What These Pages Show IF YOU HAVE ANY DOUBT WHATSOEVER ABOUT HOW TO LOOK AT THE SUN SAFELY, WITH OR WITHOUT A TELESCOPE, JUST DON'T LOOK AT IT AT ALL, OK? You're on the net already, so please use GOOGLE and do some research on "safe solar filters" from companies like Thousand Oaks, Baader, and (if you're flush) DayStar. It's easy for me to forget that this is a pretty arcane subject. One of the visitors to my sungazing pages asked some questions which others may be wondering about, too: Just looked over your new pages. What does "hydrogen alpha" mean? Reading over the text (which is very nicely written, by the way) I don't have a very good idea what you're looking through, or what the coffee can is for. And why are you using drawings instead of photographs-- is the quality better, or is there a technical problem with sun photography? What *are* these things anyway? And how big are they? The size of Alaska? Mercury? Are they just huge world-sized globs of burning sun-stuff, tossed away from the sun surface by huge explosions? Clark Here's the latest version of the post I sent back to Clark. If anyone has a better way of explaining this, or if I've made a real mess of the explanation, by all means, let me know!. Let's go through Clark's questions one point at a time. What does "hydrogen alpha" mean? The spectral line which the filter isolates is that of the first Balmer transition of hydrogen. Electrons orbit the nuclei of atoms at discrete distances -- only particular orbital radii are possible. When electrons drop from one orbit to another, the difference in orbital energy is shed in the form of light. Light of very particular wavelengths is produced when electrons change orbits. When the lone electron of the hydrogen atom falls from the third available orbit (counting out from the nucleus) to the second available orbit, it emits a photon with a wavelength of 6,563 Angstroms. (An Angstrom is a tenth of a nanometer and a nanometer is 1/1,000,000,000 of a meter). Light of this wavelength is a very deep, ruby red. All the solar observations on these pages are made in that particular color of light. What you see are very rarified clouds of hydrogen doing as hydrogen does when it's heated to several tens of thousands of degrees and subjected to powerful magnetic fields. Uhm, yes, but what does "hydrogen alpha" mean? Sorry... all light emitted by electron transitions is named according to the "series" of the transition. Electrons falling from outer orbits all the way to the first orbit produce the "Lyman" series. Electrons falling to the second orbit produce the "Balmer" series. Within each series, the light is further distinguished according to the orbit from which the electron has fallen. Light emitted when electrons fall from the first orbit outside the destination orbit comprises the "alpha" line of each series. Light produced when electrons fall from the next outer orbit makes the "beta" line. The 6,563A line of hydrogen is the Alpha line of the Balmer series of hydrogen. And that's where the name comes from. Strictly speaking, "hydrogen alpha" is ambiguous or vague -- it might as well refer to the Lyman alpha line of hydrogen. By custom and historical accident it doesn't. Reading over the text (which is very nicely written, by the way) I don't have a very good idea what you're looking through, or what the coffee can is for. Shucks. Thanks. I'm using the large, stubby telescope shown in the photograph on the shameless book promo page. The coffee-can fits neatly over the lens-end of the telescope. It serves as a filter-holder, as a bracket to hold two "pre-filters" which together attenuate the incoming light and discard both damaging infrared wavelengths (longer than about 7300A) and blue wavelengths which the delicate interference filter (the H-a filter proper) would also pass. Because of the pre-filters, the H-a filter only has to deal with light between about 6300A and 7300A. The Lumicon prominence filter only passes light in a 1.5A band centered on the wavelength of hydrogen-alpha light (or is it 3A, for 1.5A on either side -- whichever). (I've since improved upon the coffee-can holder. But not by much. The new one is made from several turns of thin brass strip and the lid of the coffee can. According to Home Depot, the brass was meant to be weather stripping for windows. DC, April 2004.) And why are you using drawings instead of photographs-- is the quality better, or is there a technical problem with sun photography? This kind of observing combines characteristics of planetary, deepsky, and lunar observing. One of the characteristics of planetary gazing is the collection of fine detail through turbulent air, which is abundant in the daytime. The details one can see dance around in the shimmering image -- exposures long enough to record the prominences are badly blurred by this shimmering. The observer can "collect" fine details seen during moments of steady seeing and quickly assimilate a lot of detail that would never be caught in a smeared-out photo. (Oh, but a webcam ought to be perfect for this; I just haven't tried that yet. DC, 4/2004) It's like deep-sky observing because the subjects are low-contrast, amorphous clouds like nebulae. And it's like lunar observing because you're looking for both bright and dark detail on every visible scale, and the detail changes in about the same time-scale as lunar features do when they are most interestingly lit, as the sun rises and sets on them. Solar cycle 23 is just getting started. By the years 2000 - 2001, the sun will be a much more active and spectacular place. I want to be really in practice by then to enjoy the show. (Encore! Encore! On to cycle 24! DC 4/2004) What *are* these things anyway? And how big are they? The size of Alaska? Mercury? Are they just huge world-sized globs of burning sun-stuff, tossed away from the sun surface by huge explosions? First, to a very good approximation, the Sun is nuthin' but hydrogen. At different depths in the Sun, we see only hydrogen under different conditions. The visible surface of the sun -- the photosphere -- is relatively dense hydrogen glowing at 5,000 - 6,000 degrees Kelvin (Kelvin degrees are the same as Celsius or Centigrade degrees, but the zero point is really absolute zero rather than the freezing point of water. Kelvin temperatures are just Celsius temperatures to which 273 has been added.) At temperatures of 5,000 - 6,000 degrees Kelvin, dense hydrogen (hydrogen at the pressure of Earth's atmosphere at sea level, more or less) glows brightly at all visible wavelengths just like a bar of white-hot iron. This is "continuum radiation" -- light comprised of a continuous band of wavelengths. That's the fierce white light you see when you look at, or glance at, the Sun. Above the photosphere is a region called the chromosphere in which hydrogen is heated up to about 20,000 degrees K. At the higher temperature and at the lower pressure of the chromosphere (about 1/100 of the Earth's air at sea-level, maybe less) glows via the same mechanism as neon in an electric sign -- by the excitation and de-excitation of its electrons in their atomic orbits. Practically all the visible light of the chromosphere comes from hydrogen undergoing the first Balmer transition (whose excruciating description you suffered through above). The hot, thin hydrogen at the top of the chromosphere comes into contact with even hotter, thinner hydrogen. This hydrogen is ionized: it's so hot that its single electron has been stripped away completely. Without that electron, orbital transitions are not possible, and this gas cannot shine by electron excitation. It does shine by other mechanisms but primarily in soft x-ray wavelengths and by scattering photons of visible light rising from far below. The corona (clearly visible only during total solar eclipses) is so thin it would be called a vacuum by any earthly standard and is heated to two to four million degrees Kelvin (precisely how it is heated remains a mystery, as far as I know [dc 4/2004]). The Sun's corona is beyond red- and white-hot. It's literally x-ray-hot. So now I can tell you what we're seeing: What we're seeing are "prominences," magnetically encapsulated portions of the chromosphere levitated by magnetic forces into the tenuous solar corona. The filter shows these loops and sheets and curtains and spires of relatively cool gas (about 20,000 degrees K) suspended far above the layer where the surrounding gas is of the density and temperature to produce their characteristic deep red light. An early Italian astronomer, Secchi, saw that the chromosphere consists of uncountable streamers of deep red hydrogen, crowded together like tall grass, colored as if on fire. He called it the "burning prairie" and his description was perfect. This is the layer you see in my drawings just above the well-defined limb of the sun. The "burning prairie" at the edge of the Sun is five to eight thousand miles deep. In moments of steady seeing, the individual spicules -- slender blades of prairie grass -- are visible, and then the glowing red fog resolves into Secchi's "burning prairie." It's a great show. High above the burning prairie are these towering, graceful prominences (think of thunderstorms rising above an earthly prairie). On these pages, solar prominences are drawn more or less to scale. Very few details shown here are as small as the Earth. So ends Solar Astronomy 101. There will not be a test, but I welcome questions and corrections! DC Take me home, Mr. Wizard!