Sky & Telescope - August 2020 - 15

PL A NCK M A P: ESA A ND THE PL A NCK COLL A BOR ATION;
PENZIAS A ND WILSON: AIP EMILIO SEG RÈ VISUA L
A RCHIV ES, PH YSICS TODAY COLLECTION

parabolic dish some 9 meters in diameter - in order to investigate this new phenomenon. Reber ultimately mapped the
radio emission from broad sections of the Milky Way, including the galactic center as well as Cygnus and Cassiopeia. It
was the spark that lit radio astronomy's engine.
But sources much closer to home than the galactic center
also emit radio waves. In 1942 James Hey, English physicist
and later radio astronomer, was the ﬁrst to ﬁnd that radio signals come from the Sun (S&T: Oct. 2014, p. 30). He observed
radio emission from electrons gyrating around the concentrated magnetic ﬁelds associated with sunspots. Solar ﬂares
also emit bursts of radio waves.
After World War II, accelerated by advances in radar technology, radio astronomy as a discipline took off. Observers
discovered a host of sources in subsequent decades, including
objects that astronomers had never known existed. Jocelyn
Bell (later Bell Burnell), a graduate student at the University
of Cambridge (UK), caused quite a stir when, in 1967, she discovered evenly timed radio signals emanating from one spot
in the sky that astronomers later realized come from a rapidly
rotating and highly magnetized neutron star, or pulsar. At
that time, neutron stars were largely theoretical concepts. She
famously ﬁled the signal under the label LGM - shorthand
for "Little Green Men." But the 1974 Nobel Prize in Physics
for this discovery omitted Bell Burnell. Instead, it went to her
supervisor, Antony Hewish, as well as to Sir Martin Ryle, a
trailblazer in radio observing techniques and founding director of the Mullard Radio Astronomy Observatory.
The discovery of quasars was another paradigm-shifting
milestone in the history of radio astronomy. In 1956, Geoffrey Burbidge demonstrated that synchrotron radio radiation
- photons from charged particles whirling at relativistic
speeds along magnetic ﬁeld lines - could account for the
emission from the jet of M87, conﬁrming earlier predictions
and reports of synchrotron emission detected in the optical.
But it wasn't until 1963, when the combined efforts of Cyril
Hazard, Maarten Schmidt, and Bev Oke (and their collaborators) demonstrated that a starlike pinpoint of light, 3C 273,
was in fact more than a billion light-years away, that astronomers realized something fantastic was afoot.

S COSMIC HISS Using a horn antenna, Arno Penzias and Robert Wilson
(above, with antenna) detected a hiss at 4 GHz that proved to be the
afterglow of the universe's Big Bang phase. Several instruments have
mapped that cosmic microwave background, including the recent Planck
spacecraft (top). The contour outlines our galaxy's plane and some other
nearby sources.

These quasi-stellar radio sources, or quasars for short, are
supermassive black holes accreting matter from large disks of
hot gas around them and then spewing much of this matter
out in narrow but powerful relativistic jets from their poles.
Quasars' extensive and variegated jets are the source of the
synchrotron radiation, which arises from electrons spiraling
along magnetic ﬁeld lines in the jets. The exact mechanism
for accelerating these particles is still unclear, but it's likely
associated with the sudden release of magnetic ﬁeld lines
normally anchored in the accretion disk.
More recently, astronomers have discovered miniature
counterparts to these extragalactic colossi. Instead of a supermassive black hole gobbling up vast amounts of gas, we have
a stellar-mass black hole accreting matter from a companion
star. In lieu of jets extending for up to hundreds of thousands
of light-years, the outﬂows in microquasars, as these objects
are known, are mere light-years in length. In microquasars
we witness changes in the accretion disk and jet on humanfriendly terms, i.e., minutes to days instead of the years to
centuries required for quasars.
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Sky & Telescope - August 2020

Table of Contents for the Digital Edition of Sky & Telescope - August 2020