tear one or more of the atom's outer- 

 most electrons away from the rest of 

 the atom. The remaining atom, now 

 ionized, with a net positive charge, 

 readily recombines with other, free- 

 flying electrons nearby. The new elec- 

 trons, however, can fall only to certain 

 discrete "levels" opened up by loss of 

 the earlier electrons: levels dictated 

 by the atom's quantum mechanical 

 structure. If it helps, think of the struc- 

 ture as a big pachinko machine — those 

 vertical pegboardlike games in which 

 little steel balls fall from top to bot- 

 tom — and imagine each recombining 

 electron as one of the steel balls drop- 

 ping downward through the machine. 



Every time the electron falls from 

 one atomic energy level to a lower one, 

 the atom releases energy in the form 

 of electromagnetic radiation — that is, 

 light — at a specific wavelength or col- 

 or. So within every cloud of ionized 

 gas, electrons combining with ions are 

 like balls at the top of atomic pachinko 

 machines, in which the electrons cas- 



cade downward until they reach their 

 lowest energy state. And the more like- 

 ly a particular bounce, the more light 

 ot that color emerges from the cloud. 



For hydrogen atoms, the most like- 

 ly intra-atomic step downward is 

 the one that takes the electron from its 

 second-lowest energy state to its low- 

 est, or "ground," state. The light emit- 

 ted by this bounce, whose wavelength 

 was first measured by Lyman, is the 

 brightest spectroscopic feature emitted 

 by any cloud of ionized hydrogen gas. 

 Because most of the atoms in the uni- 

 verse, by far, are hydrogen, you would 

 think that "Lyman-alpha" radiation 

 would be easy to detect. It would be 

 worth detecting, too, because Lyman- 

 alpha radiation is a good measure ofhow 

 much ionized hydrogen there is in any 

 particular place in the universe. 



There are, however, two obstacles to 

 detecting Lyman-alpha transitions. First, 

 their emitted light shines in the ultravi- 

 olet part of the electromagnetic spec- 



POWERFUL NEW ELECTRIC 

 WOOD SPLITTER! 



The Amazing DR® Electric/Hydraulic WOOD 

 SPLITTER handles logs up to 16" thick! 



• ELECTRIC POWER 



means no loud noise or fumes, 

 so you can split ANY TIME, even in a 

 garage or basement — night or day, rain or shine! 



• POWERFUL 1800-watt electric (llOv) motor 

 splits logs up to 16" thick with 6 TONS of 

 hydraulic pressure! 



• FITS ON A WORKBENCH allowing you 



to STAND UP while splitting wood! No more sore 

 back from hunching over a big gas-powered splitter! 



• TRANSPORTS EASILY on wheels like a 

 piece of airline luggage.. .fits in almost any car trunk. 



Call Toil-Free: 



1-888-206-5311 



B' YES! Please rush me FREE full details of the New 

 DR* Electric/Hydraulic WOOD SPLITTER, including details of your 

 6 month free trial offer, low, factory-direct prices, and seasonal 

 savings now in effect 



Name 



Address 



City State Zip _ 



NHI 



SAVE YOUR BACK... 

 no more chopping! 



DR" POWER EQUIPMENT, Dept. 54886X 

 127 Meigs Road, Vergennes, VT 05491 



www.DRwoodsplitter.com 



trum, at a wavelength of 1 2 1 .6 nanome- 

 ters, or just under 1/200,000 of an inch. 

 On top of that, light at this wavelength 

 is easily absorbed by intervening mat- 

 ter — it can't even get through Earth's at- 

 mosphere, for instance. Out in deep 

 space, just a little bit of metallic or mol- 

 ecular dust mixed into a cloud of near- 

 ly pure hydrogen gas can quench 99 per- 

 cent or more of the Lyman-alpha light 

 produced in the cloud. And the universe 

 is, on the whole, a very dusty place. 



To observe the radiation from Earth, 

 Gawiser and his colleagues took ad- 

 vantage of the effects of cosmic ex- 

 pansion. At cosmic distances, the far- 

 ther an object is from Earth, the faster 

 it is moving away. The motion length- 

 ens the wavelengths of the object's light 

 beam, just as the whistle of a train 

 sounds at a lower pitch when it is re- 

 ceding than when it is approaching. 

 The cosmic wavelength-stretching is 

 called redshift. At large enough dis- 

 tances — about 10 billion light-years or 

 more — the wavelength of the ultravi- 

 olet light from Lyman-alpha transitions 

 is so stretched by the recessional speed 

 of its source that it shifts into the visi- 

 ble part of the spectrum. The redshift- 

 ed light from distant LAEs can thus be 

 seen with earthbound telescopes. 



Dust was the second problem. How 

 much dust was in the vicinity of the 

 LAEs? Would the proportion of Ly- 

 man-alpha transitions detected be suf- 

 ficient to give a clear picture of those 

 galaxies? With the 6.5-meter Magel- 

 lan-Baade Telescope at Las Campanas, 

 Chile, Gawiser's team supplemented 

 MUSYC images with spectroscopic 

 data of the same distant LAEs. He dis- 

 covered that, at a distance of around 1 1 

 billion light-years, the Lyman-alpha 

 light escaped the LAEs just fine — so 

 well, in fact, that when the light left 

 those galaxies they probably harbored 

 hardly any dust at all. Moreover, though 

 they are only about a tenth the mass of 

 the Milky Way, they form stars at rates 

 as much as ten times faster than stars 

 form in our galaxy. 



What do those results imply? Inter- 

 stellar dust is the by-product of stellar 

 aging and stellar death. No dust means 



62 NATURAL HISTORY September 2006 



