The Sun The Sun as seen in X-rays (from the Yohkoh satellite) The Sun is a star. It is a rather ordinary star - not particularly big or small, not particularly young or old. It is the source of heat which sustains life on Earth, and controls our climate and weather. It is the closest star to Earth, and the most closely studied. From it we have learned a great deal about the physical processes which determine the structure and evolution of stars in general. Below we discuss the regions of the Sun's atmosphere which we can observe and measure. The Sun's Outer Layers Only the Sun's outer layers, collectively referred to as the solar 'atmosphere', can be observed directly. There are distinct regions to the solar atmosphere: the photosphere, the chromosphere, and the corona. These three regions have substantially different properties from each other, with regions of gradual transition between them. The Photosphere The Sun has basically the same chemicalelements as found on Earth. However, the Sun is so hot that all of these elements exist in the gaseous state. There is not really a "surface" to the Sun. Think of it this way: the Sun is a bunch of gas which gets denser and denser as you move from space toward the solar core. The photosphere would then represent the depth at which we can see no deeper toward the core. Think of what a thick cloud looks like when you look down on it from an airplane - it looks solid, but it isn't. The Parts of a Star The Sun's atmosphere changes from being transparent to being opaque over a distance of only a few hundred kilometers. This is remarkable given the size of the Sun, and represents such a huge change that we often think of it as a true boundary. When we speak of the size of the Sun, we usually mean the size of the region surrounded by the photosphere. The photosphere is slightly different from one place on the Sun to another, but in general is has a pressure about a few hundredths of the sea-level pressure on Earth, a density of about a ten-thousandth of the Earth's sea-level atmospheric density, and a temperature in the range 4500-6000 Kelvin. The Chromosphere The gases which extend away from the photosphere make up the chromosphere. These gases are transparent to most visible radiation. The chromosphere is about 2500 km thick. The density of the gases decreases as you move away from the photosphere into the chromosphere, but the temperature increases! From the bottom to the top of the chromosphere, the average temperature goes from 4500 to 10,000 Kelvin! Needless to say, this rise was not anticipated by scientists when they first measured it. Throughout the rest of the Sun, temperature decreases as you move further away from the core. The Solar Corona seen during a total eclipse The Corona The chromosphere merges into the outermost region of the Sun's atmosphere, the corona. The corona extends for millions of miles into space above the photosphere. Usually, we cannot see the corona because of the brightness of the photosphere. However, during a total solar eclipse, the corona shines beautifully against the dark sky. The corona has a density about 0.0000000001 times that of the Earth's sea-level atmosphere. It is very hot - millions of Kelvin. Because of this high temperature, the bulk of the radiation from the corona is emitted at ultraviolet and X-ray wavelengths. Magnetic fields on the Sun seem to play an important part in heating the gas to such a high temperature. However, the exact way that this happens is not well understood. The image you see to the left was taken during a solar eclipse in 1980; light from the photosphere is blocked out by the Moon (the dark disk). A Few Other Solar Features The Solar Wind The solar wind is nothing more than a stream of charged particles flowing outward from the Sun with an average velocity of about 400 km/sec. It is a natural consequence of the Sun being so hot - the corona gas has too much energy to be gravitationally bound to the Sun. Sunspots and Their Cycle Sunspots are cooler regions on the Sun's photosphere (about 1500 K cooler) and so appear to be darker than the photosphere. A given sunspot can have a lifetime ranging from a few hours to a few months. It consists of two parts - the dark inside region called the umbra and the surrounding less dark region called the penumbra. Their sizes vary over a wide range, with a few having been measured to be 50,000 km in diameter! A German amateur astronomer, Heinrich Schwabe, published a paper in 1851 which stated that the number of sunspots visible on average varied with a period of about 10 years. This conclusion has been substantiated by observations over the 140 years since. The period of repetition on average is 11.1 years, but has been as short as 8 years and as long as 16 years. A plot of the relative number of sunspots as a function of time from 1645 until 1991. During the maximum of the cycle, more than 100 sunspots can be seen on the Sun at once. During the minima, the Sun sometimes has no spots at all. This cycle is closely related to the magnetism of the Sun. In fact, it is the changing magnetic field of the Sun which governs many aspects of solar activity. Life-Giving Star Compared with the billions of other stars in the universe, the sun is unremarkable. But for Earth and the other planets that revolve around it, the sun is a powerful center of attention. It holds the solar system together; pours life-giving light, heat, and energy on Earth; and generates space weather. The sun is a big star. At about 864,000 miles (1.4 million kilometers) wide, it could hold 109 planet Earths across its surface. If the sun were a hollow ball, more than a million Earths could stuff inside it. But the sun isn't hollow. It's filled with scorching hot gases that account for more than 99.8 percent of the total mass in the solar system. How hot? The temperature is about 10,000 degrees Fahrenheit (5,500 degrees Celsius) on the surface and more than 28 million degrees Fahrenheit (15.5 million Celsius) at the core. Deep in the sun's core, nuclear fusion reactions convert hydrogen to helium, which generates energy. Particles of light called photons carry this energy through the sun's spherical shell, called the radiative zone, to the top layer of the solar interior, the convection zone. There, boiling motions of gases (like in a lava lamp) transfer the energy to the surface. This journey takes more than a million years. The sun's surface, or atmosphere, is divided into three regions: the photosphere, the chromosphere, and the solar corona. The photosphere is the visible surface of the sun and the lowest layer of the atmosphere. Just above the photosphere are thechromosphere and the corona, which also emit visible light but are only seen during a solar eclipse, when the moon passes between the Earth and sun. Solar Wind and Flares In addition to light, the sun radiates heat and a steady stream of charged particles known as the solar wind. The wind blows about 280 miles (450 kilometers) a second throughout the solar system. Every so often, a patch of particles will burst from the sun in a solar flare, which can disrupt satellite communications and knock out power on Earth. Flares usually stem from the activity of sunspots, cool regions of the photosphere related to a shifting magnetic field inside the sun. Like many energy sources, the sun is not forever. It is already about 4.5 billion years old and has used up nearly half of the hydrogen in its core. The sun will continue to burn through the hydrogen for another five billion years or so, and then helium will become its primary fuel. The sun will expand to about a hundred times its current size, swallowing Earth and other planets. It will burn as a red giant for another billion years and then collapse into a white dwarf about the size of planet Earth.