Here's What You Should Know about the Sun

The Sun casts its glow over the ocean. NOAA/Cmdr. John Bortniak

That sunlight you enjoy basking in on a lazy afternoon? It comes from a star, the closest one to Earth. The Sun is the most massive object in the solar system and provides the warmth and light that life needs to survive on Earth. It also warms and influences a collection of planets, asteroids, comets, and Kuiper Belt Objects and cometary nuclei in the distant Oört Cloud.

As important as it is to us, the Sun is really sort of average when you put it in the grand hierarchy of stars.

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. It's middle-aged 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. There are sunspots, solar prominences, and 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. 

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

The Sun's Structure 

The Core: The central part of the Sun is called the core. 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. The Sun gives off the equivalent energy of 100 billion nuclear bombs each second.

The Radiative Zone: Outside 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. During this process the temperature drops from 7,000,000 K to about 2,000,000 K.

The Convection Zone: Once the hot gas has cooled enough, just outside the radiative zone, the heat transfer mechanism changes to 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 and it will give you 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 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 (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.

Edited by Carolyn Collins Petersen.