How Perseid meteor shower helps produce amazing images of galaxies
The Perseid meteor shower and its ilk provide the sodium atoms in the middle atmosphere that light up when astronomers tickle them with a laser – a key step in adaptive optics, which are used by ground-based observatories.
The next time you leaf through one of those spectacular coffee-table books on astronomy, with their images of glowing nebulae or tightly wound spiral galaxies, don't forget to thank the Perseid meteor shower and its ilk for their contribution to those cosmic mug shots.Skip to next paragraph
In Pictures Looking into the skies: Telescopes
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If those images came from ground-based observatories, it's likely the telescopes were using technology known as adaptive optics, which remove distortions that the atmosphere imparts to light from celestial objects.
What do meteors have to do with this? They provide the sodium atoms in the middle atmosphere that light up when astronomers tickle them with a laser. The tiny dot of light, invisible to humans on the ground, becomes the reference "star" that adaptive optics need to remove distortions.
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The sodium is abundant. It's at the right altitude for use as artificial stars. It lights up at two closely spaced wavelengths within the yellow portion of the visible spectrum, which makes it brighter than other elements for a given amount of laser power. And those lasers are relatively inexpensive to make.
"It's a happy coincidence," says Dennis Wellnitz, a research astronomer at the University of Maryland in College Park, of the factors that have led to a technology that has revolutionized ground-based optical astronomy.
The sodium layer was first detected in 1929 by Vesto Slipher, an astronomer at the Lowell Observatory in Flagstaff, Ariz. He detected it as a weak glow, which has since become known as night glow. A decade later, researchers identified the source as meteors.
The sodium appears in a layer of the atmosphere known as the mesosphere, which starts about 31 miles up and, depending on latitude, rises to 53 miles above Earth. The sodium itself appears to be confined to the upper three miles of the mesosphere.
This is the layer where most meteors vaporize. Estimates of the amount of material entering this layer as meteors are as high as 15,000 tons a year.
The sodium atoms that meteors leave behind are energetic enough to emit a weak glow on their own. It doesn't take much to really light them up, researchers say.
Which is just fine for adaptive optics, which give ground-based telescopes abilities rivaling those of space-based telescopes.
Fuzzy blobs in images without adaptive optics become distinct points of starlight. Galaxies show far more detail. Objects that appear solitary without adaptive optics can reveal companions with the additional sharpness that adaptive optics provide.
Adaptive optics initially achieved this by taking light from a star that appeared near the object that astronomers wanted to observe and manipulating it to cancel the atmosphere's distorting effect. But bright stars aren't always so conveniently located.
Laser light tuned to excite the sodium atoms in the middle atmosphere generates an artificial star that is available wherever astronomers point their telescopes.