1. Why Observe the Sun
Most stargazers spend most of their time observing sights in the night sky. But the daytime sky offers an opportunity to examine—close up—the seething face of a major star, the Sun, the nearest star to Earth. As you are about to discover, when you become a regular observer of our home star, there's quite a bit for you to see. With the right equipment, many fascinating and beautiful features can be seen on the visible face of the Sun with a small telescope or binoculars, and you can enjoy special events such as total and partial solar eclipses and transits of planets and other objects across the face of the Sun.
Solar observing has many benefits for amateur astronomers. Unlike many faint objects in the night sky, the Sun is easy to locate and track during the day. There's no need to stay up late and lose sleep when observing the Sun. And with modern solar filters and accessories, it's possible to get amazing views of the Sun from your own backyard that were accessible only to professional astronomers twenty years ago.
1.1 How the Sun Works: An Overview
The Sun is a star, the nearest star to Earth. Like all stars, it's a massive sphere of mostly hydrogen and helium gas and a brew of ionized hydrogen in the form of protons and electrons which together are called a plasma. The Sun is held together by its own gravity, and the mass of its outer layers pushes down on the interior region and core, heating it and igniting nuclear reactions that convert hydrogen into helium and causing the release of huge amount of energy. This energy pushes back against the internal gravitation of the Sun and prevents it from collapsing further.
The Sun has no solid "surface" like the Earth and other rocky planets in the solar system. The gas and plasma that makes up the Sun simply gets denser and hotter from the outer regions towards the center. The light generated in the core of the Sun is scattered in many directions by the dense soup of plasma within the vast majority of the Sun's volume, so we can't see very far into the Sun. Only in the very outer layers, when the gas becomes cool enough for some of the electrons to reunite with the atoms of hydrogen and helium, is the light free to flow outwards from the Sun and into your telescope.
Although astronomers can't see inside the Sun, they can determine, through observation and calculation, distinct sections in the Sun based on the temperature, pressure. Energy is transferred through these sections from the hot core, where it is created, to the cooler outer layers that we can see. Let's have a look at the main parts of the Sun.
1.2 The Insides of the Sun
The Core. The core of the Sun is where hydrogen turns into helium through the process of nuclear fusion. Every second, the Sun turns about 600 billion kg of hydrogen into helium in the core. That's the energy equivalent of about 100 billion megatons of TNT. The core accounts for about 20% of the solar radius and 99% of the energy production in the Sun. Its temperature is about 15 million K and its density is some 150 g/cm3. The density of lead, by comparison is about 11.3 g/cm3.
Most of the energy is released as tiny particles called neutrinos and as an energetic form of light called gamma rays. The neutrinos, which interact very weakly with matter, emerge from the core and out into space in a matter of seconds. The gamma rays take much longer to escape from the dense solar interior. They tend to scatter about for 100,000 years or so, losing energy all the while, before they finally emerge from the Sun as visible light. So the sunlight falling on your garden flowers was produced in the center of the Sun about 100,000 years ago, on average.
Radiative Zone. Above the core lies a zone where very little nuclear fusion occurs. Here, the atoms of hydrogen and helium are still ripped apart from their electrons because of the intense heat of some 2 million to 7 million K. Energy escapes outward here from the hotter lower layers to the cooler outer regions by thermal radiation, the same effect that causes your hand to lose heat when placed, for example, near a cold window. The radiative zone extends from about 20% to about 70% of the solar radius.
The Convective Zone. Moving out from the radiative zone, it becomes cool enough for some atoms to recombine with their electrons. This results in the formation of cells of convection, where hot gas rises in convective cells or bubbles, a little like bubbles in a pot of boiling soup. As hot gas rises, it releases heat and light to the outer layers of the Sun, cools and becomes denser, and falls back down to the lower layers of the convective zone.
The sharp division between the radiative and convective zones is called the tachocline. It lies about 200,000 km below the Sun's visible surface. The convective zone takes up most of the rest of the solar radius. You can glimpse the top layers of these deep convective zones in the Sun's photosphere. They look like circular granules on the visible surface of the Sun.
1.3 The Visible Parts of the Sun
The Photosphere. Moving further out from the convective zone, the plasma cools and the hydrogen and helium gas become mostly neutral atoms again. The light moving upward is no longer scattered around in all directions and visible light of all wavelengths escapes freely into space. The thin layer at which this happens is called the photosphere. The bright white light we can see with our eyes and telescopes comes primarily through the photosphere—Greek for the "sphere of light". It's just a few hundred kilometers thick and is about as transparent to light as Earth's atmosphere. At the photosphere, the Sun's gas has cooled to a temperature of about 5,700 K, and the light we see is similar to that coming from a solid glowing body of that temperature. The density of the photosphere is less than 1% that of Earth's atmosphere.
The magnetic fields within the Sun and the underlying convection zones result in many interesting features within the Sun's photosphere that are visible with small telescope and an appropriate solar filter. These features include:
- Sunspots. Large dark spots caused by strong magnetic fields inside the Sun that reach the photosphere, sunspots are dark patches several thousand kilometers across that last for one to two weeks. They are among the easiest features to observe.
- Granules. The hot gas in the Sun's photosphere rises and falls in large bubbles or cells about a thousand kilometers across. Because these cells look like little grains across the visible surface of the Sun, they are called solar granules.
- Pores. Small dark features that appear to be the beginnings of new sunspots.
- Faculae. Hotter, brighter patches in the photosphere which, like sunspots, are also caused by magnetic fields.
- Limb Darkening. An effect in the photosphere where the extreme edge of the solar disk appears darker than the center.
The Chromosphere. Just above the photosphere lies an even more rarified region called the chromosphere, a region so-named because of its colorful appearance. The light from the chromosphere comes mostly from hydrogen atoms excited into higher energy levels. These excited hydrogen atoms emit light at specific wavelengths, particularly a wavelength 656.3 nm, the so-called "hydrogen alpha" wavelength, that corresponds to red-orange light. The chromosphere can be spotted as a reddish ring when the Moon completely covers the face of the Sun during a total solar eclipse.
The bright white light from the Sun's photosphere can overwhelm the fascinating and dynamic features in the chromosphere. However, by using a filter that passes only light at 656.3 nm and blocks all other wavelengths of visible light, you can get a detailed view of the chromosphere. With such a filter and with a small telescope, many features in an around the chromosphere become visible including:
- Prominences and Filaments. Immense loops of hot gas suspended over the chromosphere by magnetic fields, they are perhaps the most dramatic features visible in a small telescope with an H-alpha filter.
- Plages. Bright patches associated with sunspots but well above them in the chromosphere.
- Flares. Gigantic and extremely bright ejections of material from the Sun with the energy of millions of hydrogen bombs.
- Chromospheric Network. A subtle weblike structure in the chromosphere.
- Spicules. Small short-lived jets of material that move directly upward from the Sun's surface.
All of these features, those visible in white light and in hydrogen-alpha, will be described in more detail in the next articles in this series.
The Corona. The temperature drops as you move from the Sun's core to the chromosphere. But it the upper layers of the chromosphere, the temperature begins to increase again. No one knows why. It may be a result of magnetic effects. Above the chromosphere lies the Sun's ethereal corona, a tenuous vapor of plasma that extends millions of kilometers into space and has an effective temperature of 1 million K, nearly 200 times the temperature of the photosphere. The chromosphere can only be seen directly as a ghostly-white glow during a total solar eclipse.
1.4 White-Light vs. Narrowband Solar Filters
Now that you have an understanding the workings of the Sun and its visible layers, the photosphere and the chromosphere, you're equipped to understand the two main types of solar filters available to amateur astronomers.
The most prominent visible layer of the Sun, the photosphere, emits brilliant light at all visible wavelengths, as well as the infrared and ultraviolet. So to safely observe the photosphere, with or without a telescope, you need a broadband or white-light solar filter that reduces the intensity of all colors of light entering your eye to a safe level suitable for visual observation or imaging. These filters, which reduce the brightness of light by 99.999%, are available for most types of telescopes, binoculars, and even camera lenses. Many white-light solar filters typically look like mirrors because they reflect most of the visible light that falls on them.
With a white-light solar filter, you can see features in the Sun's photosphere. Such features, as mentioned above, include sunspots, faculae, solar granules, and limb darkening. Later articles in this series will discuss white-light solar filters and what to see with them in greater detail.
Light from the Sun's chromosphere comes from atoms that emit light not over a broad spectrum but at discrete wavelengths. To see light from the chromosphere, you need a filter that passes light from hydrogen and other atoms and blocks the white light from the much brighter photosphere. That's the purpose of narrowband solar filters.
The most common type of narrowband filter, a hydrogen alpha solar filter, passes light around a very narrow band near 656.3 nm. Because hydrogen-alpha solar filters block the much brighter white light from the photosphere, they allow the direct observation of events and features on the Sun that are not visible with white-light solar filters, especially the dramatic solar prominences and filaments that loop and arc thousands of miles above the Sun's visible surface. You will learn more about narrowband filters and solar features in subsequent articles in this series on observing the Sun.