Basic Facts About The Sun Part 2

The Sun is a typical G2 star. G stars are classified as having a temperature in the range of 5000 to 6000 K, and a color ranging from white to yellow. Spectrally, G stars show most predominantly the lines of ionized calcium. Lines from ionized and neutral metals are present. Lines from ionized hydrogen show up weakly.

What are the physical characteristics of the Sun?

What we see as the surface of the Sun, the photosphere, has a temperature of about 5780 K. The interior of the Sun probably has a temperature around 16 million K, and the extended outer atmosphere of the Sun, the solar corona, has a temperature around 2 million K. Between the photosphere and the corona is a layer called the chromosphere. It is in this region where most of the temperature rise from the surface to the corona takes place. The rapid heating from the surface of the Sun to the chromosphere and then of the corona, is one of the very interesting problems in solar physics. Especially since most of this change occurs in a distance of less than 200 km!

The composition of the Sun is primarily hydrogen, followed by rapidly decreasing amounts of almost every element. Below is a list showing the fractional amounts of the most common elements.

         0.9396  hydrogen
        0.05919  helium
      0.0006483  oxygen
      0.0003946  carbon
      0.0000817  nitrogen
      0.0000423  silicon
      0.0000376  magnesium
      0.0000348  neon
      0.0000301  iron
      0.0000150  sulfur
      0.0000028  aluminum
      0.0000019  calcium
      0.0000019  sodium
      0.0000019  nickel
      0.0000009  argon

The diameter of the Sun is 1.4 million kilometers, and is about 150 million km away from the Earth. In contrast, the diameter of the Earth is 12735 km, about 1/100 the size of the Sun.

The distance from the Earth to the Sun varies throughout the year. At perihelion (closest approach) the distance is 147 million km, and at aphelion (farthest) the distance is 152 million km. Due to this distance variation, the Sun will appear about 3% bigger at perihelion than at aphelion. At this point in geological time, perihelion occurs in early January, and aphelion in early July. Slowly but surely, however, the perihelion point precesses, so in 23,000 or so years, perihelion will occur in July.

Note that the Earth's seasons are due to the inclination of the Earth's equator with respect to the Earth's orbital plane, which is about 23 degrees. The Earth-Sun distance variation has only an incredibly small effect in temperature. Here's a way you can test this. Put yourself about 152 feet away from a friend, and then have them yell at you. Then, move 5 feet (3%) closer, and have them yell again. You will notice almost no difference in how loud they sound. (The metric equivalent is: 46 meters away, and then 1.5 meters closer.)

The Sun's age is estimated to be around 4.5 billion years. It should remain more or less as it is for another 5.5 billion years, although it will continually be undergoing changes as it consumes its fuel through fusion.

What kind of light does the Sun emit?

The light or photons emitted from the Sun cover a broad spectrum from very long wavelengths such as radio to very short wavelengths such as xray. Long term exposure to UV and xrays are very damaging. So, it is a good thing the Earth's atmosphere shields us from the harmful portions of the Suns photons, otherwise there would be very little life on Earth. (At least as we know it!)

What are some historical observations of the Sun?

The Sun has been studied by humans for a long time. Eclipses were probably first "recorded" prior to 1948 B.C., and telescopic observations of the Sun's surface began around 1610. It was at this time that the sunspots could be systematically observed and were, by Galileo, Fabricius, Scheiner and Harriot. For Galileo this was actually unlucky. Western religions expected the Sun to have no "blemishes" and the observations of such did not further his career at all. He was forced to recant and placed under house arrest.

Nevertheless, observations of sunspots and the Sun continued. Around 1645 the sunspot count became very low until 1715. During these 70 years, there were likely less than 15 sunspots observed. These days, the minimum number observed per year is more like 15, even when the Sun is in its "inactive" phase. Interestingly, at this same time, cooler than normal temperatures were had in Europe. So, there is some indication that variations occurred in the Sun at the same time which cooled Earth's climate. This time period is referred to as the "Little Ice Age", and the sunspot absence as the Maunder minimum.

Why is it important to study the Sun?

As can be seen from the above, variations of the Sun's output do occur, and these affect the Earth's climate. Indeed, tree ring studies and ice core studies indicate a correlation between Earth's ice ages and the Sun's activity.

On an more timely basis, the Sun often undergoes rapid magnetic field reconfigurations. When this occurs, large amounts of material are ejected into interplanetary space. These events are called Coronal Mass Ejections or CMEs. A large CME will carry a million tons of material out towards the planets at a million miles an hour. (Almost a billion kg at 27 million m/s.) When a portion of this material reaches the Earth's outer atmosphere, it impacts satellites, perturbing their orbits, scoring their surfaces, and disrupting communications. The solar material can slide down the Earth's magnetic field lines and cause the phenomenon known as Aurora, and also burn out and totally disrupt power grids.

CMEs occur perhaps once a month or so during solar minimum, and up to twice or more a day at solar maximum. The Sun's most recent max was in 1989 and the next will be in approximately 2000. It is now approaching minimum which should occur in mid-1996.

Solar max and min referred initially to the number of sunspots observed on the surface of the Sun at any time. The number of sunspots is closely related to the complexity of the Sun's magnetic field that extends past its surface. The more complex the field, the higher the number of sunspots, and the higher the activity of CMEs and flares. There is some variation, but typically there are 11 years from one maximum to the next maximum.

How does one study the Sun?

Since the magnetic field of the Sun is so important, scientists observe the Sun in as many wavelengths as possible that give insight into the configuration and dynamics of the field. These are the various emission and absorption lines of atoms near the Sun's surface, and electron scattering of photons higher in the Sun's atmosphere, the corona.

In addition, the rotation of the interior of the Sun is very important to the existence of the magnetic field. Since one cannot see the interior of the Sun, one must use indirect methods. As everyone knows, if one strikes an object, one can tell something about its substance by the way it vibrates in response. For example, a bell will ring, and jello will just wiggle. The same applies to the Sun. The many dynamic flares, CMEs and rolling, boiling of the Sun cause it to vibrate. With very accurate telescopes the vibrations can be measured, and then carefully analyzed to determine properties of the interior. This is a very exciting new field.

Does the Sun have a surface?

The Sun visible to our eyes does not have a solid surface such as that of the Earth or the Moon. The visible Sun is a hot gas with a characteristic temperature of 5700 deg. K, well beyond the melting points of material on Earth. Nevertheless, we see only its very outer layers because the gas is opaque. The effect is the same as that for a cloud which we know is composed of water molecules but which appears to have a fluffy surface. This outer visible surface is only a few hundred kilometers thick on the Sun and is called the Photosphere. This layer is the top of the solar convection zone where the solar energy is carried to the outer surface by convective gas motions over the last quarter of the solar radius. Further inside lies the radiative zone where the energy is carried principally by radiation, not convection. At the very center is the nuclear core generating energy by fusion of hydrogen to helium at temperatures of 20 million deg. K.

Above the photosphere are two additional layers, the chromosphere and corona, which were first identified at eclipses of the Sun by the Moon. The chromosphere is an inhomogeneous layer extending 10,000 km above the photosphere. It is best thought of as the transition from the photosphere to the corona. The very outer extent of the Sun proper is the tenuous corona which can extend several million kilometers into the interplanetary medium. Such extensions of the solar atmosphere produce the striking images seen at the times of solar eclipses.

Does the brightness of the Sun change over time?

Yes, modern measurements between 1978 and 1995 show that the "brightness" or total irradiance of the Sun fluctuates by a few tenths of a precent over the 11 year solar cycle. This small fluctuation reflects stability of the solar photosphere as seen in the visible spectrum which extends from the blue at 400 nanometers (nm) to the deep red at 800nm. Observations from space show increasing variation from the ultraviolet below 400nm to the x-ray region down to .1nm. However, the bulk of the output solar energy is in the visible spectrum; therefore, its variation dominates fluctuations on the Sun's "brightness".

How does the Sun work (what goes on inside)?

The Sun is a giant, natural thermonuclear reactor that converts hydrogen to helium in its core to produce the heat we sense on our faces as sunshine. Why does this reactor not explode as a thermonuclear bomb? The Sun is held together in an equilibrium state by the mutual gravitational attractions between all its atoms acting to compress the solar center and, thus, produce and contain the nuclear reactions taking place there. The solar atmosphere outside the energy generating core adjusts itself to carry the enormous amount of energy that emerges from the surface in the form of radiation. This is the basic idea behind the existence of all stars beginning with primordial gravitational attraction and compression to the beginning of nuclear energy generation and, finally, to the exhaustion of the nuclear fuel and death of the star as a truly self luminous object.

What are the key areas of solar research today?

Contemporary solar research falls in two basic areas: 1) studies of the outer solar atmosphere and its variation, and 2) studies of the inside of the Sun using seismological techniques similar to those employed in oil prospecting on the Earth. Studies of the solar interior reveal for the first time the motions and thickness of the various internal zones predicted by the theory of stellar interiors such as the nuclear core, the radiative zone, and the convective zone. The interface between the radiative and convective zones appears to be the shell where the Sun generates the magnetic fields eventually seen on the surface in sunspots and other structures associated with the 11 year solar cycle. Thus, understanding the inside of the Sun is crucial to understanding solar variability due to the effects of intense magnetic fields appearing at the visible surface.

What are the northern lights, or aurorae?

The northern lights occur near the north pole of earth. The phenomena is known as the aurora borealis when it occurs in the northern hemisphere and as the aurora australis when it occurs in the southern hemisphere. Auroral phenomena occur on all planets with atmospheres and planetary magnetic fields (aurorae have been observed on Jupiter). The name aurora borealis comes from the latin for northern dawn.

We now know what causes these spectacular displays in the sky. The interaction of the solar wind with the geomagnetic field of the Earth cause energetic particles (primarily electrons and protons) to enter into the Earth's upper atmosphere where they interact with molecules of nitrogen and oxygen to produce the red and green light seen in the auroral phenomena (as seen from space, as seen from Earth, some recent research results).

How does the Sun affect communications?

In our technology-based economy we depend heavily on satellites and various forms of high frequency communication systems. Communications and navigation systems used by commercial airliners can be affected by geomagnetic storms which are caused by solar activity. Geomagnetic storms can actually cause the atmosphere of Earth to expand affecting satellite orbits. An excellent review of these issues can be found at the Space Environment Laboratory (NOAA).

How does the Sun affect our climate?

The Sun drives the weather on planet Earth. The winds and circulation of ocean patterns are all affected by the Sun's energy output. The differential heating of the planet, due to the tilt of the rotation axis of the Earth with respect to the Sun generates the winds and major ocean currents as well as providing us with our seasons. Furthermore, it is believed that the 11-year solar cycle has an impact on our climate. An excellent review of the climatic impact of the Sun on Earth can be found at Space Environment Laboratory (NOAA).

What is space weather?

One effect of the Sun's output on the geospace environment is auroral phenomena. Other phenomena affect communications, navigation and our climate. A nice presentation about the interactions of the Sun with the Earth can be found at Space Weather at Rice University.

National Center for Atmospheric Research