The Sun
The Sun is the star at the center of the Solar System and the source of nearly all the energy that reaches Earth. It is a vast sphere of hot plasma whose gravity holds the planets, asteroids, comets, and other objects in orbit.
Without the Sun, Earth would have no sunlight and would be far too cold to support life as we know it.
A Main-Sequence Star
The Sun is a G-type main-sequence star, sometimes informally called a yellow dwarf. It formed about 4.6 billion years ago from a collapsing cloud of gas and dust and is roughly halfway through its stable lifetime.
It is made mostly of hydrogen and helium. At its center, enormous pressure and temperature allow nuclear fusion to occur continuously.
How the Sun Produces Energy
Deep inside the Sun's core, hydrogen nuclei combine to form helium. This process, called nuclear fusion, releases vast amounts of energy. Every second, the Sun converts about 4 million tons of matter into energy.
That energy slowly moves outward through the Sun before radiating into space as sunlight, heat, and other forms of electromagnetic radiation.
Structure of the Sun
The Sun has several major layers: the core, radiative zone, convective zone, photosphere, chromosphere, and corona. The photosphere is the visible layer that we usually think of as the Sun's surface.
The corona is the Sun's outer atmosphere. It is surprisingly much hotter than the photosphere, reaching temperatures of millions of degrees. Scientists continue to study exactly how the corona becomes so hot.
Surface Activity
The Sun is highly active and constantly changing. Visible features include sunspots, which are cooler regions linked to strong magnetic fields; solar flares, which are sudden releases of energy; coronal mass ejections; and granules, which are bubbling convection cells on the photosphere.
Sunspot activity follows an approximately 11-year solar cycle, during which activity rises and falls. Powerful solar eruptions can affect satellites, radio communications, navigation systems, power grids, and auroras on Earth.
Key Facts About the Sun
Diameter: About 865,000 miles (1.39 million kilometers)
Mass: About 333,000 times Earth's mass
Surface Temperature: About 10,000°F (5,500°C)
Core Temperature: About 27 million°F (15 million°C)
Average Distance from Earth: About 93 million miles (1 AU)
Age: About 4.6 billion years
The Sun and Earth
Light from the Sun takes about 8 minutes and 20 seconds to reach Earth. Solar energy drives weather, the water cycle, photosynthesis, and many other processes that shape life and climate on our planet.
Earth orbits within the Sun's habitable zone, the region where temperatures can allow liquid water to exist on a planet's surface. Earth's atmosphere, oceans, and natural greenhouse effect also play essential roles in maintaining those conditions.
The Future of the Sun
The Sun will continue fusing hydrogen in its core for roughly another 5 billion years. It will then expand into a red giant, making the inner Solar System far hotter and leaving Earth uninhabitable long before that stage is complete.
Afterward, the Sun will shed its outer layers and leave behind a dense stellar remnant called a white dwarf. The expelled gas may form a glowing shell known as a planetary nebula.
Our Star
The Sun is far more than a bright object in the sky. It is the gravitational anchor of the Solar System, the main energy source for Earth's climate and ecosystems, and our closest example of a star.
By studying the Sun, scientists learn more about the billions of stars throughout the galaxy and the dynamic processes that shape planetary systems.