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The Sun Sun symbol.svg
The Sun by the Atmospheric Imaging Assembly of NASA's Solar Dynamics Observatory - 20100819.jpg
Observation data
Average distance
from Earth
1.496×108 km
8 min 19 s at light speed
Visual brightness (V) −26.74
True brightness 4.83
Angular size 31.6–32.7′
Adjective Solar
Orbit and rotation
Average distance
from Milky Way center
≈ 2.7×1017 km
27,200 light-years
Velocity ≈ 220 km/s (orbit around the center of the Milky Way)
≈ 20 km/s (relative to average velocity of other stars nearby)
≈ 370 km/s (relative to the cosmic microwave background)
Obliquity 7.25°
(to the ecliptic)
(to the galactic plane)
Rotation velocity 7.189×103 km/h
Physical characteristics
Equatorial radius 696,342±65 km
109 × Earth
Equatorial circumference 4.379×106 km
109 × Earth
Flattening 9×10−6
Surface area 6.09×1012 km2
12,000 × Earth
Volume 1.41×1018 km3
1,300,000 × Earth
Mass (1.98855±0.00025)×1030 kg
333,000 × Earth
Average density 1.408 g/cm3
0.255 × Earth
Surface gravity 274.0 m/s2
27.94 g
27,542.29 cgs
28 × Earth
Escape velocity
(from the surface)
617.7 km/s
55 × Earth
Temperature Center: 1.57×107 K
Photosphere: 5,778 K
Corona: ≈ 5×106 K
Luminosity (Lsol) 3.846×1026 W
Age ≈4.6 billion years

The Sun in the center of our solar system is a yellow dwarf star. It gives off energy as light. That includes light, infra-red energy (heat), ultraviolet light and radio waves. It also gives off a stream of particles, which reaches Earth as "solar wind". The source of all this energy is the reaction in the star which turns hydrogen into helium and makes huge amounts of energy.

The Sun is a star like many others in our Milky Way galaxy. It has existed for a little over 4.5 billion years, and is going to continue for at least as long. The Sun is about a hundred times as wide as the Earth. It has a mass which is 333,000 times the mass of the Earth. The Earth can also fit inside the Sun 1.3 million times. Without the Sun there could not be any life on Earth.

The enormous effect of the Sun on Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity. The rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of which is the predominant calendar in use today.


Scientists think that the Sun started from a very large cloud of dust and small bits of ice about 4.567 billion years ago. The Sun is by far the brightest object in the Earth's sky.

At the center of that huge cloud, gravity caused the material to build up into a ball. Once this got big enough, the huge pressure inside started a fusion reaction. The energy this released caused that ball to heat and shine.

The energy radiated from the Sun pushed away the rest of the cloud from itself, and the planets formed from the rest of this cloud

At its very center, hydrogen atoms collide together at great temperature and pressure so that they fuse to form atoms of helium. This process is called nuclear fusion. This fusion changes a very small part of the hydrogen atoms into a large amount of energy. This energy then travels from the core to the surface of the Sun. The Sun's surface is called the photosphere and is where it shines the energy into space.

The energy of this sunlight supports almost all life on Earth by photosynthesis, and drives Earth's climate and weather. The sun can also be used as a source of energy.


The Sun is composed primarily of the chemical elements hydrogen and helium. At this time in the Sun's life, they account for 74.9% and 23.8% of the mass of the Sun in the photosphere, respectively. All heavier elements, called metals in astronomy, account for less than 2% of the mass, with oxygen (roughly 1% of the Sun's mass), carbon (0.3%), neon (0.2%), and iron (0.2%) being the most abundant.

The Sun's original chemical composition was inherited from the interstellar medium out of which it formed. Originally it would have contained about 71.1% hydrogen, 27.4% helium, and 1.5% heavier elements. The hydrogen and most of the helium in the Sun would have been produced by Big Bang nucleosynthesis in the first 20 minutes of the universe, and the heavier elements were produced by previous generations of stars before the Sun was formed.

Sun poster
Diagram of the Sun

The Sun today is roughly halfway through the most stable part of its life. It has not changed dramatically for over four billion years, and will remain fairly stable for more than five billion more. However, after hydrogen fusion in its core has stopped, the Sun will undergo dramatic changes, both internally and externally. In the future, helium will continue to accumulate in the core, and in about 5 billion years this gradual build-up will eventually cause the Sun to exit the main sequence and become a red giant.


This is the surface of the Sun. The light that the Earth receives from the Sun is radiated from this layer. Below this layer, the Sun is opaque, or not transparent to light.


Coronal mass ejection erupts on the Sun, 31 August 2012
Coronal mass ejection erupts on the Sun, 31 August 2012
Total eclipse of the sun, August 21, 2017, Jackson, Wyoming, USA 02
Total eclipse of the sun, the violet color shows the sun's chromosphere

Five layers make up the atmosphere of the Sun. The chromosphere, transition region, and corona are much hotter than the outer photosphere surface of the Sun.

The minimum temperature zone, the coolest layer of the Sun, is about 500 km above the photosphere. It has a temperature of about 4100 K. This part of the Sun is cool enough to allow simple molecules such as carbon monoxide and water to form. These molecules can be seen on the Sun with special instruments called spectroscopes.

  • The chromosphere is the first layer of the Sun which can be seen, especially during a solar eclipse when the moon is covering most of the Sun and blocking the brightest light.
  • The solar transition region is the part of the Sun's atmosphere, between the chromosphere and outer part called the corona. It can be seen from space using telescopes that can sense ultraviolet light.
  • The corona is the outer atmosphere of the Sun and is much bigger than the rest of the Sun. The corona continuously expands into space forming the solar wind, which fills all the Solar System. The average temperature of the corona and solar wind is about 1,000,000–2,000,000 K. In the hottest regions it is 8,000,000–20,000,000 K. We do not understand why the corona is so hot. It can be seen during a solar eclipse or with an instrument called a coronagraph.

Visible features

NASA's SDO Sees Solar Flares (14391345485)
NASA's SDO (Solar Dynamics Observatory) Sees Solar Flares

Since the Sun is all gas, surface features come and go. If the Sun is viewed through a special solar telescope, dark areas called sunspots can be seen. These areas are caused by the Sun's magnetic field. The sunspots only look dark because the rest of the Sun is very bright.

Some space telescopes, including the ones that orbit the Sun have seen huge arches of the Sun's matter extend suddenly from the Sun. These are called solar prominences. Solar prominences come in many different shapes and sizes. Some of them are so large that the Earth could fit inside of them, and a few are shaped like hands. Solar flares also come and go.

Sunspots, prominences (large, bright, gaseous features) and flares become rare, and then numerous, and then rare again, every 11 years.


Solar eclipse 1999 4
Solar eclipse 1999

A solar eclipse appears when the moon is between the Earth and Sun. On August 21, 2017, there was a total solar eclipse visible in a belt spanning all across the US. This was the first total solar eclipse visible from anywhere on mainland United States since the total solar eclipse in March 1979

A lunar eclipse happens when the moon passes through the shadow of the Earth which can only occur during a full moon. The number of lunar eclipses in a single year can range from 0 to 3. Partial eclipses slightly outnumber total eclipses by 7 to 6.

Solar weather

Northern lights in Tromsoe
Northern lights - Norway

Solar weather affects us on Earth. Solar weather (also called space weather) includes sunlight and the solar wind. Solar flares shoot a lot of very hot gas out from the Sun. If a solar flare is aimed towards Earth, protons — subatomic particles with positive electric charge — can be shot at Earth at high speed, and a solar storm could result. This could cause electrical blackouts or block radio signals. It damages satellites in orbit. Radiation from an extreme solar storm could be very dangerous for astronauts, so they must be protected. The Earth’s magnetic field and atmosphere usually protect us from flares.

Solar flares can also cause an aurora. Auroras look like beautiful curtains of shimmering light. They are called Northern Lights (Aurora Borealis) if they are near the North Pole. They are called Southern Lights (Aurora Australis) if they are near the South Pole. Solar weather affects other planets, too. We have pictures of auroras on every planet except Mercury and Pluto.

Just like we can get Earth weather forecasts, we can get Solar weather forecasts. Forecasters study the Sun to figure out when flares will happen. They try to tell when solar storms will hit Earth. They also try to tell when solar storms will go to other parts of the Solar System.

Planetary system

Solar system scale wide
Solar system

The Sun has eight known planets. This includes four terrestrial planets (Mercury, Venus, Earth, and Mars), two gas giants (Jupiter and Saturn), and two ice giants (Uranus and Neptune). The Solar System also has at least five dwarf planets, an asteroid belt, numerous comets, and a large number of icy bodies which lie beyond the orbit of Neptune.

Observation and effects

Sunset 2007-1
During certain atmospheric conditions such as a sunset, the Sun becomes clearly visible to the naked eye, and can be observed without stress to the eyes

The brightness of the Sun can cause pain from looking at it with the naked eye; however, doing so for brief periods is not hazardous for normal non-dilated eyes. Looking directly at the Sun causes phosphene visual artifacts and temporary partial blindness. It also delivers about 4 milliwatts of sunlight to the retina, slightly heating it and potentially causing damage in eyes that cannot respond properly to the brightness.

UV exposure gradually yellows the lens of the eye over a period of years, and is thought to contribute to the formation of cataracts, but this depends on general exposure to solar UV, and not whether one looks directly at the Sun. Long-duration viewing of the direct Sun with the naked eye can begin to cause UV-induced, sunburn-like lesions on the retina after about 100 seconds, particularly under conditions where the UV light from the Sun is intense and well focused.

Viewing the Sun through light-concentrating optics such as binoculars may result in permanent damage to the retina without an appropriate filter that blocks UV and substantially dims the sunlight. When using an attenuating filter to view the Sun, the viewer is cautioned to use a filter specifically designed for that use. Some improvised filters that pass UV or IR rays, can actually harm the eye at high brightness levels.

Solar eclipse Slobozia
Solar eclipse 2011

Partial solar eclipses are hazardous to view because the eye's pupil is not adapted to the unusually high visual contrast. During partial eclipses most sunlight is blocked by the Moon passing in front of the Sun, but the uncovered parts of the photosphere have the same surface brightness as during a normal day. In the overall gloom, the pupil expands, and each retinal cell exposed to the solar image receives up to ten times more light than it would looking at the non-eclipsed Sun. 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.

Ultraviolet light from the Sun has antiseptic properties and can be used to sanitize tools and water. It also causes sunburn, and has other biological effects such as the production of vitamin D and sun tanning. Ultraviolet light is strongly reduced in force by Earth's ozone layer, so that the amount of UV varies greatly with latitude and has been partially responsible for many biological adaptations, including variations in human skin color in different regions of the Earth.

Exploration of the Sun

Helios spacecraft
Helios spacecraft prototype

Early space probes designed to collect information about the sun were NASA's Pioneers 5 through 9 and the Helios 1 and 2. Those were in the 1950s, 60s, and 70s. They collected a lot of data about the Sun. The Parker Solar Probe is a NASA robotic spacecraft launched in 2018 and currently en route to probe the outer corona of the Sun. It will approach to within 9.86 solar radii (6.9 million kilometers or 4.3 million miles) from the center of the Sun and by 2025 will travel, at closest approach, as fast as 690,000 km/h (430,000 mph), or 0.064% the speed of light.

The cost of the project is US$1.5 billion. Johns Hopkins University Applied Physics Laboratory designed and built the spacecraft, which was launched on August 12, 2018. It became the first NASA spacecraft named after a living person, honoring physicist Eugene Parker, professor emeritus at the University of Chicago.

A memory card containing the names of over 1.1 million people was mounted on a plaque and installed below the spacecraft's high-gain antenna on May 18, 2018. The card also contains photos of Parker and a copy of his 1958 scientific paper predicting important aspects of solar physics.

Parker Solar Probe
Parker Solar Probe - artist rendering

On 29 October 2018 at about 1:04 p.m. EDT, the spacecraft became the closest ever man-made object to the Sun. The previous record, 26.55 million miles from the Sun's surface, was set by the Helios 2 spacecraft in April 1976.

Other projects observed the Sun from Earth orbit, such as a Japanese satellite launched in 1991 called Yohkoh, which means Sunbeam in English. It told scientists more about solar flares, and activity on the sun's surface. Thanks to Yohkoh, they knew more about how to classify solar flares, and whether or not they will cause electrical disturbance on Earth or not.

Two important missions to study Sun are called the Solar and Heliospheric Observatory (SHO) and the Solar Dynamics Observatory (SDO). They have taken many pictures of the Sun, along with discovering many comets near the Sun. All of these observations were across the Sun's equator. The first space probe to observe the Sun's poles was named Ulysses. The Ulysses probe went all the way to Jupiter before approaching the Sun.

Fate of the Sun

Absorption of a gaseous planet by its star

Even before it becomes a red giant, the luminosity of the Sun will have nearly doubled, and Earth will receive as much sunlight as Venus receives today. Once the core hydrogen is exhausted in 5.4 billion years, the Sun will expand into a subgiant phase and slowly double in size over about half a billion years. It will then expand more rapidly over about half a billion years until it is over two hundred times larger than today and a couple of thousand times more luminous. This then starts the red-giant-branch phase where the Sun will spend around a billion years and lose around a third of its mass.

Earth's fate is still a bit of a mystery. In the long term, the Earth's future depends on the Sun, and the Sun is going to be fairly stable for the next 5 billion years. Calculations suggest that the Earth might move to a wider orbit. This is because about 30% of the Sun's mass will blow away in the solar wind. However, in the very long term the Earth will probably be destroyed as the Sun increases in size. Stars like the Sun become red giants at a later stage. The Sun will expand beyond orbits of Mercury, Venus, and probably Earth. In any event, the Earth's ocean and air would have vanished before the Sun gets to that stage.

After the Sun reaches a point where it can no longer get bigger, it will lose its layers and form a planetary nebula. Eventually, the Sun will shrink into a white dwarf. Then, over several hundred billion or even a trillion years, the Sun would fade into a black dwarf.

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