APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

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APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by APOD Robot » Sun Aug 16, 2020 4:07 am

Image NGC 6814: Grand Design Spiral Galaxy from Hubble

Explanation: In the center of this serene stellar swirl is likely a harrowing black-hole beast. The surrounding swirl sweeps around billions of stars which are highlighted by the brightest and bluest. The breadth and beauty of the display give the swirl the designation of a grand design spiral galaxy. The central beast shows evidence that it is a supermassive black hole about 10 million times the mass of our Sun. This ferocious creature devours stars and gas and is surrounded by a spinning moat of hot plasma that emits blasts of X-rays. The central violent activity gives it the designation of a Seyfert galaxy. Together, this beauty and beast are cataloged as NGC 6814 and have been appearing together toward the constellation of the Eagle (Aquila) for roughly the past billion years.

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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by BDanielMayfield » Sun Aug 16, 2020 4:44 am

Thanks to Hubble and to geckzilla for processing this image.

The explanation's 10 million sun mass estimate for NGC's Supermassive Black Hole isn't quite super enough however. Both the description of this image on the Hubble site page and wikipedia show it at 18 million suns.
wikipedia wrote:NGC 6814 is an intermediate spiral galaxy in constellation Aquila. It is located at a distance of about 75 million light years from Earth, which, given its apparent dimensions, means that NGC 6814 is about 85,000 light years across. NGC 6814 has an extremely bright nucleus and is a type 1.5 Seyfert galaxy. The galaxy is also a highly variable source of X-ray radiation. The ultraviolet and optical emission also varies, although more smoothly, with time lag of two days. The cause of the lag and the smoothing of light curves is considered to be the reprocessing of the X-rays in the accretion disk.[3] The cause of the active galactic nucleus is suspected to be a supermassive black hole with a mass about 18 million times that of the Sun.[4]
Just as zero is not equal to infinity, everything coming from nothing is illogical.

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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by Ann » Sun Aug 16, 2020 6:00 am

Today's APOD is a gorgeous image of a splendid spiral galaxy!

I wouldn't call it a grand design spiral galaxy, though.
Wikipedia wrote:

A grand design spiral galaxy is a type of spiral galaxy with prominent and well-defined spiral arms, as opposed to multi-arm and flocculent spirals which have subtler structural features. The spiral arms of a grand design galaxy extend clearly around the galaxy through many radians and can be observed over a large fraction of the galaxy's radius. As of 2002, approximately 10 percent of all currently known spiral galaxies are classified as grand design type spirals,[1] including M51, M74, M81, M83, and M101.
Wikipedia wrote:

Messier 100 (also known as NGC 4321) is a grand design intermediate spiral galaxy located within the southern part of constellation Coma Berenices.
I think that the arms of NGC 6814 are not long enough, and not well-defined enough, to earn the designation "grand design spiral". Wikipedia agrees, and doesn't call it a grand design spiral galaxy.

However, I found a tiny tiny picture by GALEX of NGC 6814 in ultraviolet light. Note the bright blue center of NGC 6814 in the GALEX image. So why is the center blue, when today's APOD seems to show us that there is no obvious star formation there?

Answer: We are clearly seeing ultraviolet light related to outbursts of the central black hole.

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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by bystander » Sun Aug 16, 2020 6:13 am

BDanielMayfield wrote:
Sun Aug 16, 2020 4:44 am
Thanks to Hubble and to geckzilla for processing this image.
I don't think this image is geckzilla's. Her image on flickr of NGC 6814 doesn't really look like this.
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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by JohnD » Sun Aug 16, 2020 11:49 am

Galaxies are clasified into "barred" and "unbarred" - NGC 6814 appears to be of the latter ilk.
Some of the former are extreme, EG NGC 1300 (QV https://hubblesite.org/image/3903/printshop)

The usual explanation for a bar is the result of a density wave in the galactic disc, but that theory was proposed in the 60s. Since then Black Holes have become far more prominent in our astronomy, as have the Super-Massive BHs at galactic cores. What if the foci of a galactic bar were two BHs? Wouldn't that produce a bar, with the same star-forming effect?
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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by neufer » Sun Aug 16, 2020 12:35 pm

JohnD wrote:
Sun Aug 16, 2020 11:49 am
  • Galaxies are clasified [sic] into "barred" and "unbarred"

    - NGC 6814 appears to be of the latter ilk.
http://allenbrowne.blogspot.com/2011/03/grover-cleveland-slept-here.html wrote:


Grover Cleveland Slept Here
by Allen Browne, Saturday, March 12, 2011

The Pennyfield Inn at Lock 22 on the C&O Canal was built in 1879.

President Grover Cleveland stayed here on several occasions to fish for bass in the nearby Potomac.

The old NPS signage at lock 22 told this story this way:
  • President Grover Cleveland enjoyed bass fishing near here, escaping from the pressures of office while staying at the Pennyfield House. Its folksy atmosphere appealed to the president. Mrs. Pennyfield, it's said once called up the stairs to President Cleveland,

    "Mr. President, do you want your eels skunned or unskunned?"

    He preferred them "skunned."
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Post by XgeoX » Sun Aug 16, 2020 1:02 pm

Just wanted to encourage everyone to get outside and observe the planets! Right now Jupiter, Saturn, Mars and Venus are all visible during the night and I was able to get great views even through my little 100mm scope so get out there and enjoy the spectacle!

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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by BDanielMayfield » Sun Aug 16, 2020 1:12 pm

bystander wrote:
Sun Aug 16, 2020 6:13 am
BDanielMayfield wrote:
Sun Aug 16, 2020 4:44 am
Thanks to Hubble and to geckzilla for processing this image.
I don't think this image is geckzilla's. Her image on flickr of NGC 6814 doesn't really look like this.
That's strange. Today's APOD image clearly gives an acknowledgement to Judy Schmidt (aka geckzilla) in the lower left corner, and so does the image's caption. :?:
Just as zero is not equal to infinity, everything coming from nothing is illogical.

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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by BDanielMayfield » Sun Aug 16, 2020 1:32 pm

Ann wrote:Today's APOD is a gorgeous image of a splendid spiral galaxy!
Certainly true!
I wouldn't call it a grand design spiral galaxy, though.
Technically accurate perhaps, but, at least IMHO it displays both qualities. :ssmile:

Bruce
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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by geckzilla » Sun Aug 16, 2020 2:32 pm

BDanielMayfield wrote:
Sun Aug 16, 2020 1:12 pm
bystander wrote:
Sun Aug 16, 2020 6:13 am
BDanielMayfield wrote:
Sun Aug 16, 2020 4:44 am
Thanks to Hubble and to geckzilla for processing this image.
I don't think this image is geckzilla's. Her image on flickr of NGC 6814 doesn't really look like this.
That's strange. Today's APOD image clearly gives an acknowledgement to Judy Schmidt (aka geckzilla) in the lower left corner, and so does the image's caption. :?:
That's ESA's version, alright. The acknowledgment is to say they basically copied my work. I've asked why they do redundant work, and the answer was something along the lines that they have an internal process that they never deviate from. (They did deviate at least once, and used my actual work a few months ago.)
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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by orin stepanek » Sun Aug 16, 2020 3:12 pm

NGC6814_HubbleSchmidt_960.jpg
That's confusing! :shock:
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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by BaldEagle » Sun Aug 16, 2020 3:48 pm

A random question on today's Grand Design Galaxy is this:

How far into the yellow color of this black hole would a normal star survive? And could that star be such that it still maintained planets -- if it had any to begin with?

it is easy to imagine those stars in the arms as normal, but once a star enters that misty yellow area, and later that blinding yellow center, imagining it still surviving as a star with planets (and perhaps still harboring a form of life) can be a stretch.

Comments, however speculative, would be appreciated.

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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by JohnD » Sun Aug 16, 2020 4:23 pm

neufer wrote:
Sun Aug 16, 2020 12:35 pm
JohnD wrote:
Sun Aug 16, 2020 11:49 am
  • Galaxies are clasified [sic] into "barred" and "unbarred"

    - NGC 6814 appears to be of the latter ilk.
http://allenbrowne.blogspot.com/2011/03/grover-cleveland-slept-here.html wrote:


Grover Cleveland Slept Here
by Allen Browne, Saturday, March 12, 2011

The Pennyfield Inn at Lock 22 on the C&O Canal was built in 1879.

President Grover Cleveland stayed here on several occasions to fish for bass in the nearby Potomac.

The old NPS signage at lock 22 told this story this way:
  • President Grover Cleveland enjoyed bass fishing near here, escaping from the pressures of office while staying at the Pennyfield House. Its folksy atmosphere appealed to the president. Mrs. Pennyfield, it's said once called up the stairs to President Cleveland,

    "Mr. President, do you want your eels skunned or unskunned?"

    He preferred them "skunned."
You surpass your nonsequetorial self there, Neufer!
But as no one else has answered, thank you.

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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by johnnydeep » Sun Aug 16, 2020 4:51 pm

BaldEagle wrote:
Sun Aug 16, 2020 3:48 pm
A random question on today's Grand Design Galaxy is this:

How far into the yellow color of this black hole would a normal star survive? And could that star be such that it still maintained planets -- if it had any to begin with?

it is easy to imagine those stars in the arms as normal, but once a star enters that misty yellow area, and later that blinding yellow center, imagining it still surviving as a star with planets (and perhaps still harboring a form of life) can be a stretch.

Comments, however speculative, would be appreciated.
Despite their general awesomeness, black hole sizes are quite small, astronomically speaking. The event horizon, aka, the Schwarzchild radius is given at https://en.wikipedia.org/wiki/Black_hol ... properties as:
Size of Event Horizon.JPG
Or, in km, about 3 times the mass of the black hole over the mass of the sun. If we use 20 million solar masses for the size of the black hole at the center of NGC 6814, we get a Schwarzchild radius of only 60 million km, which is well under 1 AU. That's barely a pinpoint at the center of NGC 6814. Only a few light years from it, you would barely know it existed...apart from being bathed in any radiation from matter falling into it. Whether the yellow at the center of NGC 6814 is caused by that radiation I don't know. I was going to write some more, but I realized I might not know what I'm talking about. So, have at it Neufer, Chris or other worthy poster!
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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by johnnydeep » Sun Aug 16, 2020 4:55 pm

APOD Robot wrote:
Sun Aug 16, 2020 4:07 am
Image NGC 6814: Grand Design Spiral Galaxy from Hubble

Explanation: In the center of this serene stellar swirl is likely a harrowing black-hole beast. The surrounding swirl sweeps around billions of stars which are highlighted by the brightest and bluest. The breadth and beauty of the display give the swirl the designation of a grand design spiral galaxy. The central beast shows evidence that it is a supermassive black hole about 10 million times the mass of our Sun. This ferocious creature devours stars and gas and is surrounded by a spinning moat of hot plasma that emits blasts of X-rays. The central violent activity gives it the designation of a Seyfert galaxy. Together, this beauty and beast are cataloged as NGC 6814 and have been appearing together toward the constellation of the Eagle (Aquila) for roughly the past billion years.
What does the text mean to convey by the last sentence, in particular, the "past billion years" part? That before or after that interval they will no longer be toward the constellation of the Eagle? Surely the black hole will be at the center of NGC 6814 for much longer than a billion years. And I would think that Aquila will evaporate as an identifiable constellation via simple stellar motion in a mere few million years.
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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by Ann » Sun Aug 16, 2020 6:42 pm

johnnydeep wrote:
Sun Aug 16, 2020 4:55 pm
APOD Robot wrote:
Sun Aug 16, 2020 4:07 am
Image NGC 6814: Grand Design Spiral Galaxy from Hubble

Explanation: In the center of this serene stellar swirl is likely a harrowing black-hole beast. The surrounding swirl sweeps around billions of stars which are highlighted by the brightest and bluest. The breadth and beauty of the display give the swirl the designation of a grand design spiral galaxy. The central beast shows evidence that it is a supermassive black hole about 10 million times the mass of our Sun. This ferocious creature devours stars and gas and is surrounded by a spinning moat of hot plasma that emits blasts of X-rays. The central violent activity gives it the designation of a Seyfert galaxy. Together, this beauty and beast are cataloged as NGC 6814 and have been appearing together toward the constellation of the Eagle (Aquila) for roughly the past billion years.
What does the text mean to convey by the last sentence, in particular, the "past billion years" part? That before or after that interval they will no longer be toward the constellation of the Eagle? Surely the black hole will be at the center of NGC 6814 for much longer than a billion years. And I would think that Aquila will evaporate as an identifiable constellation via simple stellar motion in a mere few million years.
And the Sun completes a full orbit around the center of the Milky Way in 250 million years or so. For how long will the bright stars of Aquila "follow the Sun" and remain a recognizable constellation from the Earth's point of view (never mind if there will be any humans around to admire the view of the night sky 250 million years from now or not)?

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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by Chris Peterson » Sun Aug 16, 2020 7:11 pm

johnnydeep wrote:
Sun Aug 16, 2020 4:55 pm
APOD Robot wrote:
Sun Aug 16, 2020 4:07 am
Image NGC 6814: Grand Design Spiral Galaxy from Hubble

Explanation: In the center of this serene stellar swirl is likely a harrowing black-hole beast. The surrounding swirl sweeps around billions of stars which are highlighted by the brightest and bluest. The breadth and beauty of the display give the swirl the designation of a grand design spiral galaxy. The central beast shows evidence that it is a supermassive black hole about 10 million times the mass of our Sun. This ferocious creature devours stars and gas and is surrounded by a spinning moat of hot plasma that emits blasts of X-rays. The central violent activity gives it the designation of a Seyfert galaxy. Together, this beauty and beast are cataloged as NGC 6814 and have been appearing together toward the constellation of the Eagle (Aquila) for roughly the past billion years.
What does the text mean to convey by the last sentence, in particular, the "past billion years" part? That before or after that interval they will no longer be toward the constellation of the Eagle? Surely the black hole will be at the center of NGC 6814 for much longer than a billion years. And I would think that Aquila will evaporate as an identifiable constellation via simple stellar motion in a mere few million years.
As Ann notes, constellations (well, technically asterisms) are only stable for a few million years. Perhaps the reference to "together" was the black hole and the galaxy. Active galactic nucleuses are cyclic- they tend to turn on and off. When they're off, they're not visible.
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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by neufer » Sun Aug 16, 2020 7:21 pm

JohnD wrote:
Sun Aug 16, 2020 4:23 pm
You surpass your nonsequetorial self there, Neufer!
But as no one else has answered, thank you.
You're welcome.
https://en.wikipedia.org/wiki/Non_sequitur_(literary_device) wrote:
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A non sequitur can denote an abrupt, illogical, or unexpected turn in plot or dialogue by including a relatively inappropriate change in manner. A non sequitur joke sincerely has no explanation, but it reflects the idiosyncrasies, mental frames and alternative world of the particular comic persona. Comic artist Gary Larson's The Far Side cartoons are known for what Larson calls "...absurd, almost non sequitur animal" characters, such as talking cows, which he uses to create a "...weird, zany, ...bizarre, odd, strange" effect; in one strip, "two cows in a field gaz[e] toward [a] burning Chicago, saying 'It seems that agent 6363 had accomplished her mission.'">>
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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by neufer » Sun Aug 16, 2020 7:36 pm

Chris Peterson wrote:
Sun Aug 16, 2020 7:11 pm
johnnydeep wrote:
Sun Aug 16, 2020 4:55 pm
APOD Robot wrote:
Sun Aug 16, 2020 4:07 am
Image NGC 6814: Grand Design Spiral Galaxy from Hubble

Together, this beauty and beast are cataloged as NGC 6814 and have been appearing together toward the constellation of the Eagle (Aquila) for roughly the past billion years.
What does the text mean to convey by the last sentence, in particular, the "past billion years" part? That before or after that interval they will no longer be toward the constellation of the Eagle? Surely the black hole will be at the center of NGC 6814 for much longer than a billion years. And I would think that Aquila will evaporate as an identifiable constellation via simple stellar motion in a mere few million years.
As Ann notes, constellations (well, technically asterisms) are only stable for a few million years. Perhaps the reference to "together" was the black hole and the galaxy. Active galactic nucleuses are cyclic- they tend to turn on and off. When they're off, they're not visible.
  • Peak emissions from active galactic "nucleuses" are of relatively short (a few hundred million years) duration:
https://en.wikipedia.org/wiki/Seyfert_galaxy wrote:
<<By the end of the 1950s, more important characteristics of Seyfert galaxies were discovered, including the fact that their nuclei are extremely compact (< 100 pc, i.e. "unresolved"), have high mass (≈109±1 solar masses), and the duration of peak nuclear emissions is relatively short (> 108 years).>>
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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by Chris Peterson » Sun Aug 16, 2020 7:44 pm

neufer wrote:
Sun Aug 16, 2020 7:36 pm
Chris Peterson wrote:
Sun Aug 16, 2020 7:11 pm
johnnydeep wrote:
Sun Aug 16, 2020 4:55 pm


What does the text mean to convey by the last sentence, in particular, the "past billion years" part? That before or after that interval they will no longer be toward the constellation of the Eagle? Surely the black hole will be at the center of NGC 6814 for much longer than a billion years. And I would think that Aquila will evaporate as an identifiable constellation via simple stellar motion in a mere few million years.
As Ann notes, constellations (well, technically asterisms) are only stable for a few million years. Perhaps the reference to "together" was the black hole and the galaxy. Active galactic nucleuses are cyclic- they tend to turn on and off. When they're off, they're not visible.
  • Peak emissions from active galactic "nucleuses" are of relatively short (a few hundred million years) duration:
https://en.wikipedia.org/wiki/Seyfert_galaxy wrote:
<<By the end of the 1950s, more important characteristics of Seyfert galaxies were discovered, including the fact that their nuclei are extremely compact (< 100 pc, i.e. "unresolved"), have high mass (≈109±1 solar masses), and the duration of peak nuclear emissions is relatively short (> 108 years).>>
Indeed, I've generally understood that these active nucleuses are only active for periods quite a bit less than a billion years. Just trying to figure alternate explanations for the sentence.
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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by Ann » Sun Aug 16, 2020 8:09 pm

johnnydeep wrote:
Sun Aug 16, 2020 4:51 pm
BaldEagle wrote:
Sun Aug 16, 2020 3:48 pm
A random question on today's Grand Design Galaxy is this:

How far into the yellow color of this black hole would a normal star survive? And could that star be such that it still maintained planets -- if it had any to begin with?

it is easy to imagine those stars in the arms as normal, but once a star enters that misty yellow area, and later that blinding yellow center, imagining it still surviving as a star with planets (and perhaps still harboring a form of life) can be a stretch.

Comments, however speculative, would be appreciated.
Despite their general awesomeness, black hole sizes are quite small, astronomically speaking. The event horizon, aka, the Schwarzchild radius is given at https://en.wikipedia.org/wiki/Black_hol ... properties as:

Size of Event Horizon.JPG

Or, in km, about 3 times the mass of the black hole over the mass of the sun. If we use 20 million solar masses for the size of the black hole at the center of NGC 6814, we get a Schwarzchild radius of only 60 million km, which is well under 1 AU. That's barely a pinpoint at the center of NGC 6814. Only a few light years from it, you would barely know it existed...apart from being bathed in any radiation from matter falling into it. Whether the yellow at the center of NGC 6814 is caused by that radiation I don't know. I was going to write some more, but I realized I might not know what I'm talking about. So, have at it Neufer, Chris or other worthy poster!
It is true that most stars and planets may remain completely unaffected by the antics of a black hole. My amateur opinion (or guess) is that the Earth has never been adversely affected by the black hole in the center of the Milky Way.

But I wouldn't be so sure that the effects of the outbursts of a feeding black hole might not be felt over a much longer distance than just a few light-years. At least the "Death Star Galaxy", 3C 321, is zapping its companion galaxy with a lethal (?) jet of X-rays from its black hole!














So what's the yellow stuff at the center of NGC 6814? Why, that's just the combined light of billions of mostly small red and yellow stars of spectral classes G, K and M.

We can, however, be sure that there is an accretion disk around the central black hole of this galaxy. And the black hole is feeding from that accretion disk, while some parts of the accretion disk grind against other parts of the accretion disk at terrible speeds and are heated to millions of degrees.

The accretion disk of the black hole is much too small to be seen in a picture like today's APOD. Note that the center of the galaxy appears to be overexposed anyway in today's APOD.

But the bright accretion disk can be seen, at least indirectly, at other wavelengths than the visual ones. Check out this ultraviolet picture of NGC 6814 from GALEX. The blue light from the center of the galaxy is ultraviolet light from the super-hot accretion disk of the active black hole.

Ann
Last edited by Ann on Sun Aug 16, 2020 8:54 pm, edited 1 time in total.
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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by neufer » Sun Aug 16, 2020 8:37 pm

Chris Peterson wrote:
Sun Aug 16, 2020 7:44 pm
neufer wrote:
Sun Aug 16, 2020 7:36 pm
Chris Peterson wrote:
Sun Aug 16, 2020 7:11 pm

As Ann notes, constellations (well, technically asterisms) are only stable for a few million years. Perhaps the reference to "together" was the black hole and the galaxy. Active galactic nucleuses are cyclic- they tend to turn on and off. When they're off, they're not visible.
  • Peak emissions from active galactic "nucleuses" are of relatively short (a few hundred million years) duration:
https://en.wikipedia.org/wiki/Seyfert_galaxy wrote:
<<By the end of the 1950s, more important characteristics of Seyfert galaxies were discovered, including the fact that their nuclei are extremely compact (< 100 pc, i.e. "unresolved"), have high mass (≈109±1 solar masses), and the duration of peak nuclear emissions is relatively short (> 108 years).>>
Indeed, I've generally understood that these active nucleuses are only active for periods quite a bit less than a billion years. Just trying to figure alternate explanations for the sentence.
  • Perhaps it is in reference to an estimate of the starburst activity duration concentrated at its center [a la Messier 100:]
https://en.wikipedia.org/wiki/Messier_100 wrote:
<<Messier 100 (also known as NGC 4321) is a grand design intermediate spiral galaxy located within the southern part of constellation Coma Berenices. It is one of the brightest and largest galaxies in the Virgo Cluster, located approximately 55 million light-years distant from Earth. Messier 100 is considered a starburst galaxy with the strongest star formation activity concentrated in its center, within a ring - actually two tightly wound spiral arms attached to a small nuclear bar with a radius of 1 kilo-parsec – where star formation has been taking place since at least 500 million years ago in separate bursts.>>
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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by Ann » Mon Aug 17, 2020 7:47 am

BaldEagle asked about the blinding yellow center of NGC 6814, and I can't resist the chance to talk a little about yellow and blue stars in spiral galaxies, and why we find them where we find them.

To start off, we need to talk about star formation. Stars form in bursts, forming clusters. Take a look at this Hubble image of a cluster (or two clusters) in nebula NGC 602 in the Small Magellanic Cloud:

As you can see, there are two clusters in the picture at left, one in the upper left corner, one at center right. There is a moderately equal number of bright blue stars in both clusters, but only the cluster at right contains huge numbers of small stars. Why is that?

At least one part of the answer is that the cluster at right is very young. Small faint stars are born whenever stars form, but bright massive stars are formed only in massive clusters. There are always more small faint stars than big bright stars formed in any cluster. So whenever there is a very young cluster of stars, there is always going to be large numbers of small stars in that cluster.

So why don't we see any small stars in the cluster at left?

Answer: They may have scattered, because of the violent tidal forces of the big bright (and massive) stars orbiting one another. Clusters also scatter when they encounter molecular clouds. The cluster at right is so young that the small stars have not yet had time to scatter. Another possibility, which is perhaps more likely, is that the cluster at right was photographed through an infrared filter, while the cluster at left was not. That would make a huge difference in whether or not you can see the small red and yellow stars in the cluster.


Click to play embedded YouTube video.

The more massive a cluster is, the better it will be at holding on to its stars. The globular clusters of the Milky Way were born with hundreds of thousands of stars, and the combined gravity of all these stars makes them stick together as clusters after, typically, 8-12 billion years.

Take a look at the video at right, where you can see the evolution of a (relatively) massive star cluster. You can see that some stars change color, become red, and then disappear. Why?














The answer is that the more massive (and bright and blue) a star is, the faster it uses up its core hydrogen. When the star's core hydrogen is gone, it swells and turns into a red giant. Now its supports itself by fuelling less energy-efficient elements than hydrogen. Relatively soon it runs out, and then it either explodes as a supernova (if it is massive enough), or else it sheds its outer layers and becomes a white dwarf.

What does a rich cluster look like when all its bright blue stars are gone?

At left you can see the old open cluster Trumpler 5. I have not managed to find out how old it is, but my guess is that it is 2-4 billion years old. At that age, all of its blue stars have died. The cluster still holds on to most of its stars, because the cluster was clearly born quite massive. It is also almost certain that the cluster is located in a "quiet" part of the Milky Way, where it is not disturbed by the passage of large molecular clouds. It is located behind the nebulosity that you can see in this image.

When clusters like Trumpler 5 finally disperse, where do their (small red and yellow) stars end up?








Well, the small red and yellow stars will quietly disperse and mostly follow the orbital motion of their parent cluster. The illustration at right shows the dispersal of a globular cluster. Open clusters will disperse in the same way, only faster, because they have less total gravity to hold them together.

Where will most of the small red and yellow stars that escaped from their parent cluster end up?









Take a look at spiral galaxy M74. Its center is yellow, and its arms are blue. Why is that?

The central part of M74, its bulge, is bright and yellow because it contains billions and billions of small red and yellow stars, which are the remnants of past star formation in this part of the galaxy. The formation of so many clusters in this part on the galaxy used up and scattered the available fuel for star formation, the cold and concentrated hydrogen gas, and no more stars are formed in the bulge. But the enormous number of small faint yellow stars in the bulge still makes the bulge shine bright and yellow.

While the bulge of M74 is yellow, its spiral arms are blue. They are blue because massive, bright and blue stars are born here. You can see individual clusters and possibly even individual bright stars in the arms. These bright stars are all young and will die soon, and they will leave behind the kind of yellow stars that crowd together in the bulge. But before even the arms of M74 turn yellow, they will turn a more dilute blue color from intermediate-mass stars like Sirius and Vega. Stars like Sirius and Vega last much longer than brilliant blue stars like the ones in Orion's Belt, but they still die much sooner than stars like the Sun.

You can see that the arms of M74 are bluer the farther away they are from the center of the galaxy. You can see, too, that the pink nebulas surrounding newborn stars are larger the farther away they are from the galactic center. You can also see that the galaxy gets ever darker the farther away we get from its center, because there are much fewer stars here. The fact that bright blue stars are born here can't compensate for the fact that the number of these stars is so small.




















A great example of how galaxies form small numbers of hot bright and ultraviolet stars in an outer set of arms is the galaxy NGC 1512. In visual light, we see an oval-shaped galaxy, with a yellow bar-shaped bulge, a bright inner ring of hot stars surrounding a yellow nucleus, and a relatively faint outer ring. In ultraviolet light, however, we see long outer arms shining by the light of small numbers of hot bright stars lined up along elongated structures of dust and gas.


Finally, take a look at the picture at left of the very center of the Milky Way. The small bright white swirl of gas surrounds the black hole of our galaxy. Note the number of stars outside this gaseous swirl. There are young blue stars born here, and the white swirl is probably the brightest feature in the entire Milky Way.

Ann
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Bill Bruehl

Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by Bill Bruehl » Mon Aug 17, 2020 11:41 am

When I stare and wonder at this beautiful image of NGC 6814, I see it as three dimensional. It goes down in the center to a great area of light and a maybe black hole. Has anyone figured the "depth" in light years? Or the size of that area of light in the center?

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Re: APOD: NGC 6814: Grand Design Spiral Galaxy... (2020 Aug 16)

Post by neufer » Mon Aug 17, 2020 1:28 pm


Bill Bruehl wrote:
Mon Aug 17, 2020 11:41 am

When I stare and wonder at this beautiful image of NGC 6814, I see it as three dimensional. It goes down in the center to a great area of light and a maybe black hole. Has anyone figured the "depth" in light years? Or the size of that area of light in the center?
I regret to inform you that everything outside of the solar system is two dimensional. :cry:
Art Neuendorffer