Sun

The sun is the star at the center of the solar system, and accounts for almost all (99.86 percent) of the mass in the solar system. It is 330,000 times the mass of Earth, and 1.3 million times its volume, being primarily gaseous. That gas is mainly hydrogen (75 percent) and helium (23 percent) with trace amounts of oxygen, carbon, neon, iron, and other elements.

Stars are classified according to characteristics like color and brightness. The sun is a G2 main sequence star, which means it is in the part of its life cycle in which it converts its hydrogen to helium by nuclear fusion. This fusion process releases considerable amounts of energy, which is one of the sources of the sun’s heat. The surface temperature of the sun is approximately 5,505 degrees Celsius. It is believed to be one of the brightest stars in the Milky Way, in the top 15 percent. Unlike solid planets like the Earth, the sun has no definite boundary; it simply becomes less and less dense at its outermost layers.

The sun was formed about 4.5 billion years ago as a result of a gravitational collapse, in the same process that formed the rest of the solar system. It orbits the center of the Milky Way galaxy (about 25,000 light years away) in a clockwise direction, completing an orbit every 225 to 250 million years. It also rotates, although the difficulty of looking at the sun—much less making out surface characteristics—makes this impossible to detect without special equipment. The sun takes 25.6 days to rotate at the equator, and because of the Earth’s orbit, its apparent rotation as seen from Earth takes 28 days.

Relationship with Earth

From the Earth, the sun appears yellow because the atmosphere scatters blue light. Its visible radiation is actually primarily in the yellow-green part of the spectrum. Sunrise and sunset are marked by more brilliant and varied colors due to the scattering of light particles as their trip through the atmosphere is lengthened. Atmospheric particles enhance this effect; the brilliance of sunsets in the aftermath of forest fires or volcanic eruptions, as a result of the prevalence of airborne particulates, has long been noted. Just before sunrise or just after sunset, the light bending over the horizon sometimes causes a brief green flash, which was first caught in color photographs in 1960.

The average distance from the sun to the Earth (about 150 million kilometers) equals one astronomical unit, which is used to measure distances in the solar system. The sun in conjunction with the axial tilt of the Earth is responsible for the Earth’s seasons: The Northern Hemisphere is exposed to more sunlight in May, June, and July, while the Southern Hemisphere is exposed to more in November, December, and January. Because the large amounts of water on Earth slow the onset of the resulting warming, summer in most places is offset by one month from these solar-maximum months. In San Francisco, where water surrounds the city on three sides, the seasonal lag is almost three months, resulting in a summer peak in the middle of September instead of July.

The sun was long believed to support all life on Earth. Plants depend on photosynthesis, using sunlight for chemical energy, and non-plant life depends on plant life at some point in the food chain. However, extremophile life forms have been discovered in hydrothermal vents; these organisms have no access to the sun and depend on sulfur compounds and chemosynthesis. It is likely that even these life forms evolved from life forms that depend on the sun, and that the sun is therefore the cause of all life on Earth. Even organisms that do not depend on the sun as directly as plants do nevertheless often require sunlight to be healthy. In humans, for instance, exposure to sunlight is necessary for the production of vitamin D, which is important for bones, the immune system, and to fight cancer and depression; studies have also indicated that sun exposure reduces the risk of multiple sclerosis later in life, and lowers blood pressure.

The sun goes through an eleven-year solar cycle during which its solar radiation and ejections of material (including solar flares and coronal mass ejections) vary, resulting in changes not only to the weather of space, but on Earth. The links between solar variation and climate are not fully understood, but changes in solar activity have short-term effects on weather. One demonstrated effect is the occurrence of colder winters in the United States and warmer winters in Canada during the periods of least solar activity. Peaks in solar activity also impact radio communication on Earth.

A 2022 study predicted what will happen to the Sun when it begins to approach the end of its lifespan. According to an article from Space.com, the Sun has an estimated 5 billion years' worth of hydrogen left before it runs out. Once the hydrogen supply is depleted, it is predicted that the Sun will enter an unstable phase where it will expand, consuming several planets (including the Earth). Following this expansion, the Sun will become what is known as a red giant, another name for a dying star in its final stage of life. The Sun will remain a red giant for an estimated 1 billion years. During its phase as a red giant, the Sun will slowly consume its remaining supply of helium. Once the remaining helium is consumed, it is believed that the Sun will collapse under its own gravitational weight, transforming into what is known as a white dwarf—the final observable life stage of a star. Scientists estimate that the Sun has 7 to 8 billion years remaining in its life cycle.

History of Observation and Exploration

Many early cultures revered the sun, respecting its life-giving role. The Roman Empire even celebrated the sun’s birthday. Most ancient cultures believed the sun revolved around the Earth, and in Medieval Europe, the challenges to this geocentric system by astronomers like Nicolaus Copernicus and Galileo were controversial enough to lead to Galileo’s excommunication by the Catholic Church. Even once the heliocentric model was accepted, the sun itself was not well understood. As late as the nineteenth century, physicist Lord Kelvin proposed that the sun was a body of liquid. Only in the twentieth century was an internal source of heat generated by radioactive decay proposed, and as the subfield of nuclear physics developed, so too did science’s understanding of the sun.

Early probes launched by the US National Aeronautics and Space Administration (NASA) from 1959 to 1968, part of the Pioneer series, orbited the sun in order to take the first detailed measurements of its magnetic field and solar wind. Later spacecraft that conducted further explorations of the sun and especially the solar wind included the Helios probes in the 1970s, the Skylab space station’s solar observatory in 1973, and NASA’s Solar Maximum Mission, which was launched in 1980 to take images of the solar corona. NASA and the European Space Agency collaborated on the Solar and Heliospheric Observatory from 1995 to 2012 and the Solar Dynamics Observatory, launched in 2010, which positioned probes at a Lagrangian point in order to observe both the sun and comets that approach it.

NASA made history in December 2021 by piloting a solar probe into the Sun's atmosphere. Launched in August 2018, the Parker Solar Probe captured images of Venus in October 2018 before completing its first full orbit of the sun in January 2019. NASA announced on December 14, 2021, that the Parker Solar Probe entered the Sun's corona—the outermost element of the Sun's atmosphere. The Parker Solar Probe is named after solar astrophysicist Eugene Newman Parker. In 2022, the European Space Agency's Gaia space observatory mission reported that the Sun will reach its hottest point when it is eight billion years old.

Bibliography

Chambers, John, and Jacqueline Mitton. From Dust to Life: The Origin and Evolution of Our Solar System. Princeton UP, 2013.

Dick, Steven J. Discovery and Classification in Astronomy. Cambridge UP, 2013.

"Gaia Reveals the Past and Future of the Sun." The European Space Agency, 8 Nov. 2022, www.esa.int/Science‗Exploration/Space‗Science/Gaia/Gaia‗reveals‗the‗past‗and‗future‗of‗the‗Sun. Accessed 13 Nov. 2024.

Johnson-Groh, Mara. “NASA Enters the Solar Atmosphere for the First Time, Bringing New Discoveries.” NASA, 14 Dec. 2021, www.nasa.gov/feature/goddard/2021/nasa-enters-the-solar-atmosphere-for-the-first-time-bringing-new-discoveries. Accessed 13 Nov. 2024.

Phillips, K.J.H. Guide to the Sun. Cambridge UP, 1995.

Schilling, Govert. Deep Space. Black Dog and Leventhal, 2014.

Sturrock, P. A., editor. Physics of the Sun. Springer, 2014.

"The Sun." NASA, 2024, science.nasa.gov/sun/. Accessed 13 Nov. 2024.

Wendel, JoAnna. “When Will the Sun Die?” Space.com, 6 Mar. 2024, www.space.com/14732-sun-burns-star-death.html. Accessed 13 Nov. 2024.

Zirker, J. B. Journey from the Center of the Sun. Princeton UP, 2002.