Sky and Telescope - June 2016 - 26

Supernova Remnants

The Elusive

Cassiopeiae
In which we track and capture a supernova remnant.

David Tosteson

For astronomers of 16th-century Europe, the heavens
were unchanging and immutable. It took the dramatic
appearance of a new object in the sky to challenge that
opinion. Danish astronomer Tycho Brahe was 26 years
old on November 11, 1572, when a "new star" in Cassiopeia exploded into, and forever altered, the collective
human understanding of the allegedly static universe.
Although not the ﬁrst to spot the event now cataloged
as Supernova (SN) 1572, Tycho was the most thorough
of all observers to document its light curve. His precise
data would be used not only in his time, but more than
three and a half centuries later to determine the explosion's origin. Copernicus ﬁred the ﬁrst shot against the
fortiﬁed concepts of Aristotelian and Ptolemaic dogma,
but this new, inexplicable star that ultimately inspired
Tycho's 1573 book, De nova stella, convinced many of his
contemporaries that a new cosmology was needed.

Star magnitudes

1h 00m

0h 40m

0h 20m

0h 00m
+64°

B Cas

3
4
5
6
7

133

κ
+62°
225
136

7790

γ
129

υ2
υ1

CASSIOPEIA

+60°

β
+58°

ρ
7789

281

June 2016 sky & telescope

+56°

In the pre-telescopic era of the late 1500s, the light
from this stellar explosion remained visible to the naked
eye for 16 months, from November 1572 until March
1574. When the nova was at its estimated -4.0 maximum
magnitude around the middle of November, it was visible even in daylight. Even as its light faded from view,
the remnant of the supernova expanded. It continued to
do so, unobserved by human eyes for the next 375 years.

The Arc of Observational History
The optical observation of Supernova Remnant (SNR)
1572, also cataloged as B Cassiopeiae, or B Cas, started
in the 20th century. Walter Baade, the German astronomer who deﬁned the category of bright stellar explosions
as "supernovae" in 1934, reexamined Tycho's data to
produce a light curve smoothed by quarter-magnitude
steps. After comparing it with similar curves, such as
that of Kepler's Supernova of 1604, he categorized SN
1572 as a Type I object that showed a linear decrease
in brightness. In his 1945 Astrophysical Journal paper,
Baade noted that no apparent remnant of SN 1572 was
visible on existing photographic plates. However, he
held out hope for future observations to be made with
the Hale telescope, then under construction at Palomar
Observatory. Dedicated in 1948, the 200-inch reﬂector
possessed four times the collecting power of the largest
instrument then extant, the Hooker 100-inch reﬂector at
Mount Wilson.
Baade imaged the area around SN 1572 with this
powerful new instrument on November 23, 1949, and on
the red plates detected nebulosity on the eastern edge of
the SNR. But he didn't report these ﬁndings, concluding instead that the ﬁ laments were too distant from the
point of origin to be related to Tycho's exploded star.
Thus, credit for the discovery of the SNR goes to Hanbury Brown and Hazard (Jodrell Bank), who detected it
in radio wavelengths in 1952. Baldwin and Edge (Cambridge University) corrected position errors in 1957, and

Sky and Telescope - June 2016

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