APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

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APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby APOD Robot » Wed May 10, 2017 4:05 am

Image UGC 1810: Wildly Interacting Galaxy from Hubble

Explanation: What's happening to this spiral galaxy? Although details remain uncertain, it surely has to do with an ongoing battle with its smaller galactic neighbor. The featured galaxy is labelled UGC 1810 by itself, but together with its collisional partner is known as Arp 273. The overall shape of the UGC 1810 -- in particular its blue outer ring -- is likely a result of wild and violent gravitational interactions. This ring's blue color is caused by massive stars that are blue hot and have formed only in the past few million years. The inner galaxy appears older, redder, and threaded with cool filamentary dust. A few bright stars appear well in the foreground, unrelated to UGC 1810, while several galaxies are visible well in the background. Arp 273 lies about 300 million light years away toward the constellation of Andromeda. Quite likely, UGC 1810 will devour its galactic sidekick over the next billion years and settle into a classic spiral form.

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Re: APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby RocketRon » Wed May 10, 2017 5:47 am

APOD Robot wrote:This ring's blue color is caused by massive stars that are blue hot


Very scenic.

Is there any good discussion anywhere about WHY stars 'burn' at different temperatures.

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Re: APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby Ann » Wed May 10, 2017 6:08 am

Arp 273 with UGC 1810 at top.
Photo: NASA/ESA, HST
M101. Photo: R. Jay GaBany.


















UGC 1810 is a truly fascinating galaxy. I'm posting a picture of it next to a picture of M101, because these two galaxies have two interesting things in common. Both are big galaxies, and both are actually quite extremely blue for their large size.

In the nearby universe, the very bluest galaxies - with a B-V of 0.3 or even less - are always small. That is because the smallest galaxies never have a really large population of old yellow stars, so they don't need to produce a huge amount of young blue stars to look blue overall. But big galaxies always do. If a big galaxy is to have a B-V index of less than 0.5, it needs a tremendous amount of young blue stars to outshine its own yellow population.

The B-V of UGC 1810 is 0.47, and the B-V of M101 is 0.45. That's amazing, and it speaks volumes of the blue fireworks they are creating! (And if you ask me, UGC 1810 looks like a water sprinkler!)

By the way, and if you ask me, UGC is actually bluer "in reality" than it is made to look in most processed versions of the Hubble data, like today's APOD. That is because of the filters used by Hubble to image this galaxy (or pair of galaxies). The filters were F390 W (ultraviolet), F475X, centered on blue, and F600LP, centered on orange. If you are going to map ultraviolet, blue and orange as blue, green and red, you are going to make the picture look redder than the data actually vouch for.

M51 and NGC 5195. Photo: NASA/ESA and HST.
Structurally though, UGC 1810 and M101 are very different. M101 is slightly "messy", because its spiral arms are neither very bright nor well-formed. The arms of UGC 1810 look almost unnaturally sharp and well-formed in places, almost as if you could take a pair of scissors and cut them out. It is of course the interaction with UGC 1813 that winds UGC 1810 into that fantastic spiral shape.

It is better to compare the structure of UGC 1810 with the structure of M51. You can see, when you look at M51, how the inner arm that originates to the east (left) of the nucleus and extends upwards and to the right, suddenly loses its inner dust lane after having made half a turn around the nucleus. You can also see how the other spiral arm that extends downwards and to the left, comes to a "fork in the road" after half a turn. I think those are the places where the original arms ended, and the rest of the spiral structure is the result of the interaction with NGC 5195.

Similarly, you can see what to me appears to be an original arm of UGC 1810. Look at the inner arm that originates to the right of the nucleus and extends upward and to the left. See how that arm traces a beautiful, regular, rather thin arc, until it comes to an end after about a full turn? I think that is an original arm. And look at the other arm, that originates near the nucleus and extends down and to the right, how it seems to dip behind the other inner arm and disappear behind it? I think this is the other original arm.

Note how yellow the original arms seem to be. The rest, the storm, the thunder, the blue froth, the sound and the fury, is the tornado of star formation whipped up by the encounter between UGC 1810 and 1813.

Oh, and one more thing... that tiny little barred spiral galaxy(?), also visible in today's APOD, and seemingly creating its own little maelstrom in the violently blue and starforming breakers of UGC 1810, is certainly fascinating. It's at 4 o'clock in today's APOD, and at 2 o'clock in the picture I posted at the top of my post.

Ann
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Re: APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby Boomer12k » Wed May 10, 2017 6:21 am

Awesome image... so that is what a merger looks like.

It is also interesting that the smaller galaxy is decimated and in "shock" as it were, while the whirlpool is still quite formed...yet they are so near.

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Re: APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby ta152h0 » Wed May 10, 2017 8:13 am

WOW ! Scream saver
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Re: APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby heehaw » Wed May 10, 2017 9:17 am

What a lovely loose spiral!

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Re: APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby Chris Peterson » Wed May 10, 2017 12:51 pm

RocketRon wrote:Is there any good discussion anywhere about WHY stars 'burn' at different temperatures.

Basically, it's just a question of mass. The more massive the star, the hotter it is.
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Re: APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby Ann » Wed May 10, 2017 1:55 pm

RocketRon wrote:
APOD Robot wrote:This ring's blue color is caused by massive stars that are blue hot


Very scenic.

Is there any good discussion anywhere about WHY stars 'burn' at different temperatures.


For stars on the main sequence, the color of the star depends (almost) entirely on its mass.

Main sequence stars shine by fusing hydrogen to helium in their cores. When the star has used up the hydrogen in its core it will turn into a giant, and although most stars will spend all of their lifetimes as giants as red stars, some stars will "turn blue again", like blue horizontal branch stars and some blue supergiants. For example, the progenitor of supernova 1987A was Sanduleak −69° 202, a blue supergiant. It is believed that Sanduleak −69° 202 had evolved into a red supergiant after leaving the main sequence, and then it shed quite a lot of its mass and turned into a blue supergiant. As a rule of thumb, evolved stars that shine with a blue light are smaller than evolved stars that shine with a red or yellow light. Note that this does not apply to degenerate stars like white dwarfs.

Look at the star cluster at left, M44 in Cancer. According to Wikipedia, the age of M44 is 600-700 million years. Again according to Wikipedia, M44 contains red giants and white dwarfs, as well as main sequence stars of spectral classes A, F, G, K, and M.

Again take a look at the picture. The bright yellow stars in the picture are red giants of spectral class K. Their temperature is about 4,000K or a little more, but not as high as 5,000K. The bright blue stars are main sequence stars of spectral class A. Their temperature is about 8,000-9,000K. If we consider the photospheres of the stars to be their "surfaces", then each "square mile" or "square kilometer" of the surface of the main sequence A-type stars shines brighter than each square mile or square kilometer of the red giants stars. But because the red giant stars are bigger, their surfaces have many more square miles or square kilometers than the main sequence A-typs stars. As a consequence, both types of stars shine about equally bright in this cluster.

Incidentally, the red giants are every bit as hot in their centers as the A-type main sequence stars. Actually, the centers of the red giants are even hotter - sometimes many times hotter - than the centers of the main sequence A-type stars. But precisely because the red giants are so big and "swollen", their atmospheres have a reddening effect on the highly energetic photons being generated in fusion processes in their centers. These photons have to "fight their way through so much fog", and undergo so many energy-stealing collisions with protons inside the star before they reach the photosphere where they can escape, that most of them leave the star as relatively low-energy photons which emit red, orange or yellow light.

Blackbody curves of star of different temperatures.
Source: http://astronomy.swin.edu.au/cosmos/B/B ... +Radiation
But let's return to the main sequence stars, which are easier to understand. For each such star, its color is dependent on its mass. The higher the mass, the faster and more intensely the hydrogen fusion in the center of the star must proceed in order to counteract the strong gravity that presses inward on the centers of these stars. Very many high-energy photons are created in their very hot centers, and although blue main sequence stars are bigger than, say, the Sun, they are still small enough to let many high-energy photons escape from their photospheres.

Consider the brightest-looking star in the sky, Sirius. Sirius is a main sequence A-type star, not unlike the brightest blue stars in M44. It is almost exactly twice as massive as the Sun, but it is almost 23 times brighter and considerably bluer and a lot more ultraviolet than the Sun. It is the extra mass of Sirius that forces its hydrogen fusion in its center to work so fast and so intensely. By contrast, in the case of small light-weight red main sequence stars, their hydrogen fusion works very slowly, producing relatively few photons of rather low energy, and the photons that leave the photospheres of the small red stars are typically low-energy photons corresponding to red, orange and yellow light.

Stars are blackbodies, producing light as blackbodies of a certain temperature. For every temperature, there is a blackbody curve that peaks at exactly that temperature. Blueward of the peak, the curve dips down relatively quickly, but redward of the peak the curve rises rather slowly. What this means is that a sufficiently cool star may not produce any blue light at all, or hardly any blue light, because its blackbody curve dips down to almost zero before it reaches the region of blue light. By contrast, a very hot star will always produce green, yellow, orange and red light as well as blue and ultraviolet light, because its blackbody curve is far from zero as it passes through these colors.

Finally, you might want to consider the picture at left, showing the evolution of stars of different masses. Perhaps you would also like to visit the page that I have linked to below the picture.

Ann
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Re: APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby sillyworm » Wed May 10, 2017 3:18 pm

Smashing! Not sure if it has been mentioned here in this discussion....WIKI mentions ,in reference to ARP 273, "it is thought that the smaller galaxy has actually passed through the larger one."[5] AMAZING to contemplate.

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Re: APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby Ann » Thu May 11, 2017 12:35 am

I should mention that there is another amazing object in UGC 1810 that is visible in today's APOD. At 1 o'clock, inside a wide gap between two of UGC 1810's spiral arms, is a very red, bright, small object surrounded by two concentric rings. A long "finger" of blue stars seems to point at the object. What can it be?

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Re: APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby sillyworm » Thu May 11, 2017 12:48 am

I saw it & imagined it to be a face on Galaxy.

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Re: APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby Ann » Thu May 11, 2017 1:41 am

Looking at it at full resolution makes it clear that the object is a background galaxy, a barred spiral galaxy with an inner ring, a set of spiral arms and a bright, redshift-reddened bulge and nucleus. But the "finger" of UGC 1810 - make that a "double finger", actually - that seems to be pointing at this background galaxy is weird!

Ann
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Re: APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby Guest » Thu May 11, 2017 3:09 am

I thought this was a joke image at first. How far is that outer arm from the next one in toward the center ? What would your galactic core look like from there ? We can see the furious, ongoing star formation, yes, but we are humans watching the behavior of existences that ridiculously transcend our lifetimes. Just pure wonder . . . like Hobbits listening to Tom Bombadil.

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Re: APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby sillyworm » Thu May 11, 2017 2:07 pm

Does anyone know the name/# of this background Galaxy?

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Re: APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby Ann » Thu May 11, 2017 2:39 pm

sillyworm wrote:Does anyone know the name/# of this background Galaxy?


I rather doubt that that the little background galaxy has a name or a designation.

Consider how faint it must be. After all, UGC 1810, which looks so bright in today's APOD, is only 13th magnitude - magnitude 13.4, to be more exact. That's faint. I quickly checked up all the Messier galaxies, and the two faintest of them, M91 and M98, are magnitude 10.1. All the others are brighter than magnitude 10.0.

I have no idea how faint that little background galaxy is, but it must be very faint indeed. Art, Chris, the rest of you math whizzes, what do you think? Magnitude 17? 18? 19? 20?

I'm sure that no one knew of the little background galaxy at all until Hubble photographed UGC 1810. After it was photographed along with UGC 1810, maybe someone gave it a name, just because they could see that it existed. Or not.

What do the rest of you think?

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Re: APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby Chris Peterson » Thu May 11, 2017 2:52 pm

Ann wrote:I have no idea how faint that little background galaxy is, but it must be very faint indeed. Art, Chris, the rest of you math whizzes, what do you think? Magnitude 17? 18? 19? 20?

There's no simple way to tell from the processed image, since this kind of processing always alters the transfer function- that is, it distorts the brightness relationship between different parts. It would be simple enough to tell from the original, linear data that was used to create the image (at least, to tell within the bandpass of any individual filter used to collect the data).
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Re: APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby sillyworm » Thu May 11, 2017 3:00 pm

After participating in the Galaxy Zoo program....I've seen some amazing Galaxy images.....hopefully we'll be studying some of them soon.

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Re: APOD: UGC 1810: Wildly Interacting Galaxy... (2017 May 10)

Postby neufer » Thu May 11, 2017 3:07 pm

Ann wrote:
UGC 1810, which looks so bright in today's APOD, is only 13th magnitude - magnitude 13.4, to be more exact. I have no idea how faint that little background galaxy is, but it must be very faint indeed. Art, Chris, the rest of you math whizzes, what do you think? Magnitude 17? 18? 19? 20?

A ballpark estimate is that it is ~20 times smaller than UGC 1810 making it ~20 [= 13.4 + 5 log(20)].
Art Neuendorffer


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