Why is the sun hot?

Our sun is the main reason that we have life on our planet. The earth is in our solar system in just the ‘right’ position so that we aren’t too close to the sun to get burned to a crisp and not too far away to become a planetary snowball. The sun is a sphere of hot gases that burn and give off heat that warms. This heat expands into the solar system and the farther away it gets, the cooler it is. So you might wonder what causes the sun to create so much heat. Heat on the sun has a very complex journey process.

When you look out into the stars you are actually seeing the suns of other solar systems. To someone way out there, our sun would look like a star as well. When our sun burst into creation, it was a mass of swirling gases that included a core or center that is compressing atoms together in a process called ‘nuclear fusion’. This intense pressure creates heat at temperatures that are around 15 million degrees C. This is hot enough to instantly vaporize anything before it had a chance to even come close to it.

The immense amount of heat radiates outwards, in the same way that you feel the heat from a room heater. The closer that you get, the hotter it gets. The sun has an ‘atmosphere’ that retains the heat. The heat molecules actually radiate out from the core, bouncing around through the first layer near the core that is called the ‘radiative zone’, for around a million years before it starts to move outward. The next layer that it reaches is called the ‘convective zone’. The temperature here is just a bit less, at 2 million degrees C. It stays there, slowly creating large bubbles of ionized atoms that form into a hot plasma. At this point it’s ready to move into the next layer called the ‘photosphere’.

You might have guessed that the temperature in the photosphere is slightly cooler, at 5,500 degrees C. The photosphere is the location on the sun that the radiation can be detected as actual sunlight. When we see sunspots on the sun it’s actually areas that are cooler and they show up as darker sections of the sun. The center of these sunspots have temperatures that can be around 4,000 degrees C.

The next level of the atmosphere of the sun continues to be cooler, around 4,320 degrees C. This is called the chromosphere. You usually can’t see the visible light in this area because it is weaker than the surrounding photosphere. The only time the photosphere is visible is during a total solar eclipse. At that time, the moon covers the photosphere and the red rim that shows up around the sun is the chromosphere.

The corona has high temperatures as the heat escapes, causing huge plasma streams that peak out like a crown’s points. It can be as hot as 2 million degrees C. As the corona cools it loses its radiation and heat and since it is now in the outer areas of the sun it is blown off as solar wind. It takes eight minutes for any of the sun’s heat to reach the earth, which is 93 million miles away.