Sky & Telescope - August 2020 - 13

LISTENING TO THE UNIVERSE
Our galaxy's plane arcs over antennas
in the Karl G. Jansky Very Large Array in
New Mexico. The telescopes can move
along rails into different conﬁgurations,
depending on the observing goals.

able upon cable upon tangled cable . . . Never had I
seen such a convoluted jumble of wires, screws, pliers,
and other whatnots in such a small space. It was like
a hardware store had imploded. In imploding, the mayhem
had become strangely alive. I could feel the thrumming of
the correlators in the pit of my stomach. A fan oscillated in
the corner, whooshing breaths of air over the back panels
of monoliths that looked like they belonged in the opening
scene of 2001: A Space Odyssey.
But I was nowhere near a ﬁlm set. I was in a radio telescope's control room, an operations central reminiscent of
the deck of the starship Enterprise. And mind you, this was
a single-dish radio telescope, nowhere near the elaborate,
multiple-dish setups of instruments such as the Very Large
Array. However, this telescope - the 14-meter radio dish of
Metsähovi Radio Observatory in Finland - was one element
in a global network that unites radio dishes across the planet
into one behemoth of a receiver, poised to capture signals
from the farthest reaches of the cosmos.
Today, we're familiar with discoveries based on radio
observations, from the silhouette of a distant galaxy's black
hole to hints of aurorae on failed stars called brown dwarfs.
But radio astronomy is a relatively new science. A century
ago, the only electromagnetic window wide open to astronomers was the visual band. Early forays had been made into

q JANSKY'S MERRY-GO-ROUND This replica of Karl Jansky's original
antenna uses the same materials as the original, down to the Model T
tires on the rotation axis. The turntable design enabled Jansky to determine the direction of any signal he detected at 20.5 MHz.

V L A WITH MILK Y WAY: NR AO / AUI / NSF,
JEFF HELLER M A N; REPLICA OF JA NSK Y'S
A N TENN A: NR AO / AUI / NSF

Sky

Long Waves and Longer Waves
Observers on Earth are poised to receive radio waves from
cosmic sources in two ways. The first is by recording the
source's ﬂux density, which is the power received per unit
area at a specific frequency. The unit of flux density is the
Jansky, in honor of Karl. One Jansky = 10-26 watt per square
meter per hertz. In some sources, such as quasars, the emitted radiation can vary with time. Astronomers can combine

flux densities from multiple frequencies to better model the
emission mechanisms, which in turn reveal the physical processes that contribute to the observed radiation.
The second way astronomers study radio sources is by
generating a map of the spatial distribution of the emitted
radio frequencies. Flux densities are used in conjunction with
point sources, while maps are useful for extended sources.
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Sky & Telescope - August 2020

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