Sun

The Sun is the star at the center of the Solar System and one of the roughly two hundred billion stars that make up the Milky Way galaxy. It is an enormous sphere of extremely hot ionized gas, known as plasma, held together by its own gravity. Deep inside the Sun, pressure and temperature become so intense that nuclear fusion reactions can occur, releasing the energy that powers the entire Solar System.

The Sun is made up of several layers, including a central core, internal transition regions, and an outer atmosphere. The visible part of this atmosphere is called the photosphere, which is the surface we normally observe from Earth. Surface temperatures average between 5700-6000°C, while the core is believed to reach temperatures of around 15-16 million degrees.

The Sun continuously emits electromagnetic radiation, including visible light, as well as streams of subatomic particles such as neutrinos. This constant flow of energy makes life on Earth possible. Plants depend on sunlight for photosynthesis, animals depend on plants either directly or indirectly, and even fossil fuels such as oil and coal are ultimately derived from solar energy captured millions of years ago.

The Size of the Sun

Although the Sun appears enormous in our sky, it is actually considered a medium-sized star by astronomical standards. It belongs to the class of G2V main-sequence stars, commonly known as yellow dwarfs.

Some of the Sun’s most important physical characteristics are the following:

• The Sun has a radius of approximately 696,500 km, about 109 times larger than Earth’s radius. If the Sun were hollow, it could contain more than one million Earths. Even though it is not among the largest stars in the universe, it looks huge from Earth because it is relatively close to our planet.
• Its mass is estimated at approximately 2x10³⁰ kg, equivalent to about 330,000 times the mass of Earth. In fact, the Sun contains more than 99% of all the mass in the Solar System. Its immense gravity is what keeps the planets and other celestial bodies in orbit.
• Because the Sun is composed mainly of hydrogen and helium, its average density is about 1.41 g/cm³, roughly one quarter of Earth’s average density. In stellar classification, the Sun is identified as a G2V main-sequence star.

Earth’s Distance from the Sun

The average distance between Earth and the Sun is approximately 149.6 million km. However, this distance changes slightly during the year because Earth follows an elliptical orbit around the Sun.

At perihelion, the point where Earth is closest to the Sun, the distance is about 147.1 million km. At aphelion, the farthest point, the distance increases to approximately 152.1 million km.

The Age of the Sun

The Sun is estimated to be 4.57 billion years old. Based on the typical lifespan of stars with similar properties, astronomers believe it is currently about halfway through its life cycle.

The Sun produces energy through nuclear fusion, a process in which hydrogen nuclei combine to form helium. The helium produced has slightly less mass than the original hydrogen involved in the reaction. The missing mass is converted into energy and radiated outward from the Sun.

Every second, the Sun transforms approximately 564 million tons of hydrogen into 560 million tons of helium. About 4 million tons of matter are converted into energy every second. This relationship between mass and energy is described by Einstein’s famous equation:

E = mc²

E = mc2 is one of the most important equations in modern physics. It describes the relationship between mass and energy in a physical system. In the equation, E represents energy, m represents mass, and c represents the speed of light in a vacuum.

Because the speed of light is extremely large, even a tiny amount of matter can produce an enormous amount of energy. This is why the Sun is able to radiate vast quantities of energy continuously over billions of years.

The amount of solar energy reaching Earth is described by the solar constant, which is approximately 1.374 kW/m². This value refers to the amount of solar radiation received under direct perpendicular sunlight. Overall, the Sun delivers an extraordinary amount of energy to Earth’s atmosphere every year, although only about 70% of that energy reaches the planet’s surface.

Solar Energy Flux

Not all of the solar energy flux reaches Earth’s surface. A significant portion of incoming solar radiation is scattered, reflected, or absorbed by the atmosphere.

Part of the sunlight is scattered by atmospheric gases and redirected back into space before it can reach the ground. Another portion is reflected by clouds, oceans, ice, and other reflective surfaces through the albedo effect.

The remaining solar energy is absorbed by the atmosphere, oceans, and land surfaces, where it plays a fundamental role in regulating Earth’s climate and sustaining life.