My first Magazine Sky & Telescope - 02.2019 | Page 35

subjective human beings. It’s ironic though, because the Crab
is the only object on Messier’s list where changes might have
been detected over several decades if these sketches had only
been more accurate.
However, photography and spectroscopy were just a few
years away from being sensitive enough to begin unravelling
the Crab Nebula’s secrets.
Deciphering the Crab
During 1913–1915 Vesto Slipher was the first to examine
the Crab’s spectrum and thus lay the groundwork for later
studies that showed the nebula to be expanding. Some years
later, in 1921, Carl Lampland discovered changes within
the Crab Nebula by examining photographs taken with the
Lowell Observatory 40-inch reflector, which he studied with
a blink comparator.
At the same time, it was suggested that the Crab was
related to the 1054 Chinese guest star, and in 1928 Edwin
Hubble proposed that the expansion rate of the Crab Nebula
and its location supported the idea that it was the debris of
the 1054 supernova. Jan Oort analyzed the Chinese astro-
nomical records in 1942 and came to the same conclusion.
That same year Walter Baade noted that the Crab’s expan-
sion rate was accelerating and that the stellar remains of the
supernova must be responsible. This supported the idea he
and Fritz Zwicky had put forth eight years earlier:
q UNFILTERED AND FILTERED VIEW (Top) The sketch shows the
unfi ltered view through the author’s 28-inch scope. Except for the fi n,
this view looks similar to the overall shape of R. J. Mitchell’s 1855 draw-
ing. Magnifi cations from 253× to 408× were used for both sketches, but
depending on how steady the seeing is, more than 1000× can be used
on the Crab. The only difference between these two sketches is the use
of an O III fi lter for the second sketch. (Bottom) This shows the view
through the 28-inch using an O III fi lter. The fi lter radically changes the
apparent shape of the Crab Nebula by bringing out its overall oval outline
as well as a few of the brighter fi laments. The O III fi lter also dims many
of the stars seen in the unfi ltered view. Note that this sketch has been left
with many pencil marks around the perimeter of the nebula to suggest
the sense of detail that was just out of reach. The scale and orientation
are the same in both sketches.
“With all reserve we advance the notion that supernovae repre-
sent the transitions from ordinary stars to neutron stars which in
their final stages consist of extremely closely packed neutrons.”
The Crab’s radio emission was discovered in 1949, which
was the first detection of these frequencies in an optical
object other than the Sun. From this point theoretical studies
and observations went hand in hand until 1967, when Franco
Pacini proposed that a strongly magnetized, rapidly spinning
neutron star was possibly energizing the Crab Nebula.
Jocelyn Bell Burnell, while still a graduate student at the
University of Cambridge, famously discovered the first pulsar
a year later, and then shortly thereafter the Crab’s pulsar (PSR
B0531+21) was identified and confirmed to be the central
engine powering the Crab Nebula. It became the first pulsar
associated with a supernova remnant, and it remains an
object of active research today.
In a Nutshell
The Crab Nebula is the youngest, brightest, and clos-
est supernova remnant, and its neutron-star pulsar is the
collapsed core of the supernova’s progenitor star. Approxi-
mately 20 kilometers in diameter, it’s a sphere of tightly
packed neutrons that rotates 30 times per second, with
a collimated beam of light blasting out from each of its
magnetic poles. The pulsar produces a relativistic outflow of
synchrotron radiation that helps accelerate the expansion of
the Crab Nebula.
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