Fireworks Galaxy (NGC 6946) with a DWARF 3: Four Hours of Cosmic Fireworks for America’s 250th

The light in this image of the Fireworks Galaxy (NGC 6946) left its source about 25.2 million years ago. When the Declaration of Independence was signed in 1776, those photons had already completed 99.999 percent of the trip to my backyard. All of American history, every one of the 250 years we are celebrating this weekend, fits inside the final 0.001 percent. I captured this with a DWARF 3 smart telescope over four hours on the night of July 3, and I cannot think of a better target for the semiquincentennial than a galaxy named for fireworks.

Why it is called the Fireworks Galaxy

NGC 6946 has produced ten observed supernovae since 1917. The Milky Way, which has roughly twice as many stars, averages one or two per century. Star for star, this galaxy detonates at about ten times our rate, and for decades it held the record for the most supernovae seen in any single galaxy. NGC 3690 has since taken that record, but NGC 6946 earned the nickname first and kept it.

The full roll call: SN 1917A, SN 1939C, SN 1948B, SN 1968D, SN 1969P, SN 1980K, SN 2002hh, SN 2004et, SN 2008S, and SN 2017eaw. Three of those went off in July, which feels appropriate.

The most recent one is the one that interests me as a smart telescope owner. SN 2017eaw was discovered in May 2017 by Patrick Wiggins, an amateur in Utah, comparing images from his 14 inch reflector against older frames of the same field. It peaked around magnitude 12.6. That is well within reach of a DWARF 3. If NGC 6946 does what it has done ten times in the last century and does it again, the telescope sitting on my patio can record it. That is not hypothetical hardware talk. An amateur found the last one by looking.

Is there a supernova in my image?

No. I checked before writing this, because I wanted there to be one. The last confirmed supernova in NGC 6946 was SN 2017eaw, nine years ago, and the transient surveys show nothing active in the galaxy right now. The remnants of the historical supernovae are still out there expanding, but detecting them takes Hubble narrowband imaging, not a 35mm aperture in a Bortle 6 backyard.

There is something stranger hiding in the frame, though. In 2009, a red supergiant in NGC 6946 flared up to over a million times the Sun’s brightness, faded over a few months, and then disappeared from optical view entirely. Fifteen years later it still has not come back. The object, cataloged as N6946-BH1, is the leading candidate for a failed supernova: a massive star that collapsed directly into a black hole without the explosion. So the Fireworks Galaxy contains at least one firework that was lit and never went off. It is somewhere in this image, invisible, which I find better than any object I can actually see.

The capture

The session ran 4 hours 20 minutes at 60 second exposures, gain 50. Following my tried and true setup guide I was up and running quickly, and avoiding mosquitoes. Of the roughly 260 frames captured, 248 made it into the stack for 4 hours 8 minutes of integration, a keep rate of about 95 percent. Polar alignment settled at 1 degree of deviation, which is looser than I would like, but at 60 second subs on the DWARF 3 it did not produce visible trailing.

The Moon rose partway through the session and I kept shooting anyway. NGC 6946 is a broadband target, so moonlight costs real signal, and the textbook answer is to stop. I had a July 4 publication date and one clear night. Sometimes the calendar wins the argument with the histogram. The moonlit frames stayed in the stack, and the cost showed up as a washed out look in the raw result, with the galaxy sitting lower in contrast against the sky than the integration time would normally deliver.

One thing worth knowing before you shoot this target: NGC 6946 sits close to the plane of the Milky Way, on the border of Cepheus and Cygnus, and our own galaxy’s dust dims it by roughly a full magnitude. The galaxy is listed at magnitude 9.6 but it photographs fainter than that number suggests. Between that foreground dust and the moonlight, my stack came out flatter than 4 hours 8 minutes should look. A contrast and ambiance pass in Snapseed on the phone recovered most of it. Not the workflow I would defend in a processing tutorial, but the arms are there and the result holds up.

250 years, 25 million light-years

William Herschel found this galaxy on September 9, 1798, twenty-two years into the American experiment, sweeping the sky from England with a telescope he built himself. The country was on its first president’s successor. The light Herschel recorded that night had been traveling since before the genus Homo existed.

I keep coming back to the ratio. 250 years is a real milestone, and this weekend is worth marking. What struck me while processing this image is the scale sitting right next to it. The light on my sensor left NGC 6946 about 25.2 million years ago. Two timelines, both real, in the same frame. One measured in generations, one measured in geologic time.

Happy 250th. The galaxy brought its own fireworks.

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