Our Sun

Our Sun is a 4.5 billion-year-old yellow dwarf star – a hot glowing ball of hydrogen and helium – at the center of our solar system. It’s about 93 million miles (150 million kilometers) from Earth and it’s our solar system’s only star. Without the Sun’s energy, life as we know it could not exist on our home planet. 

From our vantage point on Earth, the Sun may appear like an unchanging source of light and heat in the sky. But the Sun is a dynamic star, constantly changing and sending energy out into space. The science of studying the Sun and its influence throughout the solar system is called heliophysics.

The Sun is the largest object in our solar system. Its diameter is about 865,000 miles (1.4 million kilometers). Its gravity holds the solar system together, keeping everything from the biggest planets to the smallest bits of debris in orbit around it.

Even though the Sun is the center of our solar system and essential to our survival, it’s only an average star in terms of its size. Stars up to 100 times larger have been found. And many solar systems have more than one star. By studying our Sun, scientists can better understand the workings of distant stars.

The hottest part of the Sun is its core, where temperatures top 27 million °F (15 million °C). The part of the Sun we call its surface – the photosphere – is a relatively cool 10,000 °F (5,500 °C). In one of the Sun’s biggest mysteries, the Sun’s outer atmosphere, the corona, gets hotter the farther it stretches from the surface. The corona reaches up to 3.5 million °F (2 million °C) – much, much hotter than the photosphere.

The Sun orbits the center of the Milky Way, bringing with it the planets, asteroids, comets, and other objects in our solar system. Our solar system is moving with an average velocity of 450,000 miles per hour (720,000 kilometers per hour). But even at this speed, it takes about 230 million years for the Sun to make one complete trip around the Milky Way.

The Sun rotates on its axis as it revolves around the galaxy. Its spin has a tilt of 7.25 degrees with respect to the plane of the planets’ orbits. Since the Sun is not solid, different parts rotate at different rates. At the equator, the Sun spins around once about every 25 Earth days, but at its poles, the Sun rotates once on its axis every 36 Earth days.

Formation

The Sun formed about 4.6 billion years ago in a giant, spinning cloud of gas and dust called the solar nebula. As the nebula collapsed under its own gravity, it spun faster and flattened into a disk. Most of the nebula's material was pulled toward the center to form our Sun, which accounts for 99.8% of our solar system’s mass. Much of the remaining material formed the planets and other objects that now orbit the Sun. (The rest of the leftover gas and dust was blown away by the young Sun's early solar wind.)

Like all stars, our Sun will eventually run out of energy. When it starts to die, the Sun will expand into a red giant star, becoming so large that it will engulf Mercury and Venus, and possibly Earth as well. Scientists predict the Sun is a little less than halfway through its lifetime and will last another 5 billion years or so before it becomes a white dwarf.

Structure

The Sun is a huge ball of hydrogen and helium held together by its own gravity.

The Sun has several regions. The interior regions include the core, the radiative zone, and the convection zone. Moving outward – the visible surface or photosphere is next, then the chromosphere, followed by the transition zone, and then the corona – the Sun’s expansive outer atmosphere.

Once material leaves the corona at supersonic speeds, it becomes the solar wind, which forms a huge magnetic "bubble" around the Sun, called the heliosphere. The heliosphere extends beyond the orbit of the planets in our solar system. Thus, Earth exists inside the Sun’s atmosphere. Outside the heliosphere is interstellar space.

The core is the hottest part of the Sun. Nuclear reactions here – where hydrogen is fused to form helium – power the Sun’s heat and light. Temperatures top 27 million °F (15 million °C) and it’s about 86,000 miles (138,000 kilometers) thick. The density of the Sun’s core is about 150 grams per cubic centimeter (g/cm³). That is approximately 8 times the density of gold (19.3 g/cm³) or 13 times the density of lead (11.3 g/cm³).

Energy from the core is carried outward by radiation. This radiation bounces around the radiative zone, taking about 170,000 years to get from the core to the top of the convection zone. Moving outward, in the convection zone, the temperature drops below 3.5 million °F (2 million °C). Here, large bubbles of hot plasma (a soup of ionized atoms) move upward toward the photosphere, which is the layer we think of as the Sun's surface.

Surface

The Sun doesn’t have a solid surface like Earth and the other rocky planets and moons. The part of the Sun commonly called its surface is the photosphere. The word photosphere means "light sphere" – which is apt because this is the layer that emits the most visible light. It’s what we see from Earth with our eyes. (Hopefully, it goes without saying – but never look directly at the Sun without protecting your eyes.)

Although we call it the surface, the photosphere is actually the first layer of the solar atmosphere. It's about 250 miles thick, with temperatures reaching about 10,000 degrees Fahrenheit (5,500 degrees Celsius). That's much cooler than the blazing core, but it's still hot enough to make carbon – like diamonds and graphite – not just melt, but boil. Most of the Sun's radiation escapes outward from the photosphere into space.

Atmosphere

Above the photosphere is the chromosphere, the transition zone, and the corona. Not all scientists refer to the transition zone as its own region – it is simply the thin layer where the chromosphere rapidly heats and becomes the corona. The photosphere, chromosphere, and corona are all part of the Sun’s atmosphere. (The corona is sometimes casually referred to as “the Sun’s atmosphere,” but it is actually the Sun’s upper atmosphere.)

The Sun’s atmosphere is where we see features such as sunspots, coronal holes, and solar flares.