Science, Tech, Math › Science Sun Facts: What You Need to Know Share Flipboard Email Print The layered structure of the Sun and its outer surface and atmosphere. NASA Science Astronomy Solar System An Introduction to Astronomy Important Astronomers Stars, Planets, and Galaxies Space Exploration Chemistry Biology Physics Geology Weather & Climate By John P. Millis, Ph.D Professor of Physics and Astronomy Ph.D., Physics and Astronomy, Purdue University B.S., Physics, Purdue University our editorial process John P. Millis, Ph.D Updated January 10, 2020 That sunlight we all enjoy basking in on a lazy afternoon? It comes from a star, the closest one to Earth. It's one of the great features of the Sun, which is the most massive object in the solar system. It efficiently provides the warmth and light that life needs to survive on Earth. It also influences a collection of planets, asteroids, comets, Kuiper Belt Objects, and cometary nuclei in the distant Oört Cloud. As important as it is to us, in the grand scheme of the galaxy, the Sun is really sort of average. When astronomers put it in its place in the hierarchy of stars, it's not too big, nor too small, nor too active. Technically, it's classified as a G-type, main sequence star. The hottest stars are type O and the dimmest are type M on the O, B, A, F, G, K, M scale. The Sun falls more or less in the middle of that scale. Not only that, but it's a middle-aged star and astronomers refer to it informally as a yellow dwarf. That's because it's not very massive when compared to such behemoth stars as Betelgeuse. The Sun's Surface The Sun may look yellow and smooth in our sky, but it actually has quite a mottled "surface." Actually, the Sun doesn't have a hard surface as we know it on Earth but instead has an outer layer of an electrified gas called "plasma" that appears to be a surface. It contains sunspots, solar prominences, and sometimes gets roiled up by outbursts called flares. How often do these spots and flares happen? It depends on where the Sun is in its solar cycle. When the Sun is most active, it is in "solar maximum" and we see lots of sunspots and outbursts. When the Sun quiets down, it is in "solar minimum" and there is less activity. In fact, during such times, it can look pretty bland for long periods of time. The Life of the Sun Our Sun formed in a cloud of gas and dust about 4.5 billion years ago. It will continue to consume hydrogen in its core while emitting light and heat for another 5 billion years or so. Eventually, it will lose much of its mass and sport a planetary nebula. What's left over will shrink to become a slowly cooling white dwarf, an ancient object that will take billions of years to cool down to a cinder. What's Inside the Sun The Sun has a layered structure that helps it create light and heat and diffuse them out to the solar system. The core is the central part of the Sun is called the core. It's where the Sun's power plant resides. Here, the 15.7 million-degree (K) temperature and extremely high pressure are enough to cause hydrogen to fuse into helium. This process supplies nearly all of the energy output of the Sun, which allows it to give off the equivalent energy of 100 billion nuclear bombs each second. The radiative zone lies outside of the core, stretching to a distance of about 70% of the Sun's radius, the hot plasma of the Sun helps radiate energy away from the core through a region called the radiative zone. During this process, the temperature drops from 7,000,000 K to about 2,000,000 K. The convective zone helps transfer solar heat and light in a process called "convection." The hot gas plasma cools as it carries energy to the surface. The cooled gas then sinks back to the boundary of the radiative and convection zones and the process begins again. Imagine a bubbling pot of syrup to get an idea of what this convection zone is like. The photosphere (the visible surface): normally when viewing the Sun (using only proper equipment of course) we see only the photosphere, the visible surface. Once photons get to the surface of the Sun, they travel away and out through space. The surface of the Sun has a temperature of roughly 6,000 Kelvin, which is why the Sun appears yellow on Earth. The corona (outer atmosphere): during a solar eclipse a glowing aura can be seen around the Sun. This is the Sun's atmosphere, known as the corona. The dynamics of the hot gas that surround the Sun remain somewhat a mystery, although solar physicists suspect a phenomenon known as "nanoflares" are helping to heat the corona. Temperatures in the corona reach up to millions of degrees, far hotter than the solar surface. The corona is the name given to the collective layers of the atmosphere, but it is also specifically the outermost layer. The lower cool layer (about 4,100 K) receives its photons directly from the photosphere, on which are stacked the progressively hotter layers of the chromosphere and corona. Eventually, the corona fades out into the vacuum of space. Fast Facts about the Sun The Sun is a middle-aged, yellow dwarf star. It is about 4.5 billion years old and will live anothe 5 billion years.The Sun's structure is layered, with a very hot core, a radiative zone, a convective zone, a surface photosphere, and a corona. The Sun blows a steady stream of particles out from its outer layers, called the solar wind. Edited by Carolyn Collins Petersen.