G-Type Main-Sequence Star

The Sun

The Sun is the star at the center of our solar system. It supplies nearly all the light and heat that make life on Earth possible and dominates the gravitational and energetic environment of the planets.

TYPE
G2V Star
Yellow dwarf (main sequence)
AGE
~4.6 Billion Years
Roughly halfway through its life
DIAMETER
~1.39 Million km
About 109 times Earth’s diameter
SURFACE TEMP
~5,500 °C
Photosphere temperature

What Is the Sun?

The Sun is a typical G-type main-sequence star composed primarily of hydrogen (about 73% by mass) and helium (about 25%), with small amounts of heavier elements. It formed from a collapsing cloud of gas and dust in the Milky Way roughly 4.6 billion years ago.

Nuclear fusion in its core converts hydrogen into helium, releasing the enormous energy that radiates outward as light and heat. This energy sustains Earth’s climate and drives weather, ocean currents, and photosynthesis.

Internal Structure

Core

The innermost region where temperatures reach about 15 million °C. Here, nuclear fusion converts hydrogen into helium and releases energy.

Radiative Zone

Energy from the core moves outward primarily by radiation. Photons are repeatedly absorbed and re-emitted, taking a very long time to travel through this dense region.

Convective Zone

Hot plasma rises and cooler plasma sinks in large convection cells, carrying energy toward the surface more rapidly than radiation alone.

Photosphere

The visible “surface” of the Sun. This thin layer is where most of the light we see originates and where sunspots appear.

Solar Atmosphere

Chromosphere

A thin layer above the photosphere that appears reddish during solar eclipses. Temperature rises with height in this region.

Corona

The outermost atmosphere, extending millions of kilometers into space. It is extremely hot (over a million degrees) and is the source of the solar wind. The corona is visible during total solar eclipses.

Solar Activity & Features

Sunspots

Darker, cooler regions on the photosphere caused by intense magnetic fields that inhibit convection.

Solar Flares

Sudden releases of energy from magnetic reconnection, producing intense radiation across the spectrum.

Coronal Mass Ejections

Large expulsions of plasma and magnetic field from the corona that can affect Earth’s space environment.

Influence on Earth

The Sun provides the energy that powers Earth’s climate system, drives photosynthesis, and creates the conditions necessary for liquid water and life. Variations in solar output and activity influence space weather, which can affect satellites, power grids, and communications.

Earth’s magnetic field and atmosphere protect the surface from the most harmful solar radiation and the solar wind, while still allowing the beneficial light and heat to reach the ground.

Life Cycle of the Sun

The Sun is currently a main-sequence star, steadily fusing hydrogen in its core. In approximately 5 billion years it will exhaust the hydrogen in its core and expand into a red giant, eventually shedding its outer layers and leaving behind a white dwarf.

This long, stable main-sequence phase is typical of stars of the Sun’s mass and is one reason our solar system has been able to support the development of complex life.

Key Takeaways

  • 1. The Sun is a G-type main-sequence star powered by nuclear fusion.
  • 2. Its structure includes a core, radiative zone, convective zone, and photosphere.
  • 3. The corona is the hot outer atmosphere that produces the solar wind.
  • 4. Solar activity (sunspots, flares, CMEs) affects space weather around Earth.
  • 5. The Sun provides the energy that sustains Earth’s climate and life.
  • 6. It is roughly halfway through its main-sequence lifetime of about 10 billion years.