The Sun

by Fraser Cain on March 11, 2012

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The Sun. Image credit: NASA/SDO

The Sun. Image credit: NASA/SDO

The Sun is a huge ball of incandescent plasma at the center of our Solar System. It accounts for more than 99.86% of the mass of the Solar System, and provides all the energy we need for life here on Earth. Ancient civilizations, such as the Romans worshipped the Sun because they saw it as something that brought life. It was given various names such as Sol by the Romans and Helios by the Greeks. And perhaps that worship was reasonable, for without the Sun, life on Earth just wouldn’t be possible.

Characteristics of the Sun

Earth Compared to the Sun. Image credit: NASA

Earth Compared to the Sun. Image credit: NASA

The Sun’s diameter is 1,392,000 kilometres or 865,000 miles; that’s 109 times the diameter of the Earth. You could fit 1.3 million planets the size of the Earth into the Sun. Everything in the Solar System orbits around the Sun, including 8 planets and their moons, many dwarf planets, asteroids, comets and dust. The mass of the Sun really dwarfs the mass of any other object in the Solar System; for exampel, it has 333,000 times the mass of Earth. If the Sun were hollow, you could fit more than one million Earths inside of it – you could only fit 1300 Earths inside of Jupiter.

The Life of the Sun
The Sun formed 4.6 billion years ago from a vast cloud of gas and dust called the Solar Nebula. Over millions of years, this gas and dust collected into the Sun and the planets. Once the Sun’s gravity compressed its hydrogen enough, temperatures in the core reached the point of igniting solar fusion, releasing the energy we feel here on Earth.

Although it’s currently in the main sequence phase of life, the Sun is slowly heating up. It’s expected to last for another 7 billion years or so. Once all the usable hydrogen in the core runs out, the Sun will expand outward into a red giant, consuming the inner planets (maybe even Earth). Then it will slough off its outer layers and collapse inward to become a white dwarf star.

Color of the Sun

The surface of the Sun that we can see is called the photosphere, and it has an average temperature of about 5,800 kelvin. This is the point at which photons generated inside the Sun finally reach the vacuum of space. The Sun’s surface is composed of a number of layers including the photosphere, the chromosphere, and the corona, which are all the outer layers.

Parts of the Sun

Interior of the Sun. Image credit: NASA

Interior of the Sun. Image credit: NASA

But if you could descend down into the Sun, you would find the temperature and pressure increasing until you reached the core of the Sun. At the core, the temperature is 15.7 million kelvin and the pressure is sufficient to support nuclear fusion. This is where protons are merged together to form atoms of helium, releasing tremendous amounts of energy.

Because the Sun is made of plasma, it’s also highly magnetic. It has north and south magnetic poles like the Earth, and the magnetic field lines create the activity we see on the surface. The darker sunspots are created when magnetic field lines pierce through the Sun’s photosphere. Coronal mass ejections and solar flares occur when these magnetic field lines snap and reconfigure. The amount of activity on the Sun rises and falls over an 11-year cycle. At the low point, called solar minimum, there are few, if any sunspots. And then at the high point of the cycle, solar maximum, there are the most sunspots and the greatest amount of solar activity.

The Sun does much more to protect us than just provide heat. It lets off solar winds and material that travels out into space. If we did not have the Earth’s magnetic field, then this material would kill us. However, the solar winds carry out the charged material to the edge of the Solar System where it forms a magnetic field that prevents other interplanetary material from getting in. This barrier is known as the heliopause and without it the Solar System will be battered by cosmic rays.

The charged particles carried by solar winds also interfere with satellites, power lines, and other technology on Earth as well as causing the aurora borealis. Sunlight is vital to our planet, and it takes approximately eight minutes for that light to reach Earth. It depends on where the Earth is in its orbit and how far away it is from the Sun at the time.

Although our Sun appears to be yellow, it is actually white. It merely appears to be yellow because of the effect of the atmosphere. Scientists used to believe that the Sun was not really that impressive of a star. With further studies though, they have come to realize that it is brighter than most of the other stars in the galaxy, which are red dwarfs.

The Sun takes about 1 month to rotate once on its axis; however, this is a rough estimate because the Sun is a ball of plasma. Some parts of the Sun rotate faster than other parts, so it’s hard to say when it has completed a full rotation.

The Sun is composed almost entirely of hydrogen (74%) and helium (25%), with other trace elements. The innermost layer of the Sun is the core, where the nuclear fusion reactions are taking place. Outside that is the radiative zone, where photons of gamma radiation created in the core are emitted and absorbed by hydrogen atoms. A single photon can take 100,000 years to finally get through the radiative zone. Outside the radiative zone is the convective zone, where bubbles of plasma rise and fall like a lava lamp.

The Sun is made up of different layers that actually rotate at different speeds. Because the Sun is more similar to a gas giant in that it is high in helium and hydrogen, it also experiences what is known as differential rotation. At the outer layers, near the equator it rotates about once every 25.4 days  while near the poles it takes up to 36 days to complete one rotation.  Sunspots are cooler areas on the surface of the Sun, which last up to a few months and vary greatly in size. Sunspots appear in cycles and sometimes there appear to be none on the Sun at all. Scientists are still trying to learn more about sunspots.

Only about 5% of stars in the Milky Way are larger than the Sun; the vast majority are smaller red dwarf stars. Some of the biggest stars can be 100,000 times brighter and contain 100 times more mass. The Sun is also relatively young, a member of the Population I group of stars. Older stars, which formed billions of years before the Sun are Population II stars and have less heavier elements in them. The oldest stars are Population III stars, formed just after the Big Bang, but these are purely theoretical.


Location and Movement of the Sun

How the Sun Works

Solar Activity

History of the Sun

Observing the Sun

Other articles about the Sun


Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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