APOD: M31 versus M33 (2013 Sep 26)

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APOD: M31 versus M33 (2013 Sep 26)

Post by APOD Robot » Thu Sep 26, 2013 4:16 am

Image M31 versus M33

Explanation: Separated by about 14 degrees (28 Full Moons) in planet Earth's sky, spiral galaxies M31 ,left, and M33 are both large members of the Local Group, along with our own Milky Way galaxy. This wide-angle, telescopic mosaic captures colorful details of spiral structure in both, while the massive neighboring galaxies seem to be balanced either side of bright Mirach, beta star in the constellation Andromeda. But M31, the Andromeda Galaxy, is really 2.5 million light-years distant and M33, the Triangulum Galaxy, is also about 3 million light years away. Mirach, just 200 light-years from the Sun, lies well within the Milky Way, along with the dim clouds of dust drifting through the frame only a few hundred light-years above the galactic plane. Although they look far apart, M31 and M33 are locked in a mutual gravitational embrace. Radio astronomers have found indications of a bridge of neutral hydrogen gas that could connect the two, evidence of a closer encounter in the past. Based on measurements, gravitational simulations currently predict that the Milky Way, M31, and M33 will all undergo mutual close encounters and potentially mergers, billions of years in the future.

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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by Ann » Thu Sep 26, 2013 4:46 am

This is a fun picture, and I really like the composition. And I'm always happy to see Rogelio Bernal Andreo back here.

I have two nitpicks, though. In today's APOD, the surface brightness of M33 appears to be about equal to the surface brightness of M31, the Andromeda galaxy. That is not the case, because Andromeda is much brighter.

Also, the stellar content of these two galaxies appears to be quite similar. But Andromeda has a huge population of old yellow stars, and comparatively few young blue stars. For M33, the situation is the opposite. The yellow bulge of M33 is relatively faint and comparatively star-poor, but its arms are bursting with a rich population of young blue stars.

Even so, this is a fresh new take on two of our galactic neighbours. And Rogelio Bernal Andreo gives us yet another really wide-field image. They are always a lot of fun to look at!

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Last edited by Ann on Thu Sep 26, 2013 2:15 pm, edited 2 times in total.
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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by geckzilla » Thu Sep 26, 2013 5:12 am

Ann wrote:I have two nitpicks, though. In today's APOD, the surface brightness of M33 appears to be about equal to the surface brightness of M31, the Andromeda galaxy. That is not the case, because Andomeda is much brighter.

Also, the stellar content of these two galaxies appears to be quite similar. But Andromeda has a huge population of old yellow stars, and comparatively few young blue stars. For M33, the situation is the opposite. The yellow bulge of M33 is relatively faint and comparatively star-poor, but its arms are bursting with a rich population of young blue stars.
It's not easy to image this scene. Read about some of the difficulties here. Reducing the brightness of Andromeda or at least imaging and processing it in a way to make an aesthetically pleasing image and not blow out the details is a good choice. If everything in astronomical processing was left at its natural brightness, a lot of things would either get lost in flat white bright spots or disappear into blackness. The colors, are, as always, a personal matter. One day maybe you will do your own processing and come to some new understanding. It's like criticizing someone's cooking without ever having touched a stove yourself. Sure, you can do it, but it doesn't lend you much credence.
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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by rstevenson » Thu Sep 26, 2013 11:36 am

I, on the other hand, appreciate it when someone tells me that the colours and brightnesses of the two galaxies are actually different than shown -- even while I greatly appreciate the effort the imager made to present us with this beautiful image.

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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by geckzilla » Thu Sep 26, 2013 12:28 pm

...But that is true for every single astronomical image shown. That's the point. And it's not just about the colors of the galaxy. It's about showing things other than what Ann specifically cares about. You could criticize this in exactly the same way by saying oh, but this doesn't show x-rays. I could be a big fan of x-rays and say I'd prefer if the x-ray data were emphasized for every single image. It would be more realistic that way because x-rays are real and everyone should see the x-rays.
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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by stephen63 » Thu Sep 26, 2013 1:16 pm

What a great image. I'm a fan of RBA as well. He's definitely one of the best at digging integrated flux nebulae out of the noise.

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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by saturno2 » Thu Sep 26, 2013 2:08 pm

M 31 , Andromeda Galaxy, diameter 200,000 ly, and
M33 , Triangulum Galaxy, diameter 50,000 ly, have a connection of
neutral hidrogen gas.
This connection is very long, because in the sky of Earth the astronomers
see a distance of 14 degrees, between M 31 and M 33

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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by wonderboy » Thu Sep 26, 2013 2:12 pm

this image got me thinking about dark matter. if you grab a squishy ball (dark matter in this example) and squash it between each hand (representing each galaxy) then the ball will displace accordingly on both sides. If dark matter exists then surely it should be similarly displaced by the two galaxies approaching one another?
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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by neufer » Thu Sep 26, 2013 3:00 pm

wonderboy wrote:
this image got me thinking about dark matter. if you grab a squishy ball (dark matter in this example) and squash it between each hand (representing each galaxy) then the ball will displace accordingly on both sides. If dark matter exists then surely it should be similarly displaced by the two galaxies approaching one another?
It is generally assumed that isolated balls of dark matter are mostly the product of (plasma/magnetic field sticky) galactic collisions and would therefore exist off to the sides of any interacting pair of galaxies (rather than in between them).

However, if an isolated ball of dark matter did reside in between M31 & M33 the dark matter ball would probably be the entity doing most of the gravitational pulling (given that there is 5.5 times as much dark matter mass as ordinary matter mass).
http://en.wikipedia.org/wiki/Dark_matter wrote: <<The most direct observational evidence to date for dark matter is in a system known as the Bullet Cluster. In most regions of the universe, dark matter and visible material are found together, as expected because of their mutual gravitational attraction. In the Bullet Cluster, a collision between two galaxy clusters appears to have caused a separation of dark matter and baryonic matter. X-ray observations show that much of the baryonic matter (in the form of 107–108 Kelvin gas, or plasma) in the system is concentrated in the center of the system. Electromagnetic interactions between passing gas particles caused them to slow down and settle near the point of impact. However, weak gravitational lensing observations of the same system show that much of the mass resides outside of the central region of baryonic gas. Because dark matter does not interact by electromagnetic forces, it would not have been slowed in the same way as the X-ray visible gas, so the dark matter components of the two clusters passed through each other without slowing down substantially. This accounts for the separation. Unlike the galactic rotation curves, this evidence for dark matter is independent of the details of Newtonian gravity, so it is claimed to be direct evidence of the existence of dark matter. Another galaxy cluster, known as the Train Wreck Cluster/Abell 520, appears to have an unusually massive and dark core containing few of the cluster's galaxies, which presents problems for standard dark matter models.>>
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The ghost of Andromeda's "girdle"

Post by neufer » Thu Sep 26, 2013 3:38 pm

http://en.wikipedia.org/wiki/NGC_404 wrote: <<NGC 404 is a galaxy located about 10 million light years away in the constellation Andromeda. It was discovered by William Herschel in 1784, and is visible through small telescopes. NGC 404 lies just beyond the Local Group but does not appear gravitationally bound to it. It is notable for being within 7 arc-minutes of second magnitude star Mirach, making it a difficult target to observe or photograph and granting it the nickname "Mirach's Ghost".

NGC 404 is a very isolated dwarf lenticular galaxy, a bit more luminous and smaller than the Small Magellanic Cloud and unlike many other early-type galaxies is very rich in neutral hydrogen, most of it concentrated on a pair of large rings around it. It also has star formation both on its center and on its outermost regions, albeit at a low level

Both the outer gas disk and its star formation are assumed to have been triggered by one or several mergers with smaller galaxies roughly 1 billion years ago and it has been proposed NGC 404 is a former spiral galaxy that was transformed into a lenticular one by that event.

NGC 404 contains a low-ionization nuclear emission-line region (LINER), a type of region that is characterized by spectral line emission from weakly ionized atoms. A nuclear star cluster is also present as well as -likely- a supermassive black hole, with a mass of several tens of thousands solar masses.

At least two techniques have been used to measure distances to NGC 404. The infrared surface brightness fluctuations distance measurement technique estimates distances to spiral galaxies based on the graininess of the appearance of their bulges. The distance measured to NGC 404 using this technique in 2003 is 9.9 ± 0.5 Mly. However, NGC 404 is close enough that red supergiants can be imaged as individual stars. The light from these stars and knowledge of how they should compare to nearby stars within the Milky Way galaxy allows for direct measurement of the distance to the galaxy. This method is referred to as the tip of the red giant branch (TRGB) method. The estimated distance to NGC 404 using this technique is 10.0 ± 1.2 Mly. Averaged together, these distance measurements give a distance estimate of 10.0 ± 0.7 Mly.
http://en.wikipedia.org/wiki/Beta_Andromedae wrote: <<Beta Andromedae (β Andromedae) has an average apparent visual magnitude of 2.05, which makes it the brightest star in Andromeda. Based upon parallax measurements, it is located at a distance of roughly 197 light-years from Earth. The apparent magnitude of this star is reduced by 0.06 from extinction caused by the gas and dust along the line of sight.

Beta Andromedae is a red giant with a stellar classification of M0 III. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. It is suspected of being a semiregular variable star whose apparent visual magnitude varies from +2.01 to +2.10. At this stage of the star's evolution, the outer envelope has expanded to around 100 times the size of the Sun. It is radiating 1,995 times the luminosity of the Sun at an effective temperature of 3,842 K.

The name Mirach, and its variations, such as Mirac, Mirar, Mirath, Mirax, etc. come from the star's description in the Alfonsine Tables of 1521 as super mizar. Here, mirat is a corruption of the Arabic ميزر mīzar "girdle", which appeared in a Latin translation of the Almagest. This word refers to Mirach's position at the left hip of the princess Andromeda. Medieval astronomers writing in Arabic called β Andromedae Janb al-Musalsalah (English: The Side of the Chained (Lady)).>>
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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by Ann » Thu Sep 26, 2013 4:14 pm

Geckzilla wrote:
If everything in astronomical processing was left at its natural brightness, a lot of things would either get lost in flat white bright spots or disappear into blackness.
I agree. I believe that most large or moderately large spiral galaxies have bulges and/or central parts that are much brighter than the spiral arms. Compare, for example, this ESO picture of apparently bright-armed spiral galaxy M100 (with a burnt-out center) with this ESO picture where M100 is seen to have a brilliant center but faint arms and an even fainter disk. (There is a supernova in there, too.) Or compare it with this picture where the center of M100 is not burnt out, but the arms are moderately bright anyway. I like the last picture a lot, but I'm not sure it gives us a good idea of how bright the center of M100 is compared with the arms.

As for the Andromeda galaxy, take a look at this picture which shows the brilliant bulge of M31, but the arms can hardly be seen at all. I believe that this picture gives you a good idea of what M31 looks like visually.

What about M33? In this picture, the bulge is brighter than the arms, but the difference is rather small. Also the overall color of M33 is neutral or slightly blue, whereas the dominant color of M31 is clearly yellow. This is obvious from the B-V indexes of these galaxies: 0.950 for Andromeda (considerably yellower than the Sun) versus 0.550 for M33 (bluer than the Sun).

Also M31 is about two magnitudes brighter visually than M33, and it also has a higher surface brightness, particularly if we compare the two galaxies' central parts.

So the overall colors and brightnesses of these two galaxies have been tweaked in opposite directions to produce this image. The result is a dramatic, delightfully symmetric image. It is clear from the image that M33 is smaller than M31, so the picture still tells us a very important fact about these two "islands of stars".

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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by geckzilla » Thu Sep 26, 2013 4:50 pm

Blown out, not burnt out. It makes no sense to me to compare the "color" of the Sun (or any star) to the dynamic range of both tone and hue to an entire galaxy. What is the point of that, anyway? I could understand comparing galaxies to galaxies but galaxies to the Sun? I don't get it.
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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by Boomer12k » Thu Sep 26, 2013 6:05 pm

It is a wondrous picture. The interstellar wisps are interesting, adding texture to the whole scene. I had no idea the Triangulum Galaxy was so close. It is very small, about 1/2 our size. By this article, it too appears to be approaching us, or at least, has in the distant past, and will in the far flung future.
It is going to be a bigger train wreck than I thought...and we will merge into an even larger whole...
What a GLORIOUS sight the sky is going to be some day....Alien Stars EVERYWHERE....a "marriage made in heaven"?

Many of the images that make APOD are manipulated for one reason or another. Sometimes it is Aesthetics, sometimes for scientific data. It depends on what you want to study or convey...I try for detail, but I also try to bring out color, especially in nebulae. It depends on what I am shooting. Mostly....I just try to get something.... :lol2:
If a galaxy, I want to see the structure of the arms...if I have to over expose the center....fine...I need the long exposure to get that detail. I suppose, I could take another one of less exposure for the center and then post process that in place of the other, then have both. But that is more work...
My pictures are taken from the driveway in a neighborhood of a city, with a street light about 50 feet away. My original photo is "dingy". Even a 2 minute exposure of a galaxy like M101 is faint, and rather "flat" looking...so, I up the brightness and the contrast, until it looks better to me.
There are "imperfections", heat speckles, that are a camera artifact. I remove them by hand in Photoshop...am I cheating? And yes, I take my Dark Frames, and I still get some anyway. There are also "blotches", dark areas that need to be removed, these can be dirt, or bad camera pixel areas maybe...I have tried to get rid of them from the camera, and the scope many times. I count them as a Camera Artifact, but there is sometimes still dust on the front glass of the scope, even after cleaning it, and my star diagonal.
So, try not to begrudge the astrophotographer too much...it is allot of work. Sometimes it is only Beauty you are after...
So it depends on the intention of the photographer and what type of "truth" he is after. With these "Versus" pictures...I am not quite sure what that is...is it comparison? is it just composition and position in a larger depiction of the night sky? They certainly are not "fighting". "Versus" seems to be out of place here. "And" would be more appropriate to me. We could also be like the "4 planet" photos and say there are 3 galaxies here too... :shock:
I enjoy the perspective. And the detail of what else is there besides just a single object. That I appreciate. To see the area as more of a whole. Like Bernard's Loop, and not just The Horse Head Nebula...as fascinating as that is on its own, even in Infrared or Ultraviolet.
Even the STARS are interesting in this image. I notice a "haze" around them. Like I am looking through a thin fog...well, we are...all that dust.

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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by Anthony Barreiro » Thu Sep 26, 2013 7:16 pm

Ann wrote:
Geckzilla wrote:
If everything in astronomical processing was left at its natural brightness, a lot of things would either get lost in flat white bright spots or disappear into blackness.
I agree. I believe that most large or moderately large spiral galaxies have bulges and/or central parts that are much brighter than the spiral arms. Compare, for example, this ESO picture of apparently bright-armed spiral galaxy M100 (with a burnt-out center) with this ESO picture where M100 is seen to have a brilliant center but faint arms and an even fainter disk. (There is a supernova in there, too.) Or compare it with this picture where the center of M100 is not burnt out, but the arms are moderately bright anyway. I like the last picture a lot, but I'm not sure it gives us a good idea of how bright the center of M100 is compared with the arms.

As for the Andromeda galaxy, take a look at this picture which shows the brilliant bulge of M31, but the arms can hardly be seen at all. I believe that this picture gives you a good idea of what M31 looks like visually.

What about M33? In this picture, the bulge is brighter than the arms, but the difference is rather small. Also the overall color of M33 is neutral or slightly blue, whereas the dominant color of M31 is clearly yellow. This is obvious from the B-V indexes of these galaxies: 0.950 for Andromeda (considerably yellower than the Sun) versus 0.550 for M33 (bluer than the Sun).

Also M31 is about two magnitudes brighter visually than M33, and it also has a higher surface brightness, particularly if we compare the two galaxies' central parts.

So the overall colors and brightnesses of these two galaxies have been tweaked in opposite directions to produce this image. The result is a dramatic, delightfully symmetric image. It is clear from the image that M33 is smaller than M31, so the picture still tells us a very important fact about these two "islands of stars".

Ann
Thanks Ann for the examples of different pictures of these two galaxies.

My immediate reaction to today's apod was, "I wish M33 looked that bright in the eyepiece of my telescope. Then I'd be able to see it from the back yard here in the city, rather than only when I'm out at a dark location!" I appreciate the time, effort, and skill that went into creating this picture, but I would have preferred a picture that shows M33 as much fainter than M31. I'm not saying it's a bad picture, just that this one choice doesn't sit well with me.

I didn't take Ann's comments as any sort of negative criticism, either. On the contrary, she offered positive comments on the picture. And I don't think you need to process your own astronomical images to have opinions about pictures you see. I've never painted an oil painting, but when I go to the museum I have opinions about the pictures I see.
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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by BillBixby » Thu Sep 26, 2013 7:22 pm

I like today's APOD. I like the APOD most every day. Thank you to the artistic side of the persons posting all the APOD pictures.

Thank you Ann for pointing out the differences in how much actual light/brightness would be found in these different galaxies; how dimmer and unobservable many would be if not enhanced. I had seen the note of a mosaic in the "about" but failed to realize the amount of work required to make this composition for presentation. Then the comments made by others re your comment made me realize how much I am taking the APOD pictures for granted.

WOW!

Going to the link gives an example of just some of the work Rogelio went through to develop this composition. I am confused by the 26 and 12 frame references. I think the idea was to make the picture using 12 frames; however it took 26 different frames (pictures), taken by Rogelio, at multiple sites throughout our world to capture the date required.

Now I have to go to the other links and see other things I am missing.

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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by geckzilla » Thu Sep 26, 2013 7:47 pm

Anthony Barreiro wrote:I didn't take Ann's comments as any sort of negative criticism, either. On the contrary, she offered positive comments on the picture. And I don't think you need to process your own astronomical images to have opinions about pictures you see. I've never painted an oil painting, but when I go to the museum I have opinions about the pictures I see.
Of course, but you'd have a lot more insight if you did do oil painting. And you'd probably want to be familiar with oil painting technique before criticizing someone else's technique at length. I find Ann's comments to be strangely biased toward her well-known affection for the color blue and for how she thinks certain objects should be perceived. It bothers me a lot because of the large volume of verbose comments with certain esoteric terms that could cause the average reader to mistake her superficial opinions as being more deeply qualified.
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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by walker1001 » Thu Sep 26, 2013 9:17 pm

We are told here
Although they look far apart, M31 and M33 are locked in a mutual gravitational embrace. Radio astronomers have found indications of a bridge of neutral hydrogen gas that could connect the two, evidence of a closer encounter in the past. Based on measurements, gravitational simulations currently predict that the Milky Way, M31, and M33 will all undergo mutual close encounters and potentially mergers, billions of years in the future.
And we are told this sort of thing many times. (See yesterday 25th September 2013, among others)

Yet it is accepted that ALL galaxies are flying away from us and each other at ever increasing speed and will disappear into deep infinity.

When I ask why some will interact and perhaps join up when "ALL are flying away from each other", the short answer I get is that Gravity-does-it. Why does gravity do it in some cases and not"all" is not answered. Surely all matter exerts gravity on all other matter. But why are there local galaxy clusters and superclusters if everything is moving away from everything else as evidenced by their red shifts? If gravity is doing it then how can they be flying away from each otherand forming clusters? Can you have it both ways? Am I missing something?

Can someone point me to links to articles that might clear the clusters of fog in my brain? This is a matter of utmost gravity. :)

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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by Anthony Barreiro » Thu Sep 26, 2013 9:58 pm

walker1001 wrote:We are told here
Although they look far apart, M31 and M33 are locked in a mutual gravitational embrace. Radio astronomers have found indications of a bridge of neutral hydrogen gas that could connect the two, evidence of a closer encounter in the past. Based on measurements, gravitational simulations currently predict that the Milky Way, M31, and M33 will all undergo mutual close encounters and potentially mergers, billions of years in the future.
And we are told this sort of thing many times. (See yesterday 25th September 2013, among others)

Yet it is accepted that ALL galaxies are flying away from us and each other at ever increasing speed and will disappear into deep infinity.

When I ask why some will interact and perhaps join up when "ALL are flying away from each other", the short answer I get is that Gravity-does-it. Why does gravity do it in some cases and not"all" is not answered. Surely all matter exerts gravity on all other matter. But why are there local galaxy clusters and superclusters if everything is moving away from everything else as evidenced by their red shifts? If gravity is doing it then how can they be flying away from each otherand forming clusters? Can you have it both ways? Am I missing something?

Can someone point me to links to articles that might clear the clusters of fog in my brain? This is a matter of utmost gravity. :)
I think this might be the link you're looking for:

https://en.wikipedia.org/wiki/Metric_expansion_of_space

Especially:

https://en.wikipedia.org/wiki/Metric_ex ... _scales.3F

According to the inverse square law of gravity, the gravitational attraction between two objects decreases at an exponential rate as the distance between them increases. Two bodies that are close to one another have a strong gravitational attraction to one another; two bodies that are far apart have a much weaker attraction.

The universe's expansion seems to be a quality of space itself, i.e. space is expanding a little bit everywhere. The farther apart two objects are, the more space there is between them, so two objects that are farther apart are expanding away from one another faster than two objects that are closer to one another.

In other words, as distance increases, the effect of cosmic expansion increases while gravitational attraction decreases. For massive objects like galaxies that are relatively close together in cosmic terms, gravity is stronger than cosmic expansion, so objects that are close enough to one another to be gravitationally bound are not expanding away from one another.
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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by Chris Peterson » Thu Sep 26, 2013 10:09 pm

walker1001 wrote:Yet it is accepted that ALL galaxies are flying away from us and each other at ever increasing speed and will disappear into deep infinity.
That isn't accepted at all. Indeed, it is completely false. Space over very long distances is expanding, giving the appearance that widely separated galaxies are flying apart. But locally, as within galaxy clusters, gravity is preventing the expansion of space. Galaxies which are gravitationally bound, meaning they are in closed orbits around a common center, will remain together forever (unless perturbed by some other body).
walker1001 wrote:When I ask why some will interact and perhaps join up when "ALL are flying away from each other", the short answer I get is that Gravity-does-it. Why does gravity do it in some cases and not"all" is not answered.
It's simply a matter of the masses being close enough together for the gravitational force to dominate universal expansion. To work out the actual numbers for any specific example would require some difficult GR calculations.
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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by Ann » Fri Sep 27, 2013 12:51 am

geckzilla wrote:Blown out, not burnt out. It makes no sense to me to compare the "color" of the Sun (or any star) to the dynamic range of both tone and hue to an entire galaxy. What is the point of that, anyway? I could understand comparing galaxies to galaxies but galaxies to the Sun? I don't get it.
The light from galaxies comes mostly from stars. Other factors contribute, such as jets and bright nebulae. The presence or absence of dust will affect the light we can see from a particular galaxy.

Take a look at this picture of the Milky Way. At far right, we can see a bright, yellowish area. That is a part of our galaxy's bulge. Galactic bulges are typically bright, because they are so rich in stars, and they are typically yellow, because almost all the stars that are found in bulges are old and yellow, yellower than the Sun. Therefore galactic bulges are almost always yellow.

We can see a lot of dark dust in the picture. At upper right, the dust takes on a strange orange color. In my personal opinion, the orange color here is more pronounced than it would be in most other pictures of the same area. Nevertheless, this is a clear example of "dust reddening". Dust dims and reddens the light of stars behind it. Where the dust is sufficiently thick, it blots out all optical light behind it, and the area looks black to our eyes.

Between the bright yellow bulge and the orange dust-reddened area is a length of dust which is bright from star light and nebulae. This is an example of star formation in the dust. Almost all stars are born in dusty areas, and in some patches a dust lane can be full of star formation.

We can spot a few reddish patches in this "bright dust lane". One of the prominent nebulae is the Lagoon Nebula.

The brightest part of the "bright dust lane", some distance to the left of the Lagoon Nebula, is a whitish patch made up of mostly young stars. This is M24, the Small Sagittarius Star Cloud. The overall color of this star cloud is probably somewhat bluer than the color of the Sun, because most of the light here comes from stars that are bluer than the Sun. Certainly this area also contains many stars that are yellower than the Sun, but the light from the blue stars is likely to be dominant.

This is my point. If most of the light from a galaxy comes from stars that are yellower than the Sun, then the overall color of that galaxy is going to be yellower than the color of the Sun. If most of the light comes from stars that are bluer than the Sun, then the overall color of that galaxy is probably going to be bluer than the Sun. Admittedly dust reddening could be a factor that makes such a galaxy less blue than we might expect it to be.

I should add that I define the light of the Sun as white. The Sun is the source of all our natural daylight, and our eyes have evolved in such a way that we see daylight (direct sunlight plus light from the clear sky, or light from an overcast sky) as neutral, or white.

The B-V index of the Sun is 0.656 ± 0.005, which, as I said, I define as white. The B-V index of the Andromeda galaxy is yellower, 0.950, comparable to well-known star Pollux. The B-V index of M33 is 0.550, a bit bluer than the Sun, and somewhat comparable to Mirfak, the brightest star in this picture.

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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by geckzilla » Fri Sep 27, 2013 2:44 am

But a galaxy is not an average of all the light of its stars. This is like calling a ball that is half blue and half red purple because its averaged color would be purple. For astronomical imaging it is probably better to compare an object's color to itself so that you don't get caught up in all this confusion of trying to set the Sun as your baseline for everything. As long as the shorter wavelengths are bluer and the longer wavelengths are redder then the representation of the object is accurate enough. Sometimes it is very hard to cram in all the representative colors into a single composition.
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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by Ann » Fri Sep 27, 2013 2:54 am

geckzilla wrote:But a galaxy is not an average of all the light of its stars.
That's how I see it. Of course I realize that a lot of factors come into play here, but basically, yes, that's how I see it.

When I say that I think of a galaxy as a collection of stars, then I just mean the visual appearance of it. Obviously a galaxy contains all sorts of things that are either completely invisible or so faint that they don't contribute to the overall visual appearance of it at optical wavelenghts. An interesting class of "faint objects" is planets, of course! :D

I should add that the kind of galaxies that I think of as "blue" typically have parts that are mostly yellow, such as (obviously) their bulges. But if the galaxies are "blue" to me, it means that they contain a relatively high number of hot young stars that contribute a lot of light. A yellow galaxy is poor in young stars, and an overwhelming proportion of its light comes from old yellow stars.

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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by starsurfer » Fri Sep 27, 2013 8:26 am

geckzilla wrote:
Anthony Barreiro wrote:I didn't take Ann's comments as any sort of negative criticism, either. On the contrary, she offered positive comments on the picture. And I don't think you need to process your own astronomical images to have opinions about pictures you see. I've never painted an oil painting, but when I go to the museum I have opinions about the pictures I see.
Of course, but you'd have a lot more insight if you did do oil painting. And you'd probably want to be familiar with oil painting technique before criticizing someone else's technique at length. I find Ann's comments to be strangely biased toward her well-known affection for the color blue and for how she thinks certain objects should be perceived. It bothers me a lot because of the large volume of verbose comments with certain esoteric terms that could cause the average reader to mistake her superficial opinions as being more deeply qualified.
I like that Ann offers us what's in her mind, so many people lack the courage to speak what they feel. Reading Ann's posts always makes me happy! :D I would love to meet her and give her a hug!

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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by geckzilla » Fri Sep 27, 2013 12:42 pm

starsurfer wrote:I like that Ann offers us what's in her mind, so many people lack the courage to speak what they feel.
The number of crackpots I ban from the forum tell me otherwise...
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Re: APOD: M31 versus M33 (2013 Sep 26)

Post by neufer » Fri Sep 27, 2013 1:03 pm

geckzilla wrote:
starsurfer wrote:
I like that Ann offers us what's in her mind, so many people lack the courage to speak what they feel.
The number of crackpots I ban from the forum tell me otherwise...
http://en.wikipedia.org/wiki/Crank_%28person%29 wrote:
<<"Crank" is a pejorative term used for a person who holds an unshakable belief that most of his or her contemporaries consider to be false. Common synonyms for "crank" include crackpot and kook. A crank belief is so wildly at variance with those commonly held as to be ludicrous. Cranks characteristically dismiss all evidence or arguments which contradict their own unconventional beliefs, making rational debate a futile task, and rendering them impervious to facts, evidence, and rational inference. A crank differs from a fAnnatic in that the subject of the fAnnatics obsession is either not necessarily widely regarded as wrong or not necessarily a "fringe" belief.

Although a crank's beliefs seem ridiculous to experts in the field, cranks are sometimes very successful in convincing non-experts of their views. A famous example is the Indiana Pi Bill where a state legislature nearly wrote into law a crank result in geometry.

The second book of the mathematician and popular author Martin Gardner was a study of crank beliefs, Fads and Fallacies in the Name of Science. More recently, the mathematician Underwood Dudley has written a series of books on mathematical cranks, including The Trisectors, Mathematical Cranks, and Numerology: Or, What Pythagoras Wrought. And in a 1992 UseNet post, the mathematician John Baez humorously proposed a checklist, the Crackpot index, intended to diagnose cranky beliefs regarding contemporary physics.[2]

According to these authors, virtually universal characteristics of cranks include:
  • Cranks overestimate their own knowledge and ability, and underestimate that of acknowledged experts.

    Cranks insist that their alleged discoveries are urgently important.

    Cranks rarely, if ever, acknowledge any error, no matter how trivial.

    Cranks love to talk about their own beliefs, often in inappropriate social situations, but they tend to be bad listeners, being uninterested in anyone else's experience or opinions.
Art Neuendorffer