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NGC 6946
NGC 6946 is a large, multi-armed galaxy, the prototype for de Vaucouleurs classification SAB(rs)cd (a barred spiral with an inner ring). It is also listed as Arp 29 because of its massive northern arm. Due to its intense star formation and frequent supernovae, it is nicknamed the Fireworks Galaxy. The location of the galaxy will lift some eyebrows, being split in half by the boundary between Cepheus and Cygnus constellations. Edwin Hubble considered NGC 6946 to be an outlying member of the Local Group, based on computations he did following the supernova of 1917. These investigations would eventually lead to concluding that the spiral “nebulae” all over the sky are actually galaxies far away. Today, we know that the galaxy is located somewhere between 18-22 million light-years away.
William Herschel discovered the galaxy in 1798 using his 18,7-inch telescope but noted no spiral structure. In 1850, William Parsons focused on observing exactly the spiral structure of some “nebulae” but he was somewhere cautious on this physical appearance.
The Milky Way’s average supernova rate is around 1 per century. NGC 6946 exhibited 10 between 1917 and 2017, so 10 times more often. Moreover, around 40-50 large supernova remnants have been detected in all regions of the galaxy and over 140 emission nebulae have spectral characteristics of a SNR. The center of NGC 6946 has a much smaller black hole compared to the Milky Way’s – only 100.000 solar masses. It also harbors several star clusters. The galaxy richness in star formation regions is tightly connected to its supernovae rate.
NGC 6939
This is an open cluster located almost 4.000 light-years away. It was discovered in the same year as NGC 6946 by William Herschel, being apparently close to the galaxy. NGC 6939 is an old cluster, around 1 billion years old, and it’s slightly poorer in metals than the Sun. The star cluster has an apparent diameter of about 10 arc minutes and is of the Trumpler type I1m, which means the following: I = The star cluster stands out from the surrounding stars and shows a clear central concentration. 1 = Most of the stars in the cluster are about the same magnitude. m = medium star richness (50-100 stars). NGC 6939 is moving in our direction at a speed of 19 km/s.

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