Starship Asterisk*

Star Cluster, Spiral Galaxy, Supernova

Explanation: A cosmic snapshot from May 19, this colorful telescopic field of view spans about 1 degree or 2 full moons on the sky. Spiky in appearance, foreground Milky Way stars are scattered toward the royal constellation Cepheus while stars of open cluster NGC 6939 gather about 5 thousand light-years in the distance near the top of the frame. Face-on spiral galaxy NGC 6946 is toward the lower left nearly 22 million light-years away. The helpful red lines identify recently discovered supernova SN 2017eaw, the death explosion of a massive star nestled in the galaxy's bluish spiral arms. In fact in the last 100 years, 10 supernovae have been discovered in NGC 6946. By comparison, the average rate of supernovae in our Milky Way is about 1 every 100 years or so. Of course, NGC 6946 is also known as The Fireworks Galaxy.

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Cool, but I think the link for "galaxy NGC 6946" leads to another unrelated APOD.

Curious constellation boundary in this area, separating Cepheus and Cygnus, that bisects the Fireworks Galaxy. I do wonder how they devised some of these boundaries.

APOD Robot wrote:In fact in the last 100 years, 10 supernovae have been discovered in NGC 6946.
By comparison, the average rate of supernovae in our Milky Way is about 1 every 100 years or so.

Is there thought to any particular reason for this, or is this statistics at play ?

Nitpicker wrote:Cool, but I think the link for "galaxy NGC 6946" leads to another unrelated APOD.

I think it was meant to lead to this one:
https://apod.nasa.gov/apod/ap120109.html

Nitpicker wrote:

Nitpicker wrote:Cool, but I think the link for "galaxy NGC 6946" leads to another unrelated APOD.

I think it was meant to lead to this one:
https://apod.nasa.gov/apod/ap120109.html

I agree with you, Nit.
Can't resist showing you this, a map of the locations of the nine most recent supernovas in NGC 6946, apart from the latest one, SN 2017eaw.

So where on the map would SN 2017eaw be found? Well, at top center of the image you can see 3 moderately brightish foreground stars forming an uneven triangle. One of these three stars is some distance away from the other two, to the lower right of them. SN 2017eaw would be to the lower right of that star, or, in other words, it would be found in one of the very few locations in NGC 6946 where there hasn't been a supernova recently until now!

Ann

NGC 6946 has a disk that is probably a few hundred light years thick. It is strange to imagine an observer on the other side of the disk, twice as far away from Earth. They would most likely see these 10 SNs pop in a completely different order.

RocketRon wrote:

APOD Robot wrote:In fact in the last 100 years, 10 supernovae have been discovered in NGC 6946.
By comparison, the average rate of supernovae in our Milky Way is about 1 every 100 years or so.

Is there thought to any particular reason for this, or is this statistics at play ?

That's a very hard question to answer, if there is an answer at all. I'll speculate.

Wikipedia wrote:
The diameter of the galaxy is approximately 40,000 light-years or just about a third of the size of the Milky Way.

To me, it is interesting that the galaxy is so small. Another interesting thing is that the galaxy is very dusty.

Wikipedia wrote:
NGC 6946 is highly obscured by interstellar matter of the Milky Way galaxy, as it is quite close to the galactic plane.

Well, I'm going to disagree a bit with Wikipedia. I don't doubt that NGC 6946 is somewhat reddened by dust in our own galaxy, but in my opinion, most of the reddening of NGC 6946 is caused by dust of NGC 6946's own making.

Take a look at the picture at left. I apologize for the streaks seen along the image. The reason why I chose this particular image is that it highlights the tremendous blue super star cluster of NGC 6946, seen at 9 o'clock. As you can see, this star cluster appears very blue here, not very reddened at all. James D Wray made the same observation of the super star cluster of NGC 6946 in his UBV galaxy photography book The Color Atlas of Galaxies: the very bright blue clusters of NGC 6946 don't appear very reddened at all. (There is one more bright blue cluster in NGC 6946, at 2 o'clock in the picture at left.)



My conclusion is that the formation of these huge clusters has blown away much of the dust surrounding them, leaving them mostly unreddened. This also means that the reddening that we see in much of the rest of the galaxy appears to be internal, caused by dust created by the galaxy itself. The color index of NGC 6946 is 0.800, which is really red for a starforming spiral galaxy seen face on, but it's just right for a galaxy reddened by its own almost ubiquitous dust.

In the picture at right you can see M101, a giant galaxy forming stars all over its super-sized arms. The star formation in M101 is recent and ongoing, and the galaxy contains relatively little dust (some of which is shown in brown in the picture) and it contains large numbers of fresh pink emission nebulas.

I think that the ubiquitous dust in NGC 6946 might be a remnant of very widespread star formation in this galaxy in the past, with huge numbers of supernovas exploding and scattering their copious dust all over NGC 6946. All these supernovas would also have blown much of the hydrogen gas clear out of the galaxy itself, but because NGC 6946 appears to be quite isolated in the sky, the evacuated gas would not be captured by other galaxies, and at least some of the gas may now be falling back onto NGC 6946 itself, feeding more star formation.

Perhaps we are now seeing the tail end of the latest great burst of star formation in NGC 6946 and the peak of the latest bout of supernovas.

And perhaps, because NGC 6946 is both small and isolated, it will create self-propagating cycles running through it without interruption of many outside forces.

Well, that's the end of my speculation!

Ann

In the 2012 APOD this galaxy was 10 million ly distant. Today it is 22 million. Is the Universe expanding this fast?

DL MARTIN wrote:
In the 2012 APOD this galaxy was 10 million ly distant. Today it is 22 million. Is the Universe expanding this fast?

• There is a wide range in distance measurements to NGC 6946
  (...probably due to its obscuration by the Milky Way);
  
  however, the 2012 APOD distance of 10 Mly was simply wrong:

http://ned.ipac.caltech.edu/cgi-bin/nDistance?name=NGC+6946 wrote:

Code: Select all

NGC 6946 	Distance Modulus(mag) 	Metric Distance(Mly)
----------------------------------------------------------------------------
Min. 	             28.02 	             13.1
Max. 	             30.52 	             41.4
Mean 	             28.67 	             18.1
Median 	           28.66 	             17.6

neufer wrote:

DL MARTIN wrote:
In the 2012 APOD this galaxy was 10 million ly distant. Today it is 22 million. Is the Universe expanding this fast?

• There is a wide range in distance measurements to NGC 6946
  (...probably due to its obscuration by the Milky Way);
  
  however, the 2012 APOD distance of 10 Mly was simply wrong:

http://ned.ipac.caltech.edu/cgi-bin/nDistance?name=NGC+6946 wrote:

Code: Select all

NGC 6946 	Distance Modulus(mag) 	Metric Distance(Mly)
----------------------------------------------------------------------------
Min. 	             28.02 	             13.1
Max. 	             30.52 	             41.4
Mean 	             28.67 	             18.1
Median 	           28.66 	             17.6

Thanks, Art, very interesting!

It seems to me that the true distance to NGC 6946 might be slightly less rather than slightly more than 20 million light-years.

Ann

I found an interesting article about NGC 6946.

Tammy Plotner of Universe Today wrote:

Most normally, bursts of star formation happen in galaxies which have nearby companions to lend materials. Yet, NGC 6946 appears to be alone in the field. According to a 2000 study done by Pisano (et al) ” Such gas-rich companions could include material left over from the galaxy assembly process which could persist into the current day around an isolated galaxy such as NGC 6946. NGC 6946 is prolifically forming stars, has a nuclear starburst, and has widespread high-velocity clouds associated with the disc. All of these features could be explained by the accretion of low-mass Hi clouds by NGC 6946. Our survey recovered two previously detected dwarf galaxies associated with NGC 6946, but otherwise found no signatures of interactions in the NGC 6946 system. The companions are small enough, and distant enough from NGC 6946 that they should have minimal effect on the main galaxy. Some tidal debris may be expected due to interaction between the two dwarf galaxies, but none is observed. This could be because it is at low column densities, or because the dwarf galaxies are more separated than they appear on the sky. This study of the system suggests that NGC 6946 is a gravitationally bound system with two dwarf galaxies in stable orbits about the larger primary galaxy.”

So NGC 6946 is isolated apart from two small dwarf companions in stable orbits around it, and it has high-velocity clouds of gas associated with the disk.

Ann

Oh, and by the way, the cluster...

Gloria Andreuzzi et al. wrote:

We present CCD UBVI photometry of the nearby, intermediate age open cluster NGC 6939. Using the synthetic Colour - Magnitude Diagrams technique we estimate the following parameters: age between 1.3 and 1.0 Gyr (depending on whether or not overshooting is considered), reddening 0.34<E(B-V)<0.38 and distance modulus 11.3<mod_0<11.4 .

So, NGC 6939 would be around a billion years old or a little more. That's much too old for the cluster to have any blue stars left, but not nearly old enough for the cluster to be interesting because of its advanced age.
Yawn.

Ann

Edit: Well, perhaps it looks like there are two blue stars in it.. whatever...

Ann wrote:

Wikipedia wrote:
NGC 6946 is highly obscured by interstellar matter of the Milky Way galaxy, as it is quite close to the galactic plane.

Well, I'm going to disagree a bit with Wikipedia. I don't doubt that NGC 6946 is somewhat reddened by dust in our own galaxy, but in my opinion, most of the reddening of NGC 6946 is caused by dust of NGC 6946's own making.

You're not disagreeing with Wikipedia, you're disagreeing with the primary sources that Wikipedia references.

The issue isn't reddening as such, but attenuation, and there's no doubt that NGC 6946 is highly attenuated by dust in the Milky Way. It is one of the galaxies commonly used in studies which seek photometric calibration techniques for dust-attenuated sources. It would be much brighter if it were simply located in a different part of the sky.

Whoooooooooooooo Hoo!
http://asterisk.apod.com/viewtopic.php?f=8&t=37204
Yes, I beat APoD to it! Or rather, Mr.Wiggins did, and I noticed a post, I think on Facebook about it!

Yeee-Ha!
Excuse me, most un-British. I am, however, extremely gratified. I thank you.
John

Speaking about supernovas in NGC 6946, check out bystander's post in the Breaking Science News forum.

It appears that there may have been a massive star in NGC 6946 that tried to go supernova, but it didn't manage to. Instead, it collapsed directly into a black hole.
Ann

Sometime ago APOD recorded an event, a Supernova that had erupted behind a lensing galaxy and there wasaposibility to record the light of the actual event in real time.

JohnD wrote:Whoooooooooooooo Hoo!
http://asterisk.apod.com/viewtopic.php?f=8&t=37204
Yes, I beat APoD to it! Or rather, Mr.Wiggins did, and I noticed a post, I think on Facebook about it!

Yeee-Ha!
Excuse me, most un-British. I am, however, extremely gratified. I thank you.
John

Beat the APOD to what?

SN 2017eaw was already very public knowledge on May 14, 2017, as evidenced by this Bob King article at Sky & Telescope online having the same date. I saw it visually in a scope on the morning of May 21, prior to the EPOD about it on May 22 that you mention at the "viewtopic" link -- and I was delayed a few days by bad weather and other circumstances!

ta152h0 wrote:Sometime ago APOD recorded an event, a Supernova that had erupted behind a lensing galaxy and there wasaposibility to record the light of the actual event in real time.

SN Refsdal.

I sometimes wonder if you-all were at the back of the queue when humour was handed out.

A cap o' bells looks silly on a gorilla, but you sure have to laugh at his jokes.
John

speaking of humor, my wife teaches first grade and once a week science is taught. Few weeks back it was weather patterns and sure enouugh sand storms was brought up. When the first graders heard the name assigned to some sandstorm, it took the principal a long time to round up enough thinking chairs for what followed. It was like permission was given to share what they learned in the playground

Thanks APOD for sharing!
Paolo

Ah, first grade!
I remember my teacher at the blackboard.
She would say a word, and then very slowly, stroke by stroke, she would build up each letter; one by one, and finally we had the whole word.
She did two or three words, and then said 'now the fourth word, which will be bump."
I instantly knew that she had NO idea what trouble she was headed into with us little kids!
She didn't see it until the third letter was completely ... when of course we all burst into hysterical childish laughter!

nothing like first grade boys now armed with a killer word given to them by science and they can use it freely in the playground, aned at home. Permission granted. My wife had no idea what was going to happen.

Ann wrote:Speaking about supernovas in NGC 6946, check out bystander's post in the Breaking Science News forum.

It appears that there may have been a massive star in NGC 6946 that tried to go supernova, but it didn't manage to. Instead, it collapsed directly into a black hole.
Ann

Very cool.

Joe Stieber wrote:

JohnD wrote:Whoooooooooooooo Hoo!
http://asterisk.apod.com/viewtopic.php?f=8&t=37204
Yes, I beat APoD to it! Or rather, Mr.Wiggins did, and I noticed a post, I think on Facebook about it!

Yeee-Ha!
Excuse me, most un-British. I am, however, extremely gratified. I thank you.
John

Beat the APOD to what?

SN 2017eaw was already very public knowledge on May 14, 2017, as evidenced by this Bob King article at Sky & Telescope online having the same date. I saw it visually in a scope on the morning of May 21, prior to the EPOD about it on May 22 that you mention at the "viewtopic" link -- and I was delayed a few days by bad weather and other circumstances!

Brilliant, Joe!

But hey, John D, I congratulate you on being the first person to mention it here at Starship Asterisk*.

Ann