Our Local Star - The Sun


The Sun


The Sun is an ordinary star. Its claim to fame might be the fact that we are here, living on one of its planets. The Sun dominates the Solar System by making up 99.9% of the mass of the system. Everything else in the system is in orbit around the Sun.

In case you don't have a feel for the Sun's size, remember that its diameter is about 109 times Earth, which makes the Sun about 860,000 miles diameter. It's mass is about 331,600 Earths.

The Sun appears to us as a bright disk in the sky, the disk that ancient Egyptians called the Aten. It subtends an angle of 1/2 degree (30") on the sky. This is smaller than you might think. Since it isn't safe to look at the Sun, try this with a full Moon, which is safe to look at. Find the smallest object which, held at arm's length, will cover the Moon. You may be surprised at the result.

Note that the Sun is the only star close enough for us to study in real detail. In rounded numbers, the nearest star is about 260,000 times farther away than the Sun - and that's the NEAREST other star.

The Sun's effective surface temperature is 5780K, but 6000K is an easy number to remember and is close enough for our purposes. The layer that you see as the Sun is the photosphere (literally - light sphere). It is not a "surface" like that of Earth, but rather a depth in the Sun where the gas (mostly hydrogen) becomes opaque. There's still a lot of hydrogen above the photosphere, but it's transparent (except for absorption lines, of course). The chromosphere (literally - color sphere) lies right above the photosphere. We normally don't see it because the emission lines from its excited gas are completely overpowered by the brilliant continuous spectrum light from the photosphere. It IS possible to get a glimpse of the chromosphere during a solar eclipse; at the beginning and end of totality, when the Moon is just covering the photosphere, the chromosphere can be seen just beyond the limb of the Moon. It appears for a few seconds as a reddish arc at the edge of the Moon. The red color is from the 656.3 nm emission line of hydrogen.

The next total solar eclipse in North America occurs on 21 August 2017. It enters the U.S. in Oregon, crosses through Missouri and goes out in South Carolina. Here are some links for more information.

All the energy from the Sun comes from nuclear reactions in the core, which is about 200,000 km (about 125,000 mi). That energy comes out of the core as very high energy gamma radiation, but by the time the energy reaches the surface it has cooled to about 6000K. Near the core, the temperature gradient is small enough that radiation works more effectively than convection, so the energy is carried by radiation in that region. Higher up, the density is lower and the temperature gradient larger, so convection dominates.

That nuclear reaction produces a LARGE amount of energy, an amount which has been accurately measured. The solar constant is about 1400 watts per square meter. Measuring that energy output is simple in concept. First - imagine a sphere with its center in the Sun and a radius of 1 AU. Compute the area of the sphere by using 4 times pi times the radius squared. Multiply the total square meters in the sphere by the energy per square meter to get the total energy output of the Sun.

Next - from physics we can get the amount of energy produced in the formation of one helium nucleus in the core. Divide the total energy output (ergs/second) by the energy released forming one helium nucleus (energy/helium) and you get helium/second, which is the number of helium atoms being created every second. Since 4 hydrogen nuclei go into one helium, this simple calculation will yield the number of hydrogen atoms being consumed every second! You can see this in "More Precisely" on page 263.

We need to quickly review the means of energy transport.

For conduction, imagine having a copper bar about 2 feet long and holding it in your hand while you stick the far end of it into a fire; you won't hold on to it very long. Convection requires a fluid which can move and carry heat. Remember that hot air rises, so as the air around a hot object rises, it is replaced by cooler air from below. This produces flow that carries heat away. Radiation is simply electromagnetic energy, which will travel perfectly well through space; no medium is needed. Here's an illustration of all three.

Look at Figure 9.6 on page 247. The (a) part shows how the density of the Solar material varies with depth. The Sun's average density is about 1400 kg/m&^sup3;, or 1.4 gm/cm³. This is only 1.4 times the density of water. But that's AVERAGE Sun. In the core the density is 100 times higher! We don't ever encounter material of that density.

Read the section on energy transport on page 247. Note the physical difference between the radiation and convection zones. The radiation zone is almost completely transparent to the high-energy radiation, so radiation is the most efficient way of getting the energy out. As the gas gets cooler at higher levels, more nuclei collect electrons and become whole atoms. The gas begins to become opaque, blocking radiation. The gas then absorbs the energy and is heated. This drives convection in the upper layers.

The convection in the photosphere can be seen; look at Figure 9.8 on page 249. Each of those little glowing convection cells about the size of Texas! Over a period of hours the motion can be observed. Read that section beginning on page 249. The convection stops at the photosphere; above the photosphere the gas is transparent and radiation is umimpeded.

The age of the Sun is interesting, but if you ask the Sun how old it is, it won't tell you! How to find out??? Since the Sun and planets formed in the same process and the Sun itself is likely a little bit older than the planets, we have a chance. We can use the little brother example; suppose you don't want to reveal your age (not unusual). I can't ask you how old you are because you won't tell. But - suppose you have a younger brother and I know that brother. He's a guy and doesn't mind telling how old he is, so I ask. He tells me. Now - I know that you are a few years older than your brother. This gives me a VERY GOOD estimate of your age; it's not exact, but is close enough.

We can consider the Earth as the Sun's little brother. We CAN find out about how old the Earth is by studying the oldest rocks on the surface; this yields an age for Earth of ABOUT 4.5 billion years. Add a little bit of margin and we get the Sun's age as 5 billion years, plus or minus a bit.