The Caldwell Objects

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NGC 6946
Type: Mixed Spiral Galaxy
(SABcd) Con: Cepheus
RA: 20 h 34.9 m
Dec: +60° 09'
Mag: 8.8
Dim: 11.2' x 8.8'
SB: 14.2
Dist: 18 million light-years
Disc: William Herschel, 1798
W. HERSCHEL: [Observed 9 September 1798]
Considerably faint, very large, of an irregular figure,
a sort of bright nucleus in the middle. The nebulosity
[spans] 6' or 7'. The nucleus seems to consist of stars,
the nebulosity is of the milky kind. It is a pretty
object. (HIV-76)
GC / NGC: Very faint, very large, very gradually,
then very suddenly brighter in the middle, partially
resolved, some stars seen.
NGC 6946 is A SPIRAL GALAXY WITH A PAIR of
branching arms that have a puzzling propensity
for supernova explosions. In the 20th century
terrestrial astronomers witnessed six supernovae
blazing forth in that galaxy (in 1917, 1939, 1948,
1968, 1969, and 1980). The longest duration
between these events was 22 years, suggesting
that, as of this writing, another seems about due.
Caldwell 12 could be the ticket to fame for a
dedicated amateur who decides to survey that
galaxy whenever possible. As seen from Earth,
the 1980 eruption was the galaxy's brightest. The
new star rose to a peak visual magnitude of 11.4
and outshone any foreground star near it.
Designated Supernova 1980K, the "guest star"
was classified as a Type II supernova, one that
occurs when a very massive supergiant star
The Caldwell Objects
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(one with 8 or more times the mass of our Sun)
collapses abruptly after nuclear fusion grinds to a
halt in its core. Astronomers now believe that the
core of a Type II supernova progenitor collapses,
then rebounds slightly, transferring prodigious
amounts of energy to the star's envelope. The
resulting shock wave blows much of that
envelope into outer space. Since they are
spawned by short-lived supergiant stars, Type II
supernovae generally occur in the spiral arms of
disk galaxies, where stars are continuously born.
Type II supernovae tend to show more
diversity in their light curves and absolute magnitudes
than do Type la supernovae, which make
for much better "standard candles" — even
though their cause remains unknown. Indeed,
Supernova 1980K evinced such an
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