Exploring the sun: in this year of peak solar activity, gear up to watch the closest star.
In Earth's sky, the Sun appears as big as the Moon does--they're both V20 wide, about the width of your little fingertip at arm's length. So in a solar-filtered telescope, the Sun looks just as enormous as the Moon. Its face shows much less detail, usually no more than a few little sunspots and maybe some white faculae (bright Sun-clouds) around the edge. On the other hand, while the Moon's surface features are forever fixed, those on the Sun are always changing--from week to week, day to day, and occasionally from minute to minute.
But you really need the right equipment! Looking at the Sun bare-eyed or through an improper filter can damage your vision for life. That's especially true if you look into a telescope that may concentrate sunlight enough to set paper on fire.
Fortunately, nowadays you have more inexpensive choices than ever to adapt your scope for safe solar viewing.
The First Step
The first and cheapest thing to get is a simple "white light" solar filter. It goes over the front of your telescope and blocks 99.999% of the Sun's light from entering, as well as the Sun's invisible but damaging ultraviolet and infrared rays. This allows safe, comfortable high-resolution observing.
White-light filters come in two types: metal-coated glass and metal-coated wispy plastic film, often Mylar. A glass filter may look better than wrinkly, flimsy Mylar, pictured below. But a glass filter does not necessarily give as sharp and clear a view, even though it's more expensive. A few years ago Sky & Telescope, our parent magazine, tested and reviewed whitelight solar filters. We judged Baader AstroSolar Safety Film, wrinkles and all, to give the sharpest views of fine solar details.
You can buy either kind of filter mounted in cells that are sized to fit onto the front of many telescopes. Or buy the film as a loose sheet, and cut it with scissors (between pieces of paper to prevent fingerprints) to make a do-it-yourself filter out of cardboard like the one pictured below. Just make sure that it can't blow off the scope while your eye is at the eyepiece!
The Sun Close Up
What can you see?
First off, you'll notice that the Sun is dimmer toward its edges than near the middle. This limb darkening arises from the fact that near the Sun's edge (limb), we are looking diagonally through the Sun's atmosphere into a layer that's a little less deep than when we look straight down near the center of the Sun's disk. The deeper you see into the Sun, the hotter it gets. Different kinds of stars have different amounts of limb darkening, as inferred from indirect evidence, but only on the Sun can we see it directly.
The Sun goes through an 11-year cycle of magnetic surface activity, including sunspots and solar flares, and in 2013 we should be at a peak of this cycle. On most days the Sun is likely to display at least one complex sunspot group. A group usually contains two large main spots oriented more or less east-west and several little ones. Most spots have a dark central umbra surrounded by an irregular gray penumbra. A really sharp view will show that the penumbra consists of many thin radial streaks, like grass lying sideways.
A sunspot is a place where a strong magnetic field breaks out of the Sun's interior. The field holds the surface material fairly rigidly in place, allowing it time to cool compared with the surrounding hot gas constantly boiling up from the depths.
Why do magnetic fields grasp so tightly onto the Sun's gas but not, for instance, air on Earth? Because the Sun's gas is so hot that it's partly ionized, meaning electrons are stripped off some of its atoms. These free electrons can move around, which means the Sun's hot gas conducts electricity just like metal, so a moving magnetic field holds and drags the gas just as firmly as if it were copper wire in a motor.
Often surrounding a sunspot region are faculae, irregular bright patches just above the surface that are still not fully understood. These come and go in ghostly magnetic partnership with sunspot groups. They're best seen near the Sun's limb.
With luck you might someday catch a rare white-light flare in an active region, brightening and vanishing in just a few minutes. Lesser solar flares are common, sometimes blasting protons and electrons our way that spawn bright auroras in the upper atmosphere and disrupt the power grid. But rarely does a flare reach the level of white-light visibility.
And don't ignore the Sun's blank surface itself. The visible surface is called the photosphere, literally "the layer that shines." Or to turn it around, this is the depth in the Sun to which your sight reaches before the gas becomes dense enough to be opaque.
A sharp, high-contrast telescope during a spell of good atmospheric seeing will show that the photosphere is not so blank after all. The entire surface is riddled with granulation: small convection cells of hot gas rising from below, breaking up, and sinking back down on a timescale of just a few minutes.
In addition to white-light ("broadband") views, richly detailed and rapidly changing solar phenomena await observers willing to make a bigger investment in their equipment arsenal. "Narrowband" views of the Sun isolate one specific wavelength of light that is emitted and absorbed by some crucial type of atom in the solar atmosphere. Narrowband solar telescopes were once exotic but are now in reach of amateurs. They're not cheap; a whole telescope needs to be designed for this single task.
The deep red hydrogen-alpha (H-alpha) wavelength is the most interesting. It reveals dramatically fluid motion and complex magnetic structures all across the solar chromosphere, the layer just above the photosphere. But the first things that often catch your eye in an H-alpha telescope are prominences standing along the Sun's limb. These are sprays of hydrogen plasma held aloft by magnetic fields. Sometimes looking like licks of fire, less often like massive loops, prominences can span the width of a dozen Earths or more.
Where a prominence wanders across the Sun's face, rather than being seen against the dark sky off the edge, it appears darker than the solar surface and is known as a filament.
Prominences can last from minutes to weeks and may change rapidly, depending on the stability of the magnetic fields holding them aloft. When the field holding a prominence or filament becomes unstable, the plasma either slides back down the lines of force onto the Sun, or in some cases, erupts away into space as a "coronal mass ejection."
Smaller spicules are also visible along the solar limb in H-alpha, appearing as tiny "hair" prominences, particularly around the Sun's polar regions.
Flares, sudden releases of pent-up magnetic energy, are visible more often in H-alpha than in white light.
Narrowband solar telescopes have reinvigorated amateur solar astronomy. But every telescope owner should have at least a white-light filter. With the Sun at solar maximum, this is a fine year to get acquainted with our stellar neighbor. ?
Stephen W. Ramsden operates the Charles Bates Solar Astronomy Project (solarastronomy.org), a nonprofit public outreach program sharing views of the Sun at schools and public events. Alan MacRobert is a senior editor of Sky & Telescope.
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|Title Annotation:||Visual Observing|
|Author:||Ramsden, Stephen W.; MacRobert, Alan|
|Date:||Jan 1, 2013|
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