Core The Sun’s energy source, SOLAR OBSERVING this is where hydrogen fuses into helium. Twisted field lines With a good solar filter, you can see sunspots, flares, and other hot The Sun’s rotation twists mag- Solar features netic field lines deep inside it. stuff. ⁄ ⁄ ⁄ BY MICHAEL E. BAKICH Faculae These bright areas on the photosphere appear brightest near the limb. How to observe the Sun Limb darkening Near the Sun’s limb (edge), Flares range from subflares (smaller The Sun, because it’s the brightest sky Photosphere light must travel farther than 2 square degrees) to Importance 4 This is the Sun’s through the solar atmo- object, also is the easiest to observe. Put safety first, and even flares, which cover more than 24.8 square visible surface. sphere. This effect darkens a small telescope will delight you with high-quality views. degrees. On the Sun, one square degree the limb. equals roughly 57 million square miles (150 Plus, you can make good solar observations even when con- million square kilometers). Corona This is the Sun’s outer ditions rule out seeing other celestial objects. Sunspots Dark areas in a sea of light atmosphere, the source of the solar wind. Dark spots mark Start with the disk Here, hydrogen atoms emit energy called Sunspots, which are features of the photo- where magnetic fields, The photosphere is the Sun’s visible surface Hydrogen-alpha (Hα) radiation. Hα is sphere, come in many shapes and sizes, amplified inside the and is the lowest observable layer of solar reddish-colored light with a wavelength of according to the whim of the Sun’s mag- Solar wind Sun, break through atmosphere. Observing the photosphere is 656.28 nanometers (nm). netic field. The field traps gas, slowing its This thin, ionized the surface. easy through visible-light solar filters. Through an Hα filter, which allows only motion and making it cooler than the sur- gas speeds away If the seeing (atmospheric steadiness) is Hα light through, you’ll see prominences, rounding area on the Sun’s surface. from the Sun. good, you’ll spot granulation, which bright gas clouds ejected from the Sun and Usually, sunspots consist of a dark cen- observers describe as a mottled effect. Vast shaped by its magnetic field. Prominences tral region called the umbra surrounded by Prominence Flare gas bubbles, whose centers are rising and appear as spikes, loops, “trees,” detached a lighter region known as the penumbra. Magnetic fields suspend Flares are sudden edges are sinking, create granules. regions, and more. Prominences look like The penumbra’s temperature is typically gas far above the Sun’s releases of energy surface. Prominences Faculae are bright areas visible on the dark lines silhouetted against the solar disk; 1,800° F (1,000° C) below that of the photo- stored in sunspot mag- Granulation sometimes erupt. photosphere. Facula is Latin for “little astronomers call those lines filaments. sphere, and that of the umbra between netic fields. They’re Gas bubbles whose centers are rising torch.” Faculae appear all over the disk, but Another Hα feature appears as bright areas 2,700° F (1,500° C) and 3,600° F (2,000° C) often associated with and edges are falling create a mot- observers most often see them near the around sunspots called plages. cooler than the photosphere. coronal mass ejections. tling effect called granulation. solar limb. There, the contrast between the Roughly every 11 years, solar activity ASTRONOMY: ROEN KELLY faculae and the darkened limb is highest. Solar explosions peaks, resulting in greater numbers of sun- Finally, look for a phenomenon called Also best seen through Hα filters, solar spots and flares. German astronomer Hein- a sheet of paper. Others use a box assembly. through glass filters it can appear white, Hydrogen-alpha (Hα) filters limb darkening. We observe limb darken- flares occur when the Sun’s atmosphere rich Schwabe (1789–1875) discovered the A box is a better choice because it darkens yellow, or orange. Glass filters are more Observing the Sun at the wavelength of Hα ing because the Sun is a sphere. Near what suddenly releases built-up magnetic energy. sunspot cycle in 1843. This “11-year cycle” the surrounding area and increases con- expensive but more durable. light is gaining in popularity. All Hα filters we see as the edge of the solar disk, the light Solar flares emit radiation storms and are varies from as few as 9.5 to as many as 12.5 trast. In either case, the sheet of paper dis- All solar filters fit over a telescope’s center on 656.3nm. However, such filters must travel farther through the solar atmo- the solar system’s largest explosions. years. The start of any given solar cycle is plays a 6-inch (150mm) circle — the objective (front) end. Some cover the entire have different bandpass widths. The widest sphere. This causes the limb to be dimmer Astronomers classify flares by how defined as the minimum of solar activity. standard size used by observers worldwide. objective (full-aperture filters), while others of these can be nearly 2 Angstroms (Å) and than the rest of the disk. much area they cover at the time of maxi- Since the 19th century, astronomers have Mark the four directions, focus the Sun, have smaller openings offset from center the narrowest 0.3 Å. mum brightness. recorded sunspot numbers each day. and fit it to your circle. If it doesn’t fit, (off-axis filters). Off-axis filters eliminate One Angstrom equals 0.1 nanometer. The chromosphere In 1858, Swiss astronomer Johann either adjust the eyepiece/paper distance or secondary-mirror obstructions in Newto- Prominences look great through a 1 Å- The “sphere of color” lies just Rudolf Wolf (1816–1893) published his choose an eyepiece with a different focal nian and catadioptric scopes. All solar fil- bandpass Hα filter, but chromospheric above the photosphere. formula for determining the daily sunspot length. Don’t use eyepieces with lens ele- ters should have round openings. Other detail is low. Through a filter with a 0.5 Å number: k(10g + f); where g is the number ments held together by cement because the shapes introduce distracting patterns. bandpass, you’ll see lots of chromospheric of sunspot groups; f is the number of indi- Sun’s heat will damage them. Never use a solar filter that fits into an detail but few prominences. Some Hα fil- vidual sunspots; and k is a calibration fac- eyepiece. Some of these filters have cracked ters are tunable; you can shift the bandpass’ THE CORONADO tor, different for each observer. To calculate Visual solar filters due to heat buildup (and it doesn’t take central wavelength slightly to either side. PST, available from Meade your k-number, see this story’s web extra at A good solar filter is safe — it does not long) and caused eye damage. Solar observing is addictive. Soon, you’ll Instruments, is a complete Hydrogen- www.Astronomy.com/toc. transmit harmful ultraviolet or infrared The most common solar filters transmit find yourself watching the Sun as much as alpha telescope. ASTRONOMY: WILLIAM ZUBACK radiation. It also drops the Sun’s brightness 0.001 percent of the Sun’s light. This drops the stars. Don’t forget the sunscreen. Observing by solar projection to a comfortable level. the Sun’s brightness by roughly 12.5 magni- Michael E. Bakich is a senior editor of One way to observe the Sun is by using an Visible-light filters are either coated tudes. Photographic-only solar filters with WEB To learn how to classify sunspots, Astronomy who loves both visible-light and eyepiece to project the Sun’s image. Some glass or optical-quality Mylar. The solar densities of 3 (7.5-magnitude drop) and 3.5 EXTRA be sure to log on to www.Astronomy.com/toc. Hydrogen-alpha solar observing. observers use an adjustable arm that holds image through Mylar looks pale blue; (8.75-magnitude drop) are also available. © 2010 Kalmbach Publishing Co. This material may not be reproduced in any form 64 Astronomy ⁄ ⁄ ⁄ Aprilwithout 08 permission from the publisher. www.Astronomy.com www.Astronomy.com 65.