From our Earthly perspective, the Sun might seem like a constant, unchanging source of warmth and light. However, this celestial body is far from static. It’s a dynamic star, continuously evolving and radiating energy throughout our solar system. Heliophysics, the scientific study of the Sun and its vast influence across the solar system, helps us understand this ever-changing nature.
The Sun reigns supreme as the largest entity in our solar system, boasting a diameter of approximately 865,000 miles (1.4 million kilometers). Its immense gravitational pull is the glue that binds our solar system, dictating the orbits of everything from colossal planets to minute space debris.
Despite its central role and vital importance for life on Earth, the Sun is considered an average-sized star. Scientists have discovered stars up to 100 times larger. Moreover, many solar systems are home to multiple stars. By studying our Sun in detail, we gain invaluable insights into the workings of stars far beyond our reach.
The Sun’s core is an inferno, reaching temperatures of a staggering 27 million °F (15 million °C). In contrast, the visible surface, known as the photosphere, is a comparatively cooler 10,000 °F (5,500 °C). Intriguingly, the Sun’s outer atmosphere, the corona, defies expectations by becoming hotter the further it extends from the surface, reaching up to a scorching 3.5 million °F (2 million °C) – significantly hotter than the photosphere.
The Solar and Heliospheric Observatory (SOHO) marked its 25th anniversary on December 2, 2020. Since its launch, SOHO has been instrumental in constantly monitoring the Sun, providing us with unprecedented views and data.
Names Through History
Across cultures and languages, the Sun has been revered and named. The Latin term “sol,” meaning Sun, is the root of “solar,” the adjective for all things related to the Sun. Helios, the Sun god in ancient Greek mythology, also lends his name to many solar terms, such as heliosphere and helioseismology, reflecting the enduring human fascination with our star.
Life’s Dependence on the Sun
While the Sun’s extreme temperatures and radiation preclude the possibility of life as we know it on its surface, it is undeniably the source of all life on Earth. The Sun’s light and energy are fundamental for our planet’s ecosystems and survival.
Size, Scale, and Stellar Neighbors
Our Sun is classified as a medium-sized star with a radius of about 435,000 miles (700,000 kilometers). While many stars dwarf our Sun in size, it is vastly more massive than Earth. It would take over 330,000 Earths to equal the Sun’s mass, and a staggering 1.3 million Earths to fill its volume.
The Sun is situated approximately 93 million miles (150 million kilometers) from Earth. Our nearest stellar neighbors are in the Alpha Centauri system. Proxima Centauri, a red dwarf star, is 4.24 light-years away, while Alpha Centauri A and B, two sun-like stars orbiting each other, are 4.37 light-years distant. A light-year, the distance light travels in a year, is approximately 6 trillion miles (9.5 trillion kilometers).
Orbit and Solar System Motion
The Sun resides within the Milky Way galaxy, specifically in a spiral arm called the Orion Spur, extending from the Sagittarius arm.
Alt text: Illustration depicting the Sun’s location within the Orion Spur of the Milky Way galaxy, highlighting the spiral arm structure.
The Sun, carrying along planets, asteroids, comets, and all other solar system bodies, orbits the Milky Way’s center at an average speed of 450,000 miles per hour (720,000 kilometers per hour). Even at this incredible velocity, a single orbit around the galactic center takes approximately 230 million years.
As it orbits the galaxy, the Sun also rotates on its axis, tilted at 7.25 degrees relative to the plane of planetary orbits. Being a ball of gas and plasma, the Sun doesn’t rotate uniformly. Its equator completes a rotation roughly every 25 Earth days, while the poles take about 36 Earth days.
Absence of Moons and Rings
As a star, the Sun does not possess moons. However, planets orbiting the Sun, of course, have their own moons.
In its early formation, around 4.6 billion years ago, the Sun was enveloped in a disk of gas and dust. Remnants of this primordial dust still exist as several dust rings orbiting the Sun, tracing the paths of planets whose gravity influences their positions.
The Sun’s Fiery Birth
The Sun originated approximately 4.6 billion years ago from a vast, swirling cloud of gas and dust known as the solar nebula. As gravity caused the nebula to collapse, it spun faster and flattened into a disk. The majority of the nebula’s material gravitated towards the center, giving birth to our Sun, which now accounts for 99.8% of the solar system’s total mass. The remaining material coalesced to form planets and other celestial objects, while the young Sun’s solar wind dispersed the leftover gas and dust.
Like all stars, our Sun has a finite lifespan. In its twilight years, the Sun will transform into a red giant, expanding dramatically to potentially engulf Mercury, Venus, and possibly even Earth. However, scientists estimate that the Sun is currently just under halfway through its life, with about 5 billion years remaining before it eventually becomes a white dwarf.
Internal Structure and Layers
Alt text: A detailed 3D model illustrating the Sun’s internal structure, showcasing the core, radiative zone, convection zone, photosphere, chromosphere, transition zone, and corona.
The Sun is essentially a colossal sphere of hydrogen and helium, bound together by its own gravity.
Its structure is comprised of distinct regions. The interior consists of the core, the radiative zone, and the convection zone. Moving outwards, we encounter the visible surface or photosphere, followed by the chromosphere, the transition zone, and finally the corona, the Sun’s expansive outer atmosphere.
When material escapes the corona at supersonic speeds, it becomes the solar wind, creating a vast magnetic “bubble” around the Sun called the heliosphere. The heliosphere extends far beyond the orbits of planets in our solar system, meaning Earth is actually situated within the Sun’s atmosphere. Beyond the heliosphere lies interstellar space.
The core is the Sun’s hottest region. Nuclear fusion reactions, where hydrogen atoms fuse to form helium, generate the Sun’s heat and light. Temperatures in the core exceed 27 million °F (15 million °C), and it spans approximately 86,000 miles (138,000 kilometers) in thickness. The Sun’s core is incredibly dense, about 150 grams per cubic centimeter (g/cm³), roughly 8 times denser than gold and 13 times denser than lead.
Energy produced in the core travels outwards via radiation, bouncing through the radiative zone for around 170,000 years before reaching the convection zone. In this zone, temperatures drop below 3.5 million °F (2 million °C). Here, enormous bubbles of hot plasma (ionized gas) rise towards the photosphere, the layer we perceive as the Sun’s surface.
The Sun’s “Surface” – The Photosphere
Unlike Earth and other rocky bodies, the Sun lacks a solid surface. The photosphere is what we commonly refer to as the Sun’s surface. “Photosphere” literally means “light sphere,” aptly named as this layer emits the majority of the Sun’s visible light, the light we see from Earth. (It is crucial to remember: never look directly at the Sun without proper eye protection.)
Although called the surface, the photosphere is actually the lowest layer of the solar atmosphere. It’s about 250 miles thick with temperatures around 10,000 degrees Fahrenheit (5,500 degrees Celsius). While cooler than the core, this temperature is still intensely hot, capable of boiling carbon in all its forms. Most of the Sun’s radiation escapes from the photosphere into space.
Solar Atmosphere: Chromosphere, Transition Zone, and Corona
Above the photosphere lie the chromosphere, the transition zone, and the corona. Some scientists don’t classify the transition zone as a separate region, considering it simply the thin layer where the chromosphere rapidly heats up to become the corona. The photosphere, chromosphere, and corona collectively constitute the Sun’s atmosphere. The corona is often informally called “the Sun’s atmosphere,” but more accurately, it’s the Sun’s upper atmosphere.
The Sun’s atmosphere is where we observe dynamic features like sunspots, coronal holes, and solar flares, all contributing to the ever-changing appearance of our star.
Key Features to Observe on the Sun
From our vantage point, the Sun’s appearance is dominated by various features that reveal its dynamic nature. Sunspots, darker, cooler areas on the photosphere, are indicators of magnetic activity. Solar flares are sudden bursts of energy emanating from the Sun, often associated with sunspots. Coronal holes are regions in the corona with lower density and temperature, appearing darker in certain wavelengths and being sources of high-speed solar wind. These features constantly change and evolve, making the Sun a fascinating object to observe and study, ensuring that “what our Sun will look like” is never a static picture.
