In 180 1 the German physicist and phar- 

 macist Johann Wilhelm Ritter found 

 yet another band of invisible light. But 

 instead of a thermometer, Ritter placed 

 a little pile of light-sensitive silver chlo- 

 ride in each visible color as well as in 

 the dark, unlit area next to the violet 

 end of the spectrum. Sure enough, the 

 pile in the unlit patch darkened more 

 than the pile in the violet patch. What's 

 beyond violet? "Ultra" violet. 



Sky watching didn't change over- 

 night, though. The first telescope de- 

 signed to detect invisible parts of the 

 electromagnetic spectrum wouldn't be 

 built for 130 years. That's well after ra- 

 dio waves, X rays, and gamma rays had 

 been discovered, and well atter the Ger- 

 man physicist Heinrich Hertz had 

 shown that the only real difference 

 among the various kinds of light is the 

 frequency of the waves in each band. In 

 fact, credit Hertz for recognizing that 

 there wan electromagnetic spectrum. In 

 his honor, the unit of frequency for any- 



thing that vibrates, including sound, has 

 duly been named the hertz. 



Today, telescopes operate in every in- 

 visible part of the spectrum — from 

 low-frequency radio waves a dozen 

 meters long to high-frequency gamma 

 rays no longer than a quadrillionth of a 

 centimeter. That rich palette of light 

 supplies no end of astrophysical dis- 

 coveries. Want to peek at a stellar nurs- 

 ery deep inside a gas cloud? Check it 

 out through NASA's infrared Spitzer 

 Space Telescope. Want to measure the 

 spectrum of supermassive black holes 

 colliding in the center of a galaxy? Take 

 aim with the Chandra X-Ray Observ- 

 atory. Want to watch the explosion of 

 a giant star, whose mass is as great as 

 forty suns? Catch the drama via the Eu- 

 ropean Space Agency's International 

 Gamma-Ray Astrophysics Laboratory. 



A telescope is merely a tool to aug- 

 ment our meager senses, enabling 

 us to get better acquainted with faraway 

 places. The bigger the telescope, the 

 dimmer the objects it brings into view; 

 the more perfectly shaped its mirrors, 

 the sharper the image it makes; the more 

 sensitive its detectors, the more efficient 

 its observations. But in all cases, every 

 bit of information a telescope delivers 

 to the astrophysicist comes to Earth on 

 a beam of light. 



Somehow, though, astronomers were 

 a bit slow to make the connection be- 

 tween the newfound invisible bands ot 

 light and the idea of building a telescope 

 that might detect those bands from cos- 

 mic sources. Surely hubris takes some of 

 the blame: how could the universe pos- 

 sibly send us light that our marvelous 

 eyes cannot see? For more than three 

 centuries — from Galileo's day until Ed- 

 win Hubble's — building a telescope 

 meant only one thing: making an in- 

 strument to catch visible light \scc "The 

 Light Brigade," by Neil deGrasse Tyson, 

 March 2()()6\. Celestial happenings, 

 however, don't limit themselves to 

 what's convenient for the human retina. 

 Instead, they emit varying amounts of 

 light simultaneously in multiple bands. 

 So, without telescopes and detectors 

 tuned across the spectrum, astrophysi- 



cists would still be blissfully ignorant. 



Take an exploding star — a supernova. 

 It's a cosmically common and seriously 

 high-energy event that generates pro- 

 digious quantities of X rays. Sometimes 

 bursts of gamma rays and flashes of 

 ultraviolet accompany the explosions, 

 and there's never a shortage of visible 

 light. And long after the explosive gases 

 cool, the shock waves dissipate, and the 

 visible light fades, the supernova "rem- 

 nant" keeps on shining in the infrared. 

 Most stellar explosions take place in 

 distant galaxies, but if a star blows up 

 within our own Milky Way, its death 

 throes are bright enough for everyone 

 to see, even without a telescope. No one 

 on Earth saw the X rays or gamma rays 

 from the last two supernova spectacu- 

 lars hosted by our galaxy, in 1 572 and 

 1604, but their wondrous visible light 

 was widely reported. 



Problem is, no single combination of 

 telescope and detector can see every fea- 

 ture of such explosions, because no such 

 combination can see every band oflight. 

 In fact, the range of wavelengths that 

 make up each band strongly influences 

 the design of the hardware used to de- 

 tect it. For the moment, think oflight 

 as made up of waves. Each beam of light 

 has a measurable wavelength: the dis- 

 tance between consecutive crests (or 

 troughs) of a single wave. Only after you 

 identify the wavelength range of your 

 astronomical affections can you begin 

 to think about the size of your mirror, 

 the materials you'll need to make it, the 

 shape and surface it must have, and the 

 kind of detector you'll need. 



X-ray wavelengths, for example, are 

 extremely short. So if you're gathering 

 X rays, your mirror had better be super- 

 smooth, lest it distort them. But if you're 

 gathering long radio waves, your mir- 

 ror could be made of chicken wire that 

 you've bent with your hands, because 

 the irregularities in the wire would be 

 much smaller than the wavelengths 

 you're after. Of course, you also want 

 plenty of detail — high resolution- — and 

 so your mirror should be as big as you 

 can afford to make it. In the end. your 

 telescope must be much, much wider 

 than the wavelength of light you aim to 



June 2006 NATURAL HISTORY 



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