Sun | National Geographic Society (2023)

The sun is an ordinarystar, one of about 100 billion in our galaxy, the Milky Way. The sun has extremely important influences on our planet: It drives weather, ocean currents, seasons, andclimate, and makes plant life possible throughphotosynthesis. Without the sun’s heat and light, life on Earth would not exist.

About 4.5 billion years ago, the sun began to take shape from amolecular cloudthat was mainly composed of hydrogen and helium. A nearbysupernovaemitted a shockwave, which came in contact with the

molecular cloud

and energized it. The

molecular cloud

began tocompress, and some regions of gas collapsed under their owngravitational pull. As one of these regions collapsed, it also began torotateand heat up from increasing pressure. Much of the hydrogen and helium remained in the center of this hot, rotating mass. Eventually, the gases heated up enough to beginnuclear fusion, and became the sun in oursolar system.

Other parts of the

molecular cloud

cooled into a disc around the brand-new sun and became planets, asteroids, comets, and other bodies in our

solar system

.

The sun is about 150 million kilometers (93 million miles) from Earth. This distance, called anastronomical unit(AU), is a standard measure of distance forastronomers and astrophysicists.

An AU can be measured at light speed, or the time it takes for a photon of light to travel from the sun to Earth. It takes light on the sun about eight minutes and 19 seconds to reach Earth.

Theradiusof the sun, or the distance from the very center to the outer limits, is about 700,000 kilometers (432,000 miles). That distance is about 109 times the size of Earth’s

radius

. The sun not only has a much larger

radius

than Earth—it is also much more massive. The sun’s mass is more than 333,000 times that of Earth, and contains about 99.8 percent of all of the mass in the entire

solar system

!

Composition

The sun is made up of a blazing combination of gases. These gases are actually in the form of plasma.

Plasma

is a state of matter similar to gas, but with most of the particlesionized. This means the particles have an increased or reduced number of electrons.

About three quarters of the sun is hydrogen, which is constantly fusing together and creating helium by a process called

nuclear fusion

. Helium makes up almost the entire remaining quarter. A very small percentage (1.69 percent) of the sun’s mass is made up of other gases and metals: iron, nickel, oxygen, silicon, sulfur, magnesium, carbon, neon, calcium, and chromium This 1.69 percent may seem insignificant, but its mass is still 5,628 times the mass of Earth.

The sun is not a solid mass. It does not have easily identifiable boundaries like rocky planets like Earth. Instead, the sun is composed of layers made up almost entirely of hydrogen and helium. These gases carry out different functions in each layer, and the sun’s layers are measured by their percentage of the sun’s total

radius

.

The sun is permeated and somewhat controlled by amagnetic field. The

magnetic field

is defined by a combination of three complex mechanisms: a circular electric current that runs through the sun, layers of the sun that

rotate

at different speeds, and the sun’s ability to conductelectricity. Near the sun’sequator,

magnetic field

lines make small loops near the surface.

Magnetic field

lines that flow through the poles extend much farther, thousands of kilometers, before returning to the opposite pole.

The sun

rotates

around its own axis, just like Earth. The sun

rotates

counterclockwise, and takes between 25 and 35 days to complete a single rotation.

The sunorbits clockwise around the center of the Milky Way. Its

orbit

is between 24,000 and 26,000 light-years away from the galactic center. The sun takes about 225 million to 250 million years to

orbit

one time around the galactic center.

Electromagnetic Radiation

The sun’s energy travels to Earth at the speed of light in the form of electromagnetic radiation (EMR).

Theelectromagnetic spectrumexists as waves of different frequencies andwavelengths.

Thefrequencyof a wave represents how many times the wave repeats itself in a certain unit of time. Waves with very short

wavelengths

repeat themselves several times in a given unit of time, so they are high-

frequency

. In contrast, low-

frequency

waves have much longer

wavelengths

.

The vast majority of electromagnetic waves that come from the sun are invisible to us. The most high-

frequency

waves emitted by the sun are gamma rays, X-rays, andultraviolet radiation(UV rays). The most harmful UV rays are almost completely absorbed by Earth’s atmosphere. Less potent UV rays travel through the atmosphere, and can cause sunburn.

The sun also emitsinfrared radiation—whose waves are a much lower-

frequency

. Most heat from the sun arrives as infrared energy.

Sandwiched between infrared and UV is the visible spectrum, which contains all the colors we, as humans, can see. The color red has the longest

wavelengths

(closest to infrared), and violet (closest to UV) the shortest.

The sun itself is white, which means it contains all the colors in the visible spectrum. The sun appears orangish-yellow because the blue light it emits has a shorter

wavelength

, and is scattered in the atmosphere—the same process that makes the sky appear blue.

Astronomers

, however, call the sun a “yellow dwarf”

star

because its colors fall within the yellow-green section of the

electromagnetic spectrum

.

Evolution of the Sun

The sun, although it has sustained all life on our planet, will not shine forever. The sun has already existed for about 4.5 billion years.

The process of

nuclear fusion

, which creates the heat and light that make life on our planet possible, is also the process that slowly changes the sun’s composition. Through

nuclear fusion

, the sun is constantly using up the hydrogen in its core:Every second, the sun fuses around 620 million metric tons of hydrogen into helium.

At this stage in the sun’s life, its

core

is about 74% hydrogen. Over the next five billion years, the sun will burn through most of its hydrogen, and helium will become its major source of fuel.

Over those five billion years, the sun will go from “yellow dwarf” to “red giant.” When almost all of the hydrogen in the sun’s

core

has been consumed, the

core

will contract and heat up, increasing the amount of

nuclear fusion

that takes place. The outer layers of the sun will expand from this extra energy.

The sun will expand to about 200 times its current

radius

, swallowing Mercury and Venus.

Astrophysicists debate whether Earth’s

orbit

would expand beyond the sun’s reach, or if our planet would be engulfed by the sun as well.

As the sun expands, it will spread its energy over a larger surface area, which has an overall cooling effect on the

star

. This cooling will shift the sun’s visible light to a reddish color—a

red giant

.

Eventually, the sun’s

core

reaches a temperature of about 100 million on theKelvin scale (almost 100 million degrees Celsius or 180 million degrees Farenheit), the common scientific scale for measuring temperature. When it reaches this temperature, helium will begin fusing to create carbon, a much heavier element. This will cause intense solar wind and other solar activity, which will eventually throw off the entire outer layers of the sun. The

red giant

phase will be over. Only the sun’s carbon

core

will be left, and as a “white dwarf,” it will not create or emit energy.

Sun’s Structure

The sun is made up of six layers:

core

, radiative zone, convective zone, photosphere, chromosphere, and corona.

Core

The sun’score, more than a thousand times the size of Earth and more than 10 timesdenser than lead, is a huge furnace. Temperatures in the

core

exceed 15.7 million kelvin (also 15.7 million degrees Celsius, or 28 million degrees Fahrenheit). The

core

extends to about 25% of the sun’s

radius

.

The

core

is the only place where

nuclear fusion

reactions can happen. The sun’s other layers are heated from the nuclear energy created there. Protons of hydrogen atoms violently collide and fuse, or join together, to create a helium atom.

This process, known as a PP (proton-proton) chain reaction, emits an enormous amount of energy. The energy released during one second of solar fusion is far greater than that released in the explosion of hundreds of thousands of hydrogen bombs.

During

nuclear fusion

in the

core

, two types of energy are released:

photons

and neutrinos. These particles carry and emit the light, heat, and energy of the sun.

Photons

are the smallest particle of light and other forms of electromagnetic radiation.

Neutrinos

are more difficult to detect, and only account for about two percent of the sun’s total energy. The sun emits both

photons

and

neutrinos

in all directions, all the time.

Radiative Zone

Theradiative zoneof the sun starts at about 25 percent of the radius, and extends to about 70 percent of the radius. In this broad zone, heat from the core cools dramatically, from between seven million K to two million K.

In the radiative zone, energy is transferred by a process called thermal radiation. During this process, photons that were released in the core travel a short distance, are absorbed by a nearby ion, released by that ion, and absorbed again by another. One photon can continue this process for almost 200,000 years!

Transition Zone: Tachocline

Between the

radiative zone

and the next layer, the convective zone, there is a

transition zone

called the

tachocline

. This region is created as a result of the sun’s differential rotation.

Differential rotation

happens when different parts of an object

rotate

at different velocities. The sun is made up of gases undergoing different processes at different layers and different latitudes. The sun’s

equator

rotates

much faster than its poles, for instance.

The rotation rate of the sun changes rapidly in the

tachocline

.

Convective Zone

At around 70% of the sun’s

radius

, the convective zone begins. In this zone, the sun’s temperature is not hot enough to transfer energy by thermal radiation. Instead, it transfers heat by thermalconvectionthrough thermal columns.

Similar to water boiling in a pot, or hot wax in a lava lamp, gases deep in the sun’s convective zone are heated and “boil” outward, away from the sun’s

core

, through thermal columns. When the gases reach the outer limits of the convective zone, they cool down, and plunge back to the base of the convective zone, to be heated again.

Photosphere

The

photosphere

is the bright yellow, visible "surface" of the sun. The

photosphere

is about 400 kilometers (250 miles) thick, and temperatures there reach about 6,000 k (5,700° C, 10,300° F).

The thermal columns of the

convection

zone are visible in the

photosphere

, bubbling like boiling oatmeal. Through powerful telescopes, the tops of the columns appear asgranules crowded across the sun. Each

granule

has a bright center, which is the hot gas rising through a thermal column. The

granules

’ dark edges are the cool gas descending back down the column to the bottom of the convective zone.

Although the tops of the thermal columns look like small

granules

, they are usually more than 1,000 kilometers (621 miles) across. Most thermal columns exist for about eight to 20 minutes before they dissolve and form new columns. There are also “super

granules

” that can be up to 30,000 kilometers (18,641 miles) across, and last for up to 24 hours.

Sunspots, solar flares, and solar prominences take form in the

photosphere

, although they are the result of processes and disruptions in other layers of the sun.

Photosphere: Sunspots

A

sunspot

is just what it sounds like—a dark spot on the sun. A

sunspot

forms when intense magnetic activity in the convective zoneruptures a thermal column.At the top of the

ruptured

column (visible in the

photosphere

), temperature is temporarily decreased because hot gases are not reaching it.

Photosphere: Solar Flares

The process of creating

sunspots

opens a connection between the

corona

(the very outer layer of the sun) and the sun’s interior. Solar matter surges out of this opening in formations called

solar flares

. These explosions are massive: In the period of a few minutes,

solar flares

release the equivalent of about 160 billion megatons of TNT, or about a sixth of the total energy the sun releases in one second.

Clouds of ions, atoms, and electrons erupt from

solar flares

, and reach Earth in about two days.

Solar flares

and

solar prominences

contribute tospace weather, which can cause disturbances to Earth’s atmosphere and

magnetic field

, as well as disrupt satellite and telecommunications systems.

Photosphere: Coronal Mass Ejections

Coronal mass ejections (CMEs) are another type of solar activity caused by the constant movement and disturbances within the sun’s magnetic field. CMEs typically form near the active regions of sunspots, the correlation between the two has not been proven. The cause of CMEs is still being studied, and it is hypothesized that disruptions in either the photosphere or corona lead to these violent solar explosions.

Photosphere: Solar Prominence

Solar prominences are bright loops of solar matter. They can burst far into the coronal layer of the sun, expanding hundreds of kilometers per second. These curved and twisted features can reach hundreds of thousands of kilometers in height and width, and last anywhere from a few days to a few months.

Solar prominences are cooler than the corona, and they appear as darker strands against the sun. For this reason, they are also known as filaments.

Photosphere: Solar Cycle

The sun does not constantly emit

sunspots

and solar ejecta; it goes through a cycle of about 11 years. During this

solar cycle

, the

frequency

of

solar flares

changes. During solar maximums, there can be several flares per day. During solar minimums, there may be fewer than one a week.

The

solar cycle

is defined by the sun’s

magnetic fields

, which loop around the sun and connect at the two poles. Every 11 years, the

magnetic fields

reverse, causing a disruption that leads to solar activity and

sunspots

.

The

solar cycle

can have effects on Earth’s

climate

. For example, the sun’s ultraviolet light splits oxygen in the stratosphere and strengthens Earth’s protectiveozone layer. During the solar minimum, there are low amounts of UV rays, which means that Earth’s

ozone layer

is temporarily thinned. This allows more UV rays to enter and heat Earth’s atmosphere.

Solar Atmosphere

The solar atmosphere is the hottest region of the sun. It is made up of the

chromosphere

, the

corona

, and a

transition zone

called the solar transition region that connects the two.

The solar atmosphere is obscured by the bright light emitted by the

photosphere

, and it can rarely be seen without special instruments. Only duringsolar eclipses, when the moon moves between Earth and the sun and hides the

photosphere

, can these layers be seen with the unaided eye.

Chromosphere

The pinkish-red

chromosphere

is about 2,000 kilometers (1,250 miles) thick and riddled with jets of hot gas.

At the bottom of the

chromosphere

, where it meets the

photosphere

, the sun is at its coolest, at about 4,400 k (4,100° C, 7,500° F). This low temperature gives the

chromosphere

its pink color. The temperature in the

chromosphere

increases with altitude, and reaches 25,000 k (25,000° C, 45,000° F) at the outer edge of the region.

The

chromosphere

gives off jets of burning gases calledspicules, similar to

solar flares

. These fiery wisps of gas reach out from the

chromosphere

like long, flaming fingers; they are usually about 500 kilometers (310 miles) in diameter.

Spicules

only last for about 15 minutes, but can reach thousands of kilometers in height before collapsing and dissolving.

Solar Transition Region

The solar transition region (STR) separates the chromosphere from the corona.

Below the STR, the layers of the sun are controlled and stay separate because of gravity, gas pressure, and the different processes of exchanging energy. Above the STR, the motion and shape of the layers are much more dynamic. They are dominated by magnetic forces. These magnetic forces can put into action solar events such as coronal loops and the solar wind.

The state of helium in these two regions has differences as well. Below the STR, helium is partially ionized. This means it has lost an electron, but still has one left. Around the STR, helium absorbs a bit more heat and loses its last electron. Its temperature soars to almost one million k (one million °C, 1.8 million °F).

Corona

The

corona

is the wispy outermost layer of the solar atmosphere, and can extend millions of kilometers into space. Gases in the

corona

burn at about one million k (one million° C, 1.8 million° F), and move about 145 kilometers (90 miles) per second.

Some of the particles reach anescape velocityof 400 kilometers per second (249 miles per second). They escape the sun’s

gravitational pull

and become the

solar wind

. The

solar wind

blasts from the sun to the edge of the

solar system

.

Other particles form

coronal

loops.

Coronal

loops are bursts of particles that curve back around to a nearby

sunspot

.

Near the sun’s poles are

coronal

holes. These areas are colder and darker than other regions of the sun, and allow some of the fastest-moving parts of the

solar wind

to pass through.

Solar Wind

The

solar wind

is a stream of extremely hot, charged particles that are thrown out from the upper atmosphere of the sun. This means that every 150 million years, the sun loses a mass equal to that of Earth. However, even at this rate of loss, the sun has only lost about 0.01% of its total mass from

solar wind

.

The

solar wind

blows in all directions. It continues moving at that speed for about 10 billion kilometers (six billion miles).

Some of the particles in the

solar wind

slip through Earth’s

magnetic field

and into its upper atmosphere near the poles. As they collide with our planet's atmosphere, these charged particles set the atmosphere aglow with color, creatingauroras, colorful light displays known as the Northern and Southern Lights.

Solar winds

can also cause solar storms. These storms can interfere with

satellites

and knock outpower grids on Earth.

The

solar wind

fills the heliosphere, the massive bubble of charged particles that encompasses the

solar system

.

The

solar wind

eventually slows down near the border of the heliosphere, at a theoretical boundary called theheliopause. This boundary separates the matter and energy of our

solar system

from the matter in neighboring

star

systems and theinterstellar medium.

The

interstellar medium

is the space between

star

systems. The

solar wind

, having traveled billions of kilometers, cannot extend beyond the

interstellar medium

.

Studying the Sun

The sun has not always been a subject of scientific discovery and inquiry. For thousands of years, the sun was known in cultures all over the world as a god, a goddess, and a symbol of life.

To the ancient Aztecs, the sun was a powerful deity known as Tonatiuh, who required human sacrifice to travel across the sky. In Baltic mythology, the sun was a goddess named Saule, who brought fertility and health. Chinese mythology held that the sun is the only remaining of 10 sun gods.

In 150 A.D., Greek scholar Claudius Ptolemy created a geocentric model of the

solar system

in which the moon, planets, and sun revolved around Earth. It was not until the 16th century that Polish

astronomer

Nicolaus Copernicus used mathematical and scientific reasoning to prove that planets

rotated

around the sun. This heliocentric model is the one we use today.

In the 17th century, the telescope allowed people to examine the sun in detail. The sun is much too bright to allow us to study it with our eyes unprotected.With a telescope, it was possible for the first time to project a clear image of the sun onto a screen for examination.

English scientist SirIsaac Newtonused a telescope and prism to scatter the light of the sun, and proved that sunlight was actually made of a spectrum of colors.

In 1800, infrared and ultraviolet light were discovered to exist just outside of the visible spectrum. An optical instrument called a spectroscope made it possible to separate visible light and other electromagnetic radiation into its various

wavelengths

.Spectroscopyalso helped scientists identify gases in the sun’s atmosphere—each element has its own

wavelength

pattern.

However, the method by which the sun generated its energy remained a mystery. Many scientists hypothesized that the sun was contracting, and emitting heat from that process.

In 1868, English

astronomer

Joseph Norman Lockyer was studying the sun’s

electromagnetic spectrum

. He observed bright lines in the

photosphere

that did not have a

wavelength

of any known element on Earth. He guessed that there was an element isolated on the sun, and named it helium after the Greek sun god, Helios.

Over the next 30 years,

astronomers

concluded that the sun had a hot, pressurized

core

that was capable of producing massive amounts of energy through

nuclear fusion

.

Technology continued to improve and allowed scientists to uncover new features of the sun. Infrared telescopes were invented in the 1960s, and scientists observed energy outside the visible spectrum. Twentieth-century

astronomers

used balloons and rockets to send specialized telescopes high above Earth, and examined the sun without any interference from Earth's atmosphere.

Solrad 1was the first spacecraft designed to study the sun, and was launched by the United States in 1960. That decade, NASA sent fivePioneersatellites to orbit the sun and collect information about the star.

In 1980, NASA launched a mission during the solar maximum to gather information about the high-frequency gamma rays, UV rays, and x-rays that are emitted during solar flares.

The Solar and Heliospheric Observatory (SOHO) was developed in Europe and put into orbit in 1996 to collect information. SOHO has been successfully collecting data and forecasting space weather for 12 years.

Voyager 1and2are spacecraft traveling to the edge of the heliosphere to discover what the atmosphere is made of where

solar wind

meets the

interstellar medium

. Voyager 1 crossed this boundary in 2012 and Voyager 2 did so in 2018.

Another development in the study of the sun ishelioseismology, the study of solar waves. The turbulence of the convective zone is hypothesized to contribute to solar waves that continuously transmit solar material to the outer layers of the sun. By studying these waves, scientists understand more about the sun’s interior and the cause of solar activity.

Energy from the Sun

Photosynthesis

Sunlight provides necessary light and energy to plants and other producers in thefood web. These producers absorb the sun’s radiation and convert it into energy through a process called

photosynthesis

.

Producers are mostly plants (on land) and algae (in aquatic regions). They are the foundation of the

food web

, and their energy andnutrients are passed on to every other living organism.

Fossil Fuels

Photosynthesis

is also responsible for all of the

fossil fuels

on Earth. Scientists estimate that about three billion years ago, the first producers evolved in aquatic settings. Sunlight allowed plant life to thrive and adapt. After the plants died, they decomposed and shifted deeper into the earth, sometimes thousands of meters. This process continued for millions of years.

Under intense pressure and high temperatures, these remains became what we know as

fossil fuels

. These microorganisms became petroleum, natural gas, and coal.

People have developed processes for extracting these

fossil fuels

and using them for energy. However,

fossil fuels

are anonrenewable resource. They take millions of years to form.

Solar Energy Technology

Solar energy

technology harnesses the sun’s radiation and converts it into heat, light, or

electricity

.

Solar energy

is arenewable resource, and many technologies can harvest it directly for use in homes, businesses, schools, and hospitals. Some

solar energy

technologies include solar voltaic cells and panels, solar thermal collectors, solar thermal

electricity

, and solar architecture.

Photovoltaics use the sun’s energy to speed up electrons in solar cells and generate

electricity

. This form of technology has been used widely, and can provide

electricity

for rural areas, large power stations, buildings, and smaller devices such as parking meters and trash compactors.

The sun’s energy can also be harnessed by a method called “concentrated solar power,” in which the sun’s rays are reflected and magnified by mirrors and lenses. The intensified ray of sunlight heats a fluid, which creates steam and powers an electricgenerator.

Solar power can also be collected and distributed without machinery or electronics. For example, roofs can be covered with vegetation or painted white to decrease the amount of heat absorbed into the building, thereby decreasing the amount of

electricity

needed for air conditioning. This is

solar architecture

.

Sunlight is abundant: In one hour, Earth’s atmosphere receives enough sunlight to power the

electricity

needs of all people for a year. However, solar technology is expensive, and depends on sunny and cloudless local weather to be effective. Methods of harnessing the sun’s energy are still being developed and improved.

Fast Fact

Like a Diamond in the Sky
White dwarf stars are made of crystallized carbondiamond. A typical white dwarf is about 10 billion trillion trillion carats. In about 5 billion years, says Travis Metcalfe of the Harvard-Smithsonian Center for Astrophysics, Our sun will become a diamond that truly is forever.

Fast Fact

Solar Constant
The solar constant is the average amount of solar energy reaching Earth's atmosphere. The solar constant is about 1.37 kilowatts of electricity per square meter.

Fast Fact

Solarmax
2013 will bring the next solar maximum (solarmax), a period astronomers say will bring more solar flares, coronal mass ejections, solar storms, and auroras.

Fast Fact

Sun is the Loneliest Number
The sun is pretty isolated, way out on the inner rim of the Orion Arm of the Milky Way. Its nearest stellar neighbor, a red dwarf named Proxima Centauri, is about 4.24 light-years away.

Fast Fact

Sunny Days at Space Agencies
NASA and other space agencies have more than a dozen heliophysics missions, which study the sun, heliosphere, and planetary environments as a single connected system. A few of the ongoing missions are:
ACE: observing particles of solar, interplanetary, interstellar, and galactic origins
AIM: determining the causes of the highest-altitude clouds in Earths atmosphere
Hinode: studying the sun with the worlds highest-resolution solar telescopes
IBEX: mapping the entire boundary of the solar system
RHESSI: researching gamma rays and X-rays, the most powerful energy emitted by the sun
SOHO: understanding the structure and dynamics of the sun
SDO: a crown jewel of NASA, aimed at developing the scientific understanding necessary to address those aspects of the sun and solar system that directly affect life and society
STEREO: understanding coronal mass ejections
Voyager: studying space at the edge of the solar system
Wind: understanding the solar wind

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