller outfits to sell complementary "compatible" products, whereupon competitors charged hypocrisy in Sun's call for "open systems." Although it did not at first entirely convince other workstation companies to copy Sun's SPARC design, Sun was singlehandedly making SPARC one of the international standards. By 1992 all its new workstations were based only on SPARC.

As Sun was developing its SPARCstation computer, it was also making moves to ensure the presence of improved software to take advantage of it. In 1987 Sun signed an agreement with AT&T to develop an enhanced version of the UNIX operating system to make it the software standard for workstations. AT&T even took a 19 percent equity investment in Sun in 1988 (which it sold off in 1991 upon the NCR acquisition). The product that emerged in late 1989 established a de facto high-end UNIX standard (System V Release 4.0). It was at this time that competing computer manufacturers were settling on UNIX as a universal operating system, and RISC-based hardware proved the obvious supporting standard because of its speed in handling the complexities of UNIX and its suitability for the demands of the new user interfaces and applications software. Sun Microsystems, with its RISC-based SPARCstation and involvement in upgrading UNIX, was well-positioned to take advantage of the trend. "Sun is the strongest candidate to carry the UNIX banner. It has momentum. If it can keep up the recent good work, it can continue to dominate the workstation market," wrote technology consultant Richard Shaffer in Forbes in 1990.

Despite the success of the SPARCstation, the year of its introduction, 1989, marked a temporary financial setback for Sun. It lost money during the difficult product transition period by launching the new SPARCstation 1 while at the same time trying to support two older product lines using different technologies. Meanwhile, it was encountering difficulties managing the chaos resulting from its explosive growth. Problems included rapid personnel hiring and training, communications problems, and reorganization pains. A new management information system did not accurately forecast parts needed to fill orders, and demand for SPARCstation 1 was misjudged. That year Sun also temporarily lost its market lead in workstation shipments when Hewlett-Packard purchased Apollo and combined their market shares.

Things improved rapidly the following year. The company reduced its product families from three to one, the SPARC systems. The SPARCstation 2, released in November 1990, had the power of a minicomputer. The financial outlook improved, with revenues up by 40 percent over the previous year, and for the first time in a long while Sun was spending less than it was taking in. By the end of 1990 Sun claimed more than a third of the total market share of workstation shipments, leaving Hewlett-Packard a distant second at 20 percent. Sun held a similar share of the world market of RISC technology with its SPARC product line. As the market continued to grow, Sun aimed at expanding at a similar rate, maintaining the same market share. Meanwhile, its stock doubled from a low of $14 in August 1989 to $37 in July 1990.

At the beginning of the 1990s Sun further widened its market objectives for its workstations beyond engineers, software developers, and chip designers, targeting commercial users such as insurance companies, brokerages, airlines, and publishers. In the spring of 1990 Sun announced a new line of low-end products designed to capture an increasing share of the vast commercial computing market, which was dominated by minicomputers and high-end PCs. Sun became the first workstation producer to introduce a low-end system for under $5,000. A month later the company announced the first color workstation for less than $10,000. It also began distributing its products through respected PC resellers. Sun was able to persuade software publishers to adapt over 2,800 programs for SPARC computer systems by 1991, including such major programs as Lotus 1-2-3, WordPerfect, and dBase IV, thus substantially broadening Sun's commercial market. By the end of 1992, when over a third of Sun's sales were to commercial as opposed to technical markets, there were more applications for Sun workstations than for any other UNIX workstation.

Business strategies in 1990 included streamlining the organization into two core management groups. Custom job-shop manufacturing was eliminated, allowing high volume from a single, elegantly designed product line to permit Sun's manufacturing system to attain economies of scale. More of the working capital and investment risk was pushed onto outside contractors that produce the printed circuit boards, boxes, and screens, leaving Sun with the relatively simple tasks of assembly and testing. It stayed out of the lucrative high-end of the workstation market to build on volume and market share in the lower end. By the close of 1990 Sun was one of the top ten computer hardware companies in the country, but unlike most of the others, it sold only workstations and servers: it did not sell PCs, minicomputers, or mainframes.

Sun had in the past attempted to build a critical mass for its technology and establish a de facto standard in hardware. In September 1991 it aimed at a similar broadening of its influence in operating system software when it announced plans to make the Sun OS operating system, a version of UNIX, run on more computers than just its own, including those running on Intel microprocessors. It was at this time that Kodak sold its UNIX software unit, Interactive Systems, to Sun. Interactive supplied UNIX System V release 4.0 for Intel-based computers, and thus the purchase of Interactive endowed Sun with needed expertise in the arena of Intel-based UNIX systems. Interactive had already previously agreed to install Sun's operating system, Solaris 2.0, onto Intel X86 architecture. With more computers using Sun's operating system, it would become easier to link Sun workstations with others in a network, and more software could be written for Sun's operating system. Sun needed a constant flow of new programs to keep its workstation sales booming, particularly now that it was facing challenges in hardware.

In 1991 Sun followed IBM and Apple by becoming a hybrid software-hardware company. This new strategy was an attempt to offset shrinking profit margins on hardware by selling software. A reorganization of the company transferred its software-selling operations to two new subsidiaries, SunSoft and Sun Technology Enterprises. SunSoft sold Sun's operating system to computer manufacturers, while Sun Technology Enterprises supplied software for SPARC machines, such as networking, printing, imaging, and PC emulation products. At the same time other core businesses and functions were also reorganized into subsidiaries. The largest of these was Sun Microsystems Computer Corporation, which McNealy headed in addition to his post of CEO of the parent company. Each subsidiary was set up as a separate profit and loss center having its own management to oversee product development, manufacturing, marketing, and sales.

By 1991 Sun's product line was beginning to show its age as competitors brought out machines superior in both price and performance. In the early 1990s the workstation market competition grew increasingly fierce, as it was one of the few areas of the computer industry still enjoying sales growth of more than 20 percent annually in 1991. One of the reasons for this growth was the RISC technology and the recent emphasis on serving the general business computing market. As Sun was trying to enter the office market, however, office computing companies such as IBM, Apple, Compaq, Digital Equipment, and Hewlett-Packard were pursuing the technical market, and Sun's move into the broader commercial computing market put it into competition with the bigger computer manufacturers on their home turf. Sun also reversed itself by moving into the high-end of the workstation market, where performance speeds were essential, using multiprocessors (two or more processors chained together) and special software. It introduced its first multiprocessor, the SPARCserver 600NO series, and new operating software for it in 1991.

By mid-1992, Sun had 21 subsidiaries around the world providing sales, service, and technical support, and overseas sales accounted for more than half of its revenues. Manufacturing was carried out at three sites: Milpitas, California; Westford, Massachusetts; and Linlithgow, Scotland. In February 1992 Sun became the first U.S. company to establish a significant presence in Moscow. Sun forged an agreement with a group of 50 Russian scientists, including the Russian scientist who had developed supercomputers in the Soviet Union, to work as contractors with the company.

Introduction of Java: 1995

Sun's tenth anniversary marked the conclusion of a decade of remarkable success, but not all industry experts were willing to bet that the company's second decade of business would be as successful as the first. With the enormous growth rate of the PC market and the proliferation of competitive workstations being offered by other manufacturers, Sun faced a difficult road ahead, industry pundits explained, and would be hard pressed to sustain its growth rate throughout the 1990s. The experts were wrong. Sun recorded prolific growth in the years following its tenth anniversary, demonstrating enviable success by focusing on high-end servers priced from $500,000 and up. The most significant facet of the company's business, however, was the introduction of a new product in the mid-1990s that forced analysts to quickly change their opinion about Sun's growth potential. In mid-1995, Sun introduced Java, a brand name that stood for a programming language and a set of components and tools that allowed users to write software across any computer and operating system. The potential for Java was vast, exuding the universality McNealy had preached for years. In essence, Java represented a self-sufficient computing system, emulating all the functions of the computing device, regardless of the underlying operating system.

Following the introduction of Java, McNealy found a more receptive audience to his vision of a computer world based on networks supported by powerful, high-end servers, a vision that ran counter to the approach taken by Microsoft's founder Bill Gates. McNealy reveled in his attacks against Microsoft, both in the press and in court, as he fought against "Wintel," the duopoly held by Microsoft's Windows and Intel's processing chips. "The PC is just a blip," McNealy remarked in an interview with Business Week in 1999, dismissing the significance of the PC revolution led by Microsoft. "It's a big, bright blip, but just a blip. Fifty years from now, people are going to look back and say: 'Did you really have a computer on your desk? How weird."' McNealy envisioned network computing as the future, a future in which the billions of computer chips in products ranging from refrigerators and telephones to smart cards and door locks would all be connected in a network.

By 1998, Sun's revenues had increased to $10 billion and its net income, after more than doubling since the mid-1990s, had reached $763 million. In keeping with McNealy's posture as an industry renegade, Sun operated as the only major hardware and software vendor without a cooperative relationship with Microsoft. Because of the company's independent stance, its corporate structure was reorganized in 1998 to better contend with competitors such as Microsoft. "Our goal," Sun's chief operating officer explained to Electronic News at the time of the reorganization, "is to align the organization more tightly and streamline internal processes so that we achieve greater operation efficiency and provide a unified face to the customer." The five companies that had operated as autonomous businesses were stripped of their independence and restructured into seven divisions focused on market segments and industries.

As Sun pressed ahead with turning McNealy's vision into reality, forging alliances with other companies ranked as a primary objective. To succeed in the long-term, the company needed to lead a counterrevolution and convince other computer manufacturers and electronics companies that the future was networks. Following America Online's acquisition of Netscape Communications, Sun signed a three-year alliance with America Online that bolstered Java's presence on the Internet. In 1999, Sun signed Java technology licensing agreements with Sony, Motorola, Ericsson, Samsung, Alcatel, Nortel, OpenTV, BEA Systems, Siemens-Nixdorf, and Scientific Atlanta. The last year of the decade also marked the introduction of a new software technology called Jini, which served as the cornerstone of McNealy's dream to link a vast array of electronic devices. Launched in January 1999, Jini technology eliminated many of the problems associated with connecting computers and other devices, such as printers, copiers, and fax machines, to a network.

As Sun prepared for the 21st century, much of the company's long-term success depended on the widespread acceptance of McNealy's iconoclastic perspective. Toward this end, there were positive signs supporting the Sun vision. Tele-Communications, Inc., for example, was planning to use Java to deliver telephone service, bill-paying, and other services through television set-top boxes. Java also was attracting interest from manufacturers of consumer devices such as wireless telephones, smart cards, and video game consoles. Although the company had its fair share of critics, its ability to record robust growth while exploring alternative approaches to computing earned the respect of many. "If you want to know where the computer industry is going," an analyst informed Business Week, "ask Sun." Another analyst commented to Business Week, "There have been times when Sun seemed way out of sync, yet two or three years later, we see the rest of the industry moving in their direction." Whether or not McNealy's blueprint for the future would prevail remained a question to be answered in the 21st century.

Principal Subsidiaries: Sun Microsystems Computer Corporation; SunSoft, Inc.; Sun Technology Enterprises, Inc.; Sun Express, Inc.; Sun Microsystems Laboratories, Inc.; Java Software; Sitka Corp.; SunPro Inc.; SunSelect.

Related information about Sun

The central object of our Solar System and the nearest star to the Earth. Its basic characteristics are: mass

99×10³⁰ kg; radius 696 000 km/432 500 mi; mean density 4 g/cm³; mean rotation period 2 4 days; luminosity 85×10²⁴ J/s. Its average distance from Earth is 150 million km/93 million mi, and on account of this proximity it is studied more than any other star. The source of its energy is nuclear reactions in the central core (temperature 15 million K, relative density 155) extending to a quarter of the solar radius and including half the mass. Our Sun is nearly 5000 million years old, and is about halfway through its expected life-cycle. Every second it annihilates 5 million tonnes of matter, to maintain power output of 39 × 10²⁶ watts of energy.otheruses

Observation data
Mean distance from Earth	149.6 km (92.95 mi) (8.31 minutes at the speed of light)
Visual brightness (V)	−26.8^m
Absolute magnitude	4.8^m
Spectral classification	G2V
Orbital characteristics
Mean distance from Milky Way core	17}} km (26,000-28,000 light-years)
Galactic period	8}} a
Velocity	217 km/s orbit around the center of the Galaxy, 20 km/s relative to average velocity of other stars in stellar neighborhood
Physical characteristics
Mean diameter	1.392 km (109 Earth diameters)
Circumference	4.373 km (342 Earth diameters)
Oblateness	−6}}
Surface area	6.09 km² (11,900 Earths)
Volume	1.41 km³ (1,300,000 Earths)
Mass	30}} kg(332,946 Earths)
Density	1.408 g/cm³
Surface gravity	273.95 m s^-2 (27.9 g)
Escape velocity from the surface	617.54 km/s (55 Earths)
Surface temperature	5785 K
Temperature of corona	5 MK
Core temperature	~13.6 MK
Luminosity (L_sol)	26}} W~9 cd[[cite journal	last=Menat	first=M.	year=1980	month=10	url=adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1980ApOpt..19.3458M&db_key=AST&data_type=HTML&format=&high=44b52c369030002	title=Atmospheric phenomena before and during sunset	journal=Applied Optics	volume=19	27]] cd (V band) (~100 lm/W efficacy)
Mean Intensity (I_sol)	7}} W m^-2sr^-1
Rotation characteristics
Obliquity	7.25 ° (to the ecliptic) 67.23° (to the galactic plane)
Right ascension of North pole[[cite web	url=www.hnsky.org/iau-iag.htm	title=Report Of The IAU/IAG Working Group On Cartographic Coordinates And Rotational Elements Of The Planets And Satellites: 2000	accessdate=2006-03-22	first=P. Thomas	year=2000]]	286.13° (19 h 4 min 30 s)
Declination of North pole	+63.87° (63°52' North)
Rotation period at equator	25.3800 days (25 d 9 h 7 min 13 s)
Rotation velocity at equator	7174 km/h
Photospheric composition (by mass)
Hydrogen	73.46 %
Helium	24.85 %
Oxygen	0.77 %
Carbon	0.29 %
Iron	0.16 %
Neon	0.12 %
Nitrogen	0.09 %
Silicon	0.07 %
Magnesium	0.05 %
Sulphur	0.04 %

The Sun is the star of our solar system. The Earth and other matter (including other planets, asteroids, meteoroids, comets and dust) orbit the Sun, which by itself accounts for more than 99% of the solar system's mass. Energy from the Sun?in the form of insolation from sunlight?supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather.

The Sun is sometimes referred to by its Latin name Sol or by its Greek name Helios. Its astrological and astronomical symbol is a circle with a point at its center: bigodot.

The sun's history and destiny

The Sun is about 4.6 billion years old and is about halfway through its main-sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than 4 million tonnes of matter are converted into energy within the Sun's core, producing neutrinos and solar radiation.

In about 5 billion years, the Sun will evolve into a red giant and then a white dwarf, creating a planetary nebula in the process. The Sun's magnetic field gives rise to many effects that are collectively called solar activity, including sunspots on the surface of the Sun, solar flares, and variations in the solar wind that carry material through the solar system. The effects of solar activity on Earth include auroras at moderate to high latitudes, and the disruption of radio communications and electric power. Solar activity is thought to have played a large role in the formation and evolution of the solar system, and strongly affects the structure of Earth's outer atmosphere.

Although it is the nearest star to Earth and has been intensively studied by scientists, many questions about the Sun remain unanswered, such as why its outer atmosphere has a temperature of over a million K while its visible surface (the photosphere) has a temperature of less than 6,000 K. Current topics of scientific inquiry include the sun's regular cycle of sunspot activity, the physics and origin of solar flares and prominences, the magnetic interaction between the chromosphere and the corona, and the origin of the solar wind.

Overview

About 74% of the Sun's mass is hydrogen, 25% is helium, and the rest is made up of trace quantities of heavier elements. "G2" means that it has a surface temperature of approximately 5,500 K, giving it a white color, which because of atmospheric scattering appears yellow. This means that it generates its energy by nuclear fusion of hydrogen nuclei into helium and is in a state of hydrostatic balance, neither contracting nor expanding over time. Because of logarithmic size distribution, the Sun is actually brighter than 85% of the stars in the Galaxy, most of which are red dwarfs. www.space.com/scienceastronomy/060130_mm_single_stars.html

The Sun orbits the center of the Milky Way galaxy at a distance of about 25,000 to 28,000 light-years from the galactic center, completing one revolution in about 225?250 million years. The orbital speed is 217 km/s, equivalent to one light-year every 1,400 years, and one AU every 8 days.

The Sun is a third generation star, whose formation may have been triggered by shockwaves from a nearby supernova. This is suggested by a high abundance of heavy elements such as gold and uranium in the solar system; these elements could most plausibly have been produced by endergonic nuclear reactions during a supernova, or by transmutation via neutron absorption inside a massive second-generation star.

Sunlight is the main source of energy near the surface of Earth. Sunlight on the surface of Earth is attenuated by the Earth's atmosphere so that less power arrives at the surface—closer to 1,000 watts per directly exposed square meter in clear conditions when the Sun is near the zenith. This energy can be harnessed via a variety of natural and synthetic processes—photosynthesis by plants captures the energy of sunlight and converts it to chemical form (oxygen and reduced carbon compounds), while direct heating or electrical conversion by solar cells are used by solar power equipment to generate electricity or to do other useful work. The energy stored in petroleum and other fossil fuels was originally converted from sunlight by photosynthesis in the distant past.

Sunlight has several interesting biological properties. Ultraviolet light from the Sun has antiseptic properties and can be used to sterilize tools. It also causes sunburn, and has other medical effects such as the production of Vitamin D. Its current age, determined using computer models of stellar evolution and nucleocosmochronology, is thought to be about 4.57 billion years.

The Sun does not have enough mass to explode as a supernova. The Sun is a near-perfect sphere, with an oblateness estimated at about 9 millionths, which means that its polar diameter differs from its equatorial diameter by only 10 km. While the Sun does not rotate as a solid body (the rotational period is 25 days at the equator and about 35 days at the poles), it takes approximately 28 days to complete one full rotation; Tidal effects from the planets do not significantly affect the shape of the Sun, although the Sun itself orbits the center of mass of the solar system, which is located nearly a solar radius away from the center of the Sun mostly because of the large mass of Jupiter.

The Sun does not have a definite boundary as rocky planets do; This is simply the layer below which the gases are thick enough to be opaque but above which they are transparent; Most of the Sun's mass lies within about 0.7 radii of the center.

The solar interior is not directly observable, and the Sun itself is opaque to electromagnetic radiation. However, just as seismology uses waves generated by earthquakes to reveal the interior structure of the Earth, the discipline of helioseismology makes use of pressure waves (infrasound) traversing the Sun's interior to measure and visualize the Sun's inner structure. Energy is produced by exothermic thermonuclear reactions (nuclear fusion) that mainly convert hydrogen into helium, helium into carbon, carbon into iron. All of the energy produced by fusion in the core must travel through many successive layers to the solar photosphere before it escapes into space as sunlight or kinetic energy of particles.

About 8.9 protons (hydrogen nuclei) are converted into helium nuclei every second, releasing energy at the matter-energy conversion rate of 4.26 million tonnes per second, 383 yottawatts (383 W) or 9.15 megatons of TNT per second. The rate of nuclear fusion depends strongly on density, so the fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the fusion rate and again reverting it to its present level.

The high-energy photons (gamma and X-rays) released in fusion reactions take a long time to reach the Sun's surface, slowed down by the indirect path taken, as well as by constant absorption and reemission at lower energies in the solar mantle. For many years measurements of the number of neutrinos produced in the Sun were much lower than theories predicted, a problem which was recently resolved through a better understanding of the effects of neutrino oscillation. while the material grows cooler as altitude increases, this temperature gradient is slower than the adiabatic lapse rate and hence cannot drive convection. Heat is transferred by radiation—ions of hydrogen and helium emit photons, which travel a brief distance before being reabsorbed by other ions. Convective overshoot is thought to occur at the base of the convection zone, carrying turbulent downflows into the outer layers of the radiative zone.

The thermal columns in the convection zone form an imprint on the surface of the Sun, in the form of the solar granulation and supergranulation. Sunlight has approximately a black-body spectrum that indicates its temperature is about 6,000 K (10,340°F / 5,727 °C), interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of about 10²³ m⁻³ (this is about 1% of the particle density of Earth's atmosphere at sea level).

During early studies of the optical spectrum of the photosphere, some absorption lines were found that did not correspond to any chemical elements then known on Earth. In 1868, Norman Lockyer hypothesized that these absorption lines were because of a new element which he dubbed "helium", after the Greek Sun god Helios. They can be viewed with telescopes operating across the electromagnetic spectrum, from radio through visible light to gamma rays, and comprise five principal zones: the temperature minimum, the chromosphere, the transition region, the corona, and the heliosphere. The heliosphere, which may be considered the tenuous outer atmosphere of the Sun, extends outward past the orbit of Pluto to the heliopause, where it forms a sharp shock front boundary with the interstellar medium. The increase is because of a phase transition as helium within the region becomes fully ionized by the high temperatures. Rather, it forms a kind of nimbus around chromospheric features such as spicules and filaments, and is in constant, chaotic motion. The transition region is not easily visible from Earth's surface, but is readily observable from space by instruments sensitive to the far ultraviolet portion of the spectrum.

The corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona merges smoothly with the solar wind that fills the solar system and heliosphere. While no complete theory yet exists to account for the temperature of the corona, at least some of its heat is known to be from magnetic reconnection.

The heliosphere extends from approximately 20 solar radii (0.1 AU) to the outer fringes of the solar system. Its inner boundary is defined as the layer in which the flow of the solar wind becomes superalfvénic—that is, where the flow becomes faster than the speed of Alfvén waves. The solar wind travels outward continuously through the heliosphere, forming the solar magnetic field into a spiral shape, until it impacts the heliopause more than 50 AU from the Sun. In December 2004, the Voyager 1 probe passed through a shock front that is thought to be part of the heliopause. The magnetic field gives rise to strong heating in the corona, forming active regions that are the source of intense solar flares and coronal mass ejections. The polarity of the leading sunspot alternates every solar cycle, so that it will be a north magnetic pole in one solar cycle and a south magnetic pole in the next.

The solar cycle has a great influence on space weather, and seems also to have a strong influence on the Earth's climate. During this era, which is known as the Maunder minimum or Little Ice Age, Europe experienced very cold temperatures. Earlier extended minima have been discovered through analysis of tree rings and also appear to have coincided with lower-than-average global temperatures. The Van Allen belts consist of an inner belt composed primarily of protons and an outer belt composed mostly of electrons. The most energetic particles can 'leak out' of the belts and strike the Earth's upper atmosphere, causing auroras, known as aurorae borealis in the northern hemisphere and aurorae australis in the southern hemisphere. In periods of normal solar activity, aurorae can be seen in oval-shaped regions centered on the magnetic poles and lying roughly at a geomagnetic latitude of 65°, but at times of high solar activity the auroral oval can expand greatly, moving towards the equator. Theories proposed to resolve the problem either tried to reduce the temperature of the Sun's interior to explain the lower neutrino flux, or posited that electron neutrinos could oscillate, that is, change into undetectable tau and muon neutrinos as they traveled between the Sun and the Earth. Several neutrino observatories were built in the 1980s to measure the solar neutrino flux as accurately as possible, including the Sudbury Neutrino Observatory and Kamiokande.

Coronal heating problem

The optical surface of the Sun (the photosphere) is known to have a temperature of approximately 6,000 K. The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of large solar flares and myriad similar but smaller events.

Currently, it is unclear whether waves are an efficient heating mechanism.

Faint young sun problem

Theoretical models of the sun's development suggest that 3.8 to 2.5 billion years ago, during the Archean period, the Sun was only about 75% as bright as it is today. The general consensus among scientists is that the young Earth's atmosphere contained much larger quantities of greenhouse gases (such as carbon dioxide and/or ammonia) than are present today, which trapped enough heat to compensate for the lesser amount of solar energy reaching the planet.

Magnetic field

All matter in the Sun is in the form of gas and plasma because of its high temperatures. The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field loops to erupt from the Sun's surface and trigger the formation of the Sun's dramatic sunspots and solar prominences (see magnetic reconnection). This twisting action gives rise to the solar dynamo and an 11-year solar cycle of magnetic activity as the Sun's magnetic field reverses itself about every 11 years.

The influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium creates the heliospheric current sheet, which separates regions with magnetic fields pointing in different directions. Magnetohydrodynamic (MHD) theory predicts that the motion of a conducting fluid (e.g., the interplanetary medium) in a magnetic field, induces electric currents which in turn generates magnetic fields, and in this respect it behaves like an MHD dynamo.

History of solar observation

Early understanding of the Sun

Humanity's most fundamental understanding of the Sun is as the luminous disk in the heavens, whose presence above the horizon creates day and whose absence causes night. In many prehistoric and ancient cultures, the Sun was thought to be a solar deity or other supernatural phenomenon, and worship of the Sun was central to civilizations such as the Inca of South America and the Aztecs of what is now Mexico. for example, stone megaliths accurately mark the summer solstice (some of the most prominent megaliths are located in Nabta Playa, Egypt, and at Stonehenge in England); the pyramid of El Castillo at Chichén Itzá in Mexico is designed to cast shadows in the shape of serpents climbing the pyramid at the vernal and autumn equinoxes. With respect to the fixed stars, the Sun appears from Earth to revolve once a year along the ecliptic through the zodiac, and so the Sun was considered by Greek astronomers to be one of the seven planets (Greek planetes, "wanderer"), after which the seven days of the week are named in some languages.

Development of modern scientific understanding

One of the first people in the Western world to offer a scientific explanation for the sun was the Greek philosopher Anaxagoras, who reasoned that it was a giant flaming ball of metal even larger than the Peloponnesus, and not the chariot of Helios. For teaching this heresy, he was imprisoned by the authorities and sentenced to death (though later released through the intervention of Pericles). In the early 17th century, Galileo pioneered telescopic observations of the Sun, making some of the first known observations of sunspots and positing that they were on the surface of the Sun rather than small objects passing between the Earth and the Sun. Isaac Newton observed the Sun's light using a prism, and showed that it was made up of light of many colors, while in 1800 William Herschel discovered infrared radiation beyond the red part of the solar spectrum. The 1800s saw spectroscopic studies of the Sun advance, and Joseph von Fraunhofer made the first observations of absorption lines in the spectrum, the strongest of which are still often referred to as Fraunhofer lines.

In the early years of the modern scientific era, the source of the Sun's energy was a significant puzzle. Lord Kelvin suggested that the Sun was a gradually cooling liquid body that was radiating an internal store of heat. Kelvin and Hermann von Helmholtz then proposed the Kelvin-Helmholtz mechanism to explain the energy output. Ernest Rutherford suggested that the energy could be maintained by an internal source of heat, and suggested radioactive decay as the source. However it would be Albert Einstein who would provide the essential clue to the source of a Sun's energy with his mass-energy relation E=mc².
In 1920 Sir Arthur Eddington proposed that the pressures and temperatures at the core of the Sun could produce a nuclear fusion reaction that merged hydrogen into helium, resulting in a production of energy from the net change in mass. This theoretical concept was developed
in the 1930s by the astrophysicists Subrahmanyan Chandrasekhar and Hans Bethe.

Solar space missions

The first satellites designed to observe the Sun were NASA's Pioneers 5, 6, 7, 8 and 9, which were launched between 1959 and 1968. Pioneer 9 operated for a particularly long period of time, transmitting data until 1987.

In the 1970s, Helios 1 and the Skylab Apollo Telescope Mount provided scientists with significant new data on solar wind and the solar corona. The Helios 1 satellite was a joint U.S.-German probe that studied the solar wind from an orbit carrying the spacecraft inside Mercury's orbit at perihelion. Discoveries included the first observations of coronal mass ejections, then called "coronal transients", and of coronal holes, now known to be intimately associated with the solar wind.

In 1980, the Solar Maximum Mission was launched by NASA. This spacecraft was designed to observe gamma rays, X-rays and UV radiation from solar flares during a time of high solar activity. In 1984 Space Shuttle Challenger mission STS-41C retrieved the satellite and repaired its electronics before re-releasing it into orbit. The Solar Maximum Mission subsequently acquired thousands of images of the solar corona before re-entering the Earth's atmosphere in June 1989.

Japan's Yohkoh (Sunbeam) satellite, launched in 1991, observed solar flares at X-ray wavelengths. It was destroyed by atmospheric reentry in 2005.

One of the most important solar missions to date has been the Solar and Heliospheric Observatory, jointly built by the European Space Agency and NASA and launched on December 2, 1995. In addition to its direct solar observation, SOHO has enabled the discovery of large numbers of comets, mostly very tiny sungrazing comets which incinerate as they pass the Sun.

All these satellites have observed the Sun from the plane of the ecliptic, and so have only observed its equatorial regions in detail. Once Ulysses was in its scheduled orbit, it began observing the solar wind and magnetic field strength at high solar latitudes, finding that the solar wind from high latitudes was moving at about 750 km/s (slower than expected), and that there were large magnetic waves emerging from high latitudes which scattered galactic cosmic rays.

Elemental abundances in the photosphere are well known from spectroscopic studies, but the composition of the interior of the Sun is more poorly understood. A solar wind sample return mission, Genesis, was designed to allow astronomers to directly measure the composition of solar material. Genesis returned to Earth in 2004 but was damaged by a crash landing after its parachute failed to deploy on reentry into Earth's atmosphere. UV exposure gradually yellows the lens of the eye over a period of years and can cause cataracts, but those depend on general exposure to solar UV, not on whether one looks directly at the Sun.

When looking at the sun, either with or without optical aid, using a proper filter is important as some improvised filters pass UV or IR rays that can damage the eye at high brightness levels. For example, any kind of photographic color film transmits IR light and must not be used.

Viewing the Sun through light-concentrating optics such as binoculars is very hazardous without an attenuating (ND) filter to dim the sunlight. These filters can be made of mirrored glass or metallised plastic film.

Partial solar eclipses are hazardous to view because the eye's pupil is not adapted to the unusually high visual contrast: the pupil dilates according to the total amount of light in the field of view, not by the brightest object in the field. This can damage or kill those cells, resulting in small permanent blind spots for the viewer. The hazard is insidious for inexperienced observers and for children, because there is no perception of pain: it is not immediately obvious that one's vision is being destroyed.

During sunrise and sunset, sunlight is attenuated through rayleigh and mie scattering of light by a particularly long passage through Earth's atmosphere, and the direct Sun is sometimes faint enough to be viewed directly without discomfort or safely with binoculars (provided there is no risk of bright sunlight suddenly appearing in a break between clouds).

References

Thompson, M.

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