The Basics: The Sun

At 93,000,000 miles from Earth the sun is the closest star to our planet. It is approximately 865,000 miles in diameter which is over 100 times the diameter of Earth and the volume of the sun is so large that about 1,000,000 Earths would fit inside. The sun rotates just like our planet, however the equator rotates faster that the polar region by about 9 days.

So how did our sun come to be? A star forms from large areas of gas and dust that are present throughout the universe. There is a battle to maintain equilibrium between the gravity produced by the gas and dust cloud vs. the heat escape from the gas and dust cloud. At some point gravity takes over and the cloud begins to contract. The cause that initiates the contraction is still not thoroughly understood but there are two explanations that have gained acceptance. Anyway, as the gas is compressed the temperature rises at the center of the cloud. At this stage the core is emitting radiation of which some escapes from the cloud but a lot is contained in the cloud. At this point the dust cloud is about as large as our solar system and continues to shrink as gravity continues to win the battle against heat escape. As contraction continues the core starts to really heat up and as a result appears opaque. The temperature at the core of this clump of gas and dust is now about 10,000K and it is now considered a Protostar. We are now about 50,000 years in to the process but there is more to go. The new Protostar continues to contract due to gravitational forces and it is getting hotter with time. Approximately another 50,000 years go by and the star size is now about the diameter of the orbit of the planet Mercury. Is has shrunk quite a bit and the temperature is now approaching 1,000,000K but still to cool to begin the fusion process. During all this time the battle of heat escape and gravity continues. If heat escape wins out before fusion begins the Protostar will simply burn itself out and never become a star. The contraction process continues until the central core temperature reaches about 10,000,000K which is enough to begin the fusion process. At this point we have a star but the contraction process does continue on for another 30 million years or so until our sun resembles its present condition. The newly formed star needs to eventually reach equilibrium with regard to heat escape and gravity.

Ok, but what is fusion and why is it important?  Disclaimer, I am not a nuclear physicist and this is an explanation that I understand from my reading of Astronomy textbooks. Ok, in a nutshell it is the process of converting hydrogen in to helium via the Proton-Proton Chain. The process goes like this:

1. Two protons combine to form a deuteron, a positron, an electron, and a neutrino.

2. The positron and electron collide producing gamma rays. The gamma rays produce heat. The deuteron combines with another proton to form Helium3 releasing additional energy.

3. The Helium3 nuclei collide with another Helium3 nuclei producing Helium4 and two protons. This collision also releases more energy.

Wash, rinse, and repeat until the supply is exhausted. For our sun this is estimated to last another 5 billion years. For those paying attention, what happened to the neutrino in step 1? It doesn’t seem to have been consumed in the fusion process yet it was created. Well, the little buggers escaped the suns gravity with relative ease and some of them eventually reach the Earth. Scientists have constructed neutrino detectors to study them.

Whew, I’m glad that is over! But I didn’t answer the second part of the question, why is fusion important? Did you know that if we could build a bonfire the size of the earth it would put out the same amount of heat as the sun? Yes, it is true. Only one problem though, the wood could not stay burning for billions of years and this is why fusion is so important. The fusion process takes much longer to exhaust its supply.

At this point of the suns lifespan all of the fusion occurs in the core of the sun at approximately 15,000,000K. The radiation has to escape but the journey to the surface is not as easy as it was for the neutrinos. The different layers of the sun offer resistance to the radiation leaving the core and convection has an effect as the heat reaches the surface. During the trip from the core the temperature has been reduced from 15,000,000k to about 10,000F. This convection cooling creates a lot of the granulation surface detail visible in solar images. In addition to convection cooling a lot of surface detail is caused by massive amounts of magnetic lines of flux leaving the surface and then returning. The result of these magnetic disturbances are sunspots which can be seen here. Sunspots usually appear in pairs or groups reflecting the magnetic disturbances leaving and entering the surface. Some sunspots can be many times the size of the Earth and last for several weeks. Sunspots can also generate flares, an even more concentrated area of magnetic disturbances that causes the sunspot to get very bright for a brief period of time and then disappear just as quickly. Another surface feature is a filament. This is a type of prominence that fully appears on the disk only and they appear dark grey or black against the lighter surface color. As the sun rotates the filament may make it to the disk edge and when it does it appear as a prominence.

A prominence is a magnetic disturbance that is seen on the edge of the disk. They appear as an eruption that ejects plasma from the surface in to the atmosphere. There are several different shapes associated with prominences and some appear as a bright sharp stream, some as arches, and others as curtain like. Some types can last for several days as the ejected plasma stays in the atmosphere. These types are often very large and several times the size of the earth. Other types can erupt and within a few minutes become very large and disappear just as quickly. These are often related to flares.