Chapter 14: The Sun: A Garden Variety Star

For the most of the remainder of the course we will be discussing stars in one form or another. We will begin this discussion with the closest star, our Sun.

The Sun holds special significance to residents of Earth. It is our source of light, heat, and ultimately life. It is worshiped and celebrated, even to this day.

Some of the Sun's properties that have been measured are listed in Table 14.1.

14.1 Outer Layers of the Sun

We can see only the outermost part of the Sun. Scientists divide the Sun into a number of layers, both those we can't see and those we can.

The Sun is an enormous ball of hot gas held together by its gravity. At the center is the core. Around the core is a region called the radiative zone, and surrounding that is the convective zone. These are the inner parts of the Sun, mostly invisible to our instruments.

The outer atmosphere of the Sun is visible to our telescopes. The innermost layer of the atmosphere is called the photosphere. Above the photosphere is the chromosphere, and beyond that lies the corona.

Composition of the Sun

The Sun is composed mostly of hydrogen and helium. The discovery that these are the primary consituents of the Sun was rather shocking initially. Today, we understand this fact, and it fits in very nicely with our understanding of what are the most abundant elements of the universe, also hydrogen and helium.

The Solar Photosphere

The photosphere is the opaque surface that astronomers see when they look at the Sun. (Reminder: As your mother told you, don't look directly at the Sun.) We cannot see past the photosphere, so this is what you might refer to as the "surface" of the Sun. (It is not a solid surface like the surface of the Earth, just the layer beyond which we cannot see.) The diameter of the Sun is generally taken as the diameter of the photosphere.

The photosphere is responsible for the light that we see from the Sun. Light generated inside the Sun is absorbed in the photosphere and re-emitted with a spectrum (color) determined by the temperature of the photosphere. This is "blackbody" radiation that we discussed in chapter 4. The radiation from the Sun tells us that the temperature of the photosphere is 5800K on average.

The Chromosphere

The chromosphere is a layer of gases beyond the photosphere. These gases are transparent to light, so the chromosphere is as well. Until recently, the chromosphere was only visible during a total solar eclipse. (It is a rather interesting coincidence that the Moon's size and distance is just perfectly matched to the Sun.) Observations of the chromosphere show mainly a bright red emission line due to hydrogen.

In 1868, a yellow emission line was seen in the chromosphere. This line didn't correspond to any element known at the time. Scientists realized that it must come from a new element (recall that the periodic chart had several "holes" in it from elements that had yet to be discovered) and that element was named helium after helios, the Greek word for Sun.

The transition region is a thin layer between the chromosphere and the corona where the temperature climbs (!) from about 10,000K to 1,000,000K.

The Corona

The corona (latin for crown) has been known for centuries. It is clearly visible during a solar eclipse. The temperature in the corona is known because in the corona we see spectral lines of highly ionized atoms of iron, nickel, argon, ... It requires temperatures of a million degrees kelvin to produce these ionized states.

But the corona is very rare (low in density). It is about 10 billion times less dense than the Earth's atmosphere.

The Solar Wind

A stream of charged particles called the solar wind comes out from the Sun. The solar wind is responsible for the "blowing" of a comet's tail, and for the charged particles in the Earth's magnetosphere.

14.2 The Active Sun Introduced

The Sun is not a static fixed object. It is constantly changing, with variable sunspots, coronal loops, and solar flares. Much of this seems to arise from the complicated dynamics going on inside the Sun. Scientists study the details of what can be seen in the photosphere of the Sun to gain a better understanding of the dynamics going on inside the Sun.

14.3 The Sunspot Cycle

Sunspots are the most conspicuous features of the photosphere. Sunspots were noticed thousands of years ago. They appear as dark spots on the surface of the Sun. We now understand that they are dark because they are much cooler than the surrounding region, about 1500K cooler.

The number of sunspots on the Sun goes through an 11 year cycle. The changing magnetic field of the Sun drives the sunspot cycle. The connection between magnetic fields and sunspots is seen in the high magnetic fields present in sunspots (Figure 14.15).

14.4 Activity Above the Photosphere

Above the photosphere, activity is seen by looking for emission lines of atoms. A number of different types of features are seen:

14.5 Is the Sun a Variable Star?

Something we are most certain of is that the Sun will rise tomorrow. Yet, we now understand that the Sun is not constant from year to year. The Sun undergoes an 11 year cycle of sunspots, and on longer time scales, it undergoes changes in overall activity -- that is, the amount of light and heat produced. While these changes are small, about a tenth of a percent (0.1%), this is enough to produce periods of unusual cold or heat. The period 1645 to 1715 is documented to be a period of unusually low sunspot activity and unusually cold temperatures in Europe (the "little ice age").

Observations of other stars indicate that it is normal for their activity (measured as something called luminosity) changes by 0.3 percent, or as much as 1 percent. It may be normal for the Sun's output to vary by similar amounts.

© Robert Harr 2001