In Depth | Sun – NASA Solar System Exploration

Introduction

Our Sun is a yellow dwarf star, a hot ball of glowing gases at the heart of our solar system. Its gravity holds the solar system together, keeping everything – from the biggest planets to the smallest particles of debris – in its orbit. The connection and interactions between the Sun and Earth drive the seasons, ocean currents, weather, climate, radiation belts, and auroras. Though it is special to us, there are billions of stars like our Sun scattered across the Milky Way galaxy.

Namesake

Namesake

The Sun has many names in many cultures. The Latin word for Sun is “sol,” which is the main adjective for all things Sun-related: solar.

Potential for Life

Potential for Life

The Sun itself is not a good place for living things, with its hot, energetic mix of gases and plasma. But the Sun has made life on Earth possible, providing warmth as well as the energy that organisms like plants use to form the basis of many food chains.

Size and Distance

Size and Distance

With a radius of 432,168.6 miles (695,508 kilometers), our Sun is not an especially large star – many are several times bigger – but it is still far more massive than our home planet: It would take 332,946 Earths to match the mass of the Sun. The Sun’s volume would need 1.3 million Earths to fill it.

This illustration shows the approximate size of Earth compared to the Sun. Image Credit: ESA & NASA

The Sun is about 93 million miles (150 million kilometers) from Earth. Its nearest stellar neighbor is the Alpha Centauri triple star system: Proxima Centauri is 4.24 light-years away, and Alpha Centauri A and B – two stars orbiting each other – are 4.37 light-years away. (A light-year is the distance light travels in one year, which is equal to 5,878,499,810,000 miles or 9,460,528,400,000 kilometers.)

Orbit and Rotation

Orbit and Rotation

The Sun, and everything that orbits it, is located in the Milky Way galaxy. More specifically, our Sun is in a spiral arm called the Orion Spur that extends outward from the Sagittarius arm. From there, the Sun orbits the center of the Milky Way galaxy, bringing the planets, asteroids, comets, and other objects along with it. Our solar system is moving at an average velocity of 450,000 miles per hour (720,000 kilometers per hour). But even at this speed, it takes us about 230 million years to make one complete orbit around the Milky Way.

The Sun rotates as it orbits the center of the Milky Way. Its spin has an axial tilt of 7.25 degrees with respect to the plane of the planets’ orbits. Since the Sun is not a solid body, different parts of the Sun 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.

Moons

Moons

The Sun and other stars don't have moons; instead, they have planets and their moons, along with asteroids, comets, and other objects.

Rings

Rings

The Sun does not have rings.

Formation

Formation

The Sun and the rest of the solar system formed from a giant, rotating cloud of gas and dust called a solar nebula about 4.5 billion years ago. As the nebula collapsed because of its overwhelming gravity, it spun faster and flattened into a disk. Most of the material was pulled toward the center to form our Sun, which accounts for 99.8% of the mass of the entire solar system.

Like all stars, the Sun will someday run out of energy. When the Sun starts to die, it will swell so big that it will engulf Mercury and Venus and maybe even Earth. Scientists predict the Sun is a little less than halfway through its lifetime and will last another 6.5 billion years before it shrinks down to a white dwarf.

A 3D model of the Sun, our star. Credit: NASA Visualization Technology Applications and Development (VTAD)

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Structure

The Sun, like others stars, is a ball of gas. In terms of the number of atoms, it is made of 91.0% hydrogen and 8.9% helium. By mass, the Sun is about 70.6% hydrogen and 27.4% helium.

The Sun has six regions: The core, the radiative zone, and the convective zone in the interior; the visible surface, called the photosphere; the chromosphere; and the outermost region, the corona.

The Sun's enormous mass is held together by gravitational attraction, producing immense pressure and temperature at its core. The Sun has six regions: The core, the radiative zone, and the convective zone in the interior; the visible surface, called the photosphere; the chromosphere; and the outermost region, the corona.

At the core, the temperature is about 27 million degrees Fahrenheit (15 million degrees Celsius), which is sufficient to sustain thermonuclear fusion. This is a process in which atoms combine to form larger atoms and in the process release staggering amounts of energy. Specifically, in the Sun’s core, hydrogen atoms fuse to make helium.

The energy produced in the core powers the Sun and produces all the heat and light the Sun emits. Energy from the core is carried outward by radiation, which bounces around the radiative zone, taking about 170,000 years to get from the core to the top of the convective zone. The temperature drops below 3.5 million degrees Fahrenheit (2 million degrees Celsius) in the convective zone, where large bubbles of hot plasma (a soup of ionized atoms) move upwards. The surface of the Sun – the part we can see – is 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.

Surface

VIDEO

Understanding the Sun’s behavior is an important part of life in our solar system. The Sun’s powerful outbursts can interfere with satellites, and disrupt communications signals traveling around Earth.

Surface

The surface of the Sun, the photosphere, is a 300-mile-thick (500-kilometer-thick) region, from which most of the Sun's radiation escapes outward. This is not a solid surface like the surfaces of planets. Instead, this is the outer layer of the gassy star.

We see radiation from the photosphere as sunlight when it reaches Earth about eight minutes after it leaves the Sun. The temperature of the photosphere is about 10,000 degrees Fahrenheit (5,500 degrees Celsius).

Atmosphere

Atmosphere

Above the Sun's photosphere are the tenuous chromosphere and the corona (crown), which make up the thin solar atmosphere. This is where we see features such as sunspots and solar flares.

Visible light from these top regions is usually too weak to be seen against the brighter photosphere, but during total solar eclipses, when the Moon covers the photosphere, the chromosphere looks like a red rim around the Sun, while the corona forms a beautiful white crown with plasma streamers narrowing outward, forming shapes that look like flower petals.

Strangely, the temperature in the Sun's atmosphere increases with altitude, reaching as high as 3.5 million degrees Fahrenheit (2 million degrees Celsius). The source of coronal heating has been a scientific mystery for more than 50 years.

The Sun releases a constant stream of particles and magnetic fields called the solar wind. This solar wind slams worlds across the solar system with particles and radiation — which can stream all the way to planetary surfaces unless thwarted by an atmosphere, magnetic field, or both. Here’s how these solar particles interact with a few select planets and other celestial bodies. Credit: NASA’s Goddard Space Flight Center/Mary Pat Hrybyk-KeithMagnetosphere

Magnetosphere

The electric currents in the Sun generate a complex magnetic field that extends out into space to form the interplanetary magnetic field. The volume of space controlled by the Sun's magnetic field is called the heliosphere.

The Sun's magnetic field is carried out through the solar system by the solar wind – a stream of electrically charged gas blowing outward from the Sun in all directions. Since the Sun rotates, the magnetic field spins out into a large rotating spiral, known as the Parker spiral.

The Sun doesn't behave the same way all the time. It goes through phases called the solar cycle. Approximately every 11 years, the Sun’s geographic poles change their magnetic polarity. When this happens, the Sun's photosphere, chromosphere, and corona undergo changes from quiet and calm to violently active. The height of the Sun’s activity, known as solar maximum, is a time of solar storms: sunspots, solar flares, and coronal mass ejections. These are caused by irregularities in the Sun's magnetic field and can release huge amounts of energy and particles, some of which reach us here on Earth. This space weather can damage satellites, corrode pipelines and affect power grids.

Resources

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