The Sun: Anatomy, Life Cycle, and Galactic Motion

The Sun, also known as Sol, is the central star of our solar system, providing the energy necessary to sustain life on Earth. A G2V spectral class star, it is a nearly perfect sphere of hot plasma that dominates the gravitational landscape of our local neighborhood. From its violent youth to its eventual transition into a red giant, the Sun serves as a primary example of stellar evolution.

Size comparison of major celestial objects in the Solar System, including the Sun. Distances are not to scale.

Key Facts

A photograph of the sun with a layer of fog visible in front of it. — The Sun seen through a light fog

Age: Approximately 4.6 billion years.
Mass: 332,950 times that of Earth.
Core Temperature: 15.7 million Kelvin.
Composition: Primarily hydrogen (73.46%) and helium (24.85%).
Galactic Orbit: Travels at 251 km/s around the Milky Way center.
Distance from Earth: Mean distance of 1 au (149.6 million km).

Physical Characteristics and Composition

See caption — The Sun seen from Earth, with glare from the lenses. The eye also sees glare when the Sun is looked at directly.

The Sun is an immense body of gas and plasma. Its equatorial radius is approximately 695,700 km, making it 109 times larger than Earth. Despite its size, its average density is only 0.255 times that of Earth, reflecting its gaseous nature.

The chemical makeup of the photosphere—the visible surface—is dominated by the lightest elements. Hydrogen and helium make up the vast majority of its mass, with smaller traces of oxygen, carbon, iron, neon, nitrogen, silicon, magnesium, and sulfur.

Summary of Solar Physical Properties
Property	Value	Comparison to Earth
Mass	1.988 × 1030 kg	332,950x
Volume	1.412 × 1018 km3	1,300,000x
Surface Area	6.09 × 1012 km2	12,000x
Surface Gravity	274 m/s2	27.9g

Internal Structure and Energy Production

The Sun is organized into distinct layers, each with unique physical properties. At the center lies the core, where temperatures reach 15.7 million Kelvin. Here, the Sun generates energy through the proton-proton reaction chain, a process of nuclear fusion where hydrogen nuclei fuse to form deuterium, helium-3, and eventually helium-4.

circles and arrows showing protons combining in a series of fusion reactions yielding helium-3 which breaks down tow helium-4 — Illustration of a proton-proton reaction chain, from hydrogen forming deuterium, helium-3, and regular helium-4

The Radiative and Convective Zones

Energy from the core first moves through the radiative zone, where photons are absorbed and re-emitted in a random walk toward the surface. Above this is the tachocline, a transition region, followed by the convective zone. In this outer layer, hot plasma rises and cooler plasma sinks in massive currents, transporting energy to the surface.

The Solar Atmosphere

The Sun's atmosphere consists of three primary regions: the photosphere, the chromosphere, and the corona.

Photosphere: The visible "surface" of the Sun, characterized by convection cells known as granules. It has a temperature of approximately 5,772 K.
Chromosphere: A thin layer above the photosphere that emits a reddish glow.
Corona: The outermost layer, which extends millions of kilometers into space. Surprisingly, the corona is much hotter than the surface, reaching temperatures of 5 million Kelvin.

A false-colour image of the solar photosphere — The photosphere is structured by convection cells referred to as granules.

A photograph of the surface of the sun, with flares terminating from the surface on the left. — The Sun's transition region taken by Hinode's Solar Optical Telescope

A photograph of a solar eclipse — During a total solar eclipse the solar corona can be seen with the naked eye.

The Heliosphere

The Sun's influence extends far beyond its visible edge through the heliosphere, a vast bubble of plasma and magnetic fields created by the solar wind that shields the solar system from interstellar radiation.

Solar Activity and Magnetism

The Sun is magnetically active, leading to phenomena such as sunspots—cooler, darker regions on the photosphere caused by intense magnetic activity. These activities follow a solar cycle, with variations in intensity observed over decades.

A black-and-white photograph of a group of sunspots. — A large sunspot group observed in white light

Life Cycle of a Sun-like Star

The Sun began its life as a collapsing protostar approximately 4.6 billion years ago. It is currently in the main sequence phase, where it stably fuses hydrogen into helium.

Once the hydrogen in the core is exhausted, the Sun will evolve into a red giant, expanding significantly in size and consuming the inner planets. Eventually, it will shed its outer layers and collapse into a dense white dwarf.

Galactic Context and Motion

The Sun is not stationary. It is located between 24,000 and 28,000 light-years from the center of the Milky Way. It orbits the Galactic Center at a velocity of 251 km/s, completing one full revolution (a galactic period) every 225 to 250 million years.

Location of the Sun within the Solar System, which extends to the edge of the Oort cloud where, at 125,000 to 230,000 au (2.0 to 3.6 ly), the Sun's gravitational sphere of influence ends.

Apparent motion of the Solar System barycentre with respect to the Sun

Diagram of the Local Interstellar Cloud, the G-Cloud and surrounding stars. As of 2022, the exact position of the Solar System within the interstellar clouds remains an unresolved question in astronomy.[181]

The general motion and orientation of the Sun, with Earth and the Moon as its Solar System satellites

The Sun's idealised orbit around the Galactic Centre in an artist's top-down depiction of the current layout of the Milky Way

Observational History and Space Missions

Human understanding of the Sun has evolved from religious personifications, such as the Egyptian god Ra or the Nordic sun chariot, to rigorous scientific study. Modern astronomy utilizes specialized solar telescopes and space probes to study the Sun without the interference of Earth's atmosphere.

A sculpture of the sun in a chariot being pulled by a horse that has wheels instead of hoofs. — The Trundholm sun chariot pulled by a horse is a sculpture believed to be illustrating an important part of Nordic Bronze Age mythology.

A drawing of a man wearing a crown in a chariot, being pulled by horses. — Sol, the Personification of the Sun, from a 1550 edition of Guido Bonatti's Liber astronomiae

A photograph of the sun — Sun as seen in Hydrogen-alpha light

A painting of Ra and Nefertari — Ra from the tomb of Nefertari, 13th century BC

Key missions including the Pioneer probes, the Solar Maximum Mission, and the Ulysses spacecraft have provided critical data on the solar wind, magnetic poles, and the internal structure of the star.

A photograph of Ulysses spacecraft — Ulysses spacecraft testing at the vacuum spin-balancing facility

Frequently Asked Questions

What is the primary source of the Sun's energy?

The Sun generates energy through nuclear fusion in its core, specifically the proton-proton reaction chain, which converts hydrogen into helium.

Why is the Sun's corona hotter than its surface?

This is known as the coronal heating problem. While the photosphere is about 5,772 K, the corona reaches 5 million K; this remains a subject of ongoing astronomical research.

How long will the Sun remain in its current state?

The Sun is currently in the main sequence phase of its life cycle and has been for 4.6 billion years, with a similar amount of time remaining before it exhausts its core hydrogen.

What happens to the Sun when it becomes a red giant?

The Sun will expand significantly in size, likely engulfing the inner planets, before eventually shedding its outer layers and leaving behind a white dwarf core.

How fast is the Sun moving through the galaxy?

The Sun orbits the center of the Milky Way at a velocity of approximately 251 km/s.