Charles Townes

The Nature of the Interstellar Medium

Thursday, 1 July 1971
10:00 - 11:00 CEST


Charles Townes is best known as one of the three laser pioneers who received the Nobel Prize in Physics 1964. Yet he also pioneered the fields of space spectroscopy and infrared astronomy. In 1968 he together with his young colleague William Welch at the University of Berkeley in California made an amazing discovery, which had not been considered possible by their peers: Outer space contains three-atom molecules such as ammonia and water vapor.[1] Even more complex molecules were discovered subsequently, proving evidence for a host of chemical reactions in the clouds of the interstellar medium of our Milky Way galaxy. “Such clouds may well have been the origin of our planet as they condensed”, Townes told his audience in Lindau in this “fairly general talk on material matter in our universe with some emphasis on the more recent discoveries”. One of those discoveries, as Townes initially remarks, had come from his former student at Columbia University, Arno Penzias, who had recorded the cosmic microwave background radiation as a remnant of the very early universe only some years before.

“All of the dark splotches in the center represent dark cool dust clouds, blotting out the star light, scattered all through the plain of our galaxy”, Townes explains a picture of the Milky Way. These clouds are impermeable for light, however, so that their content could not be studied much before, “but with recent technological developments in the infrared and the microwave region it is now easy to penetrate and study them so we can look in the depth of the cloud or through them to do spectroscopy and find out what is there”, says Townes and presents an inventory of the molecules, which have been found in absorption or emission spectra from these light-year long extending clouds so far, from carbon monoxide and formaldehyde to methyl alcohol and methyl acetylene, a “list growing rapidly by one new molecule per month now”. Hydrogen, although elusive and difficult to detect, is the most abundant molecule in the universe, and towards the center of our galaxy, in the direction of the Sagittarius constellation of the zodiac, the clouds of hydrogen and other molecules are “so dense, that they will gravitationally collapse after a relatively short time like thousands or millions of years”, so that in a universal cycle new stars condense out from material, which perhaps came from an earlier generation of stars.

Townes also talks about the naturally occurring water maser, which he recently had discovered. In one region in space, known as W3, water and hydroxide are “heated up” (still far in the negative Celsius temperature range, of course), so that more molecules are in an upper than in a lower energy state and the whole system amplifies like a maser, so powerful, that it sometimes broadcasts a total energy in the radio region “as large as the total luminosity of the sun.”

The astronomers Balinck and Brown discovered an even more complex and powerful radio source, Sagittarius A, in 1974. Townes developed a novel infrared detector incorporating a precision CO2 laser, which facilitated studies of this dynamic and star forming center of our galaxy. In the mid-eighties, together with his doctoral student Reinhard Genzel, he found evidence for the existence of a super-massive black hole in this region. It must have been another fulfillment of Townes’ scientific career, when Genzel, in the meantime a professor of physics in Berkeley himself, could confirm more than two decades later: “The stellar orbits in the galactic centre show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt.”[2]

Joachim Pietzsch

[1] Robert Sanders. Nobel laureate and laser inventor Charles Townes dies at 99. UC Berkeley Media Relations, January 27, 2015

[2] Ian O’Neill. Beyond any reasonable doubt: A supermassive black hole lives in centre of our galaxy. Universe today. December 10, 2008

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