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	Galaxy is a featured article; it (or a previous version of it) has been identified as one of the best articles produced by the Misplaced Pages community. Even so, if you can update or improve it, please do so.
	This article appeared on Misplaced Pages's Main Page as Today's featured article on March 3, 2007.
Article milestones Date Process Result January 23, 2007 Peer review Reviewed February 4, 2007 Good article nominee Listed February 10, 2007 Featured article candidate Promoted
Current status: Featured article

Astronomy: Astronomical objects FA‑class

This article is within the scope of WikiProject Astronomy, which collaborates on articles related to Astronomy on Misplaced Pages.AstronomyWikipedia:WikiProject AstronomyTemplate:WikiProject AstronomyAstronomy FA This article has been rated as FA-class on Misplaced Pages's content assessment scale. ??? This article has not yet received a rating on the project's importance scale. This article is supported by WikiProject Astronomical objects, which collaborates on articles related to astronomical objects.

Template:WP1.0

Miscellany

Reason for reverting: in an article about a scientific subject ("galaxy" is a word invented by and for scientists, not some olden-days "spiritual" word), it's always possible to fill pages upon pages with "This is what the scientists think .... BUT, 'more advanced isoteric sources' say that ...", with a donzen alternate theories, but we should resits that "urge".

The problem is that while thousands of scientists that study galaxies actually believe the kind of stuff written in the current article, the "esoteric" (spiritual, I'd call it) stuff you wrote about is hardly common belief, not even among any groups of spiritualists, astrologists, clerics of some religion, or anything. If you want to you explain how an esoteric school of thought called "Galaxism" (I'm just making this up, sorry) views galaxis, it would be much better if you added a "Galaxism's view on Galaxis" section in the Galaxism article, and in the Galaxy page add a link to that section, from the "Related Links". Nyh 07:39, 28 Apr 2004 (UTC)

I believe the added stuff is just someone's joke and the material was not expected to stay!

Adrian Pingstone 14:58, 28 Apr 2004 (UTC)

Would "galactographic" be an appropriate adjective equivalent to "geographical"? (A neologism, but it might be useful) Jackiespeel 19:42, 22 Mar 2005 (UTC)

There is no proof that galaxies evolve on their own - the Big Bang is a speculative theory and has quite a few problems. If anyone disagrees, let me know. Salva 02:33, 28 Apr 2005 (UTC)

Salva, you know people disagree with that. -- Temtem 03:24, Apr 28, 2005 (UTC)

Yes, thank you, "Temptem." Those that do disagree comprise a shrinking minority. One of the major problems with the Big Bang theory is that it violates several laws of physics. These include, but are not limited to: the first and second laws of thermodynamics, the conservation of angular momentum, etc. Although the theory of relativity does show us that our galaxy and the universe are expanding, it does not prove the Big Bang actually happened. If you disagree, then I suggest that you consider relearning about these important laws. By the way - if the Big Bang is true, then where did the laws of physics come from?? Salva 15:04, 28 Apr 2005 (UTC)

This is not a forum of discussion on the big bang theory but I'll respond. You cannot analyse the Big Bang in terms of Newtonian Physics or even quantum physics or general relativity. It requires quantum gravity which is a form of physics that is still not well understood. It definitely exists but is not well understood. --FK65 14:23, 13 October 2006 (UTC)

Formation of Galaxies

It appears that in empty space of sufficient size, under certain conditions which seem to be related to a torqued, twisted or curled surface boundary, the electric field splits into systems of charged, positive particulate masses and negative particular fields. These become protons and neutrons, but the separate elements form and aggregate in a separate process. It's only informed speculation though. The universe is steady state and galaxies appear and disappear slowly.

Don't all of the universe creation theories violate the first and second laws of thermodynamics when the universe is made? I mean, even a literal interpretation of Genesis woudl still require violating the laws at tthe very moment the universe is made, right? 12.202.177.193 18:47, 13 August 2005 (UTC)

Seeing as how you are neither an astrophysicist nor an astronomer Salva, I would say we should leave the theory making/breaking up to the scientists, not lawyers. --Negative3 17:32, 17 August 2005 (UTC)

Protogalaxies

Wouldn't it be a good idea to restore the reference to protogalaxies? -- Temtem 16:53, Apr 28, 2005 (UTC)

Not much to say about them; I think a link in the See Also section would be sufficient. Ben Standeven 20:12, 12 Jun 2005 (UTC)

Improvement Drive

A related topic, Astrophysics is currently nominated on Misplaced Pages:This week's improvement drive. Come and support the nomination or comment on it.--Fenice 07:31, 6 August 2005 (UTC)

The following sentences need to be reworked as they are gramattically incorrect (and confusing): The dust they acquire from feeding off the smaller galaxies acts as a net to catch more but this is not a well-known being to our Milky Way as M87 is the nearest known one. M87 also has a supermassive black hole that has a possible amount of energy for quasar level activity as a jet is seen from the 5 billion solar mass size black hole. Easily large enough for quasar-level bursts. I would correct them myself but I am not competent in the subject matter. --Negative3 17:26, 17 August 2005 (UTC)

My data table idea!

- Location Data -

This could go in with the galaxt data table.

My idea is that we could have another box set to show the galaxies, universe, cluster, & group. — Hurricane Devon (Talk) 13:36, 6 November 2005 (UTC)

I think it's unnecessary. There are few notable galaxies beyond the Virgo supercluster, and that Universe part is totally useless. Unless you've got some information on other universes. In that case, I'm all ears. ;) That template also suggests that groups are subgroups of clusters. That's not correct; group is a smaller grouping of galaxies than a cluster. Superclusters may contain several groups and clusters. Finally, the grouping where the galaxy belongs can be mentioned in the text and in the relevant galaxy group article.

--Jyril 19:16, 6 November 2005 (UTC)

I apologize about that. I didn't realize. I have another idea (see new data table).

I didn't relize that a group is just a small cluster. I'm new in this part of astronomy. I still think to have the cluster or group in the data box. Because a majority of articles about galaxies has no knoledge about the group. Puting it in the data box will help. I made up an example. I'm giving this data box a try on:

• • M109
  • Cigar Galaxy
  • Whirlpool Galaxy
  • Sunflower Galaxy
  • Dwingeloo 1
  • Bode's Galaxy

Puting the Cluster in the data box can help. Besides, all of the other info on the data box is in the artical too. (ps: that would be cool if another Universe was discovered & to find galaxies in it.)

— Hurricane Devon (Talk) 21:07, 20 November 2005 (UTC)

Galaxies vs. star clusters

I'm not sure if this belongs to the galaxies or star clusters discussion, but this one at least already exists, so...

Given the Misplaced Pages definition of a galaxy and a star cluster, it appears that star clusters as a class are proper superset of galaxies - is that the case, or are those terms not directly related? What takes a star cluster to be classified as a galaxy? (Is it just size?) I'd appreciate some clarification in the text of either article.

Thanks!

Yes, it's "just" size. But in this case, it's quite a lot of size. A star cluster might have a few dozen to thousands of solar masses of stars, while a galaxy has millions to billions. Galaxies are very very big and contain thousands of star clusters. Being so much bigger, galaxies can have considerably different effects that are important to them. For example, star clusters tend to form in a single burst of star formation and then are dead. Galaxies are big enough that their star formation has been going on since shortly after the Big Bang and continues today. Galaxies have significant dark matter contribution. Galaxies may have spiral density waves. However, there is a gray area between the two; it's unclear sometimes what is a very large globular star cluster versus a very small dwarf elliptical galaxy. -- Xerxes 14:22, 9 January 2006 (UTC)

You can have a star cluster in a galaxy but not the other way round. --Korandder 07:53, 10 July 2006 (UTC)

--Cannot find any systematic references to this but in empty space of sufficient size, under certain conditions which seem to be related to a torqued or curled surface boundary, the electric field splits into systems of charged, positive particulate masses and negative particular fields. These become the proton and neutron, but the separate elements form and aggregate in a separate process. It's only informed speculation though.

Antimatter Galaxies

If antimatter galaxies exist, is there any way we can verify their existence? Can we create a section within this article regarding the possible existence of antimatter stars and galaxies?

This question is effectively answered in the "Antiuniverse" section of the Antimatter article. Good question, but there's little point in having a section under just Galaxy when it is equally applicable to smaller (i.e. stars) and larger (i.e. galaxy clusters) bodies as well. It makes the most sense to have it as a sub-section of the article on antimatter. Cheers! Captmondo 13:57, 14 September 2006 (UTC)

Contradiction?

I found something in this article that might contradict the milky way and andromeda articles, but I am not knowledgable enough to feel confident making any changes.

The Milky way article states: "Current measurements suggest the Andromeda Galaxy is approaching us at 300 kilometers per second, and that the Milky Way might collide with it in several (3-4) billion years."

The Andromeda galaxy article states: "The Andromeda Galaxy is approaching the Sun at about 300 kilometres per second..." "...If it is on a collision course, the impact is predicted to occur in about 3 billion years."

This article states: "Studies show that the Milky Way Galaxy is moving towards the nearby Andromeda Galaxy at about 130 km/s, and depending upon the lateral movements, the two may collide in about five to six billion years."

So this article appears to have a different speed listed for Andromeda's approach. Anyone know what to be done about this, or am I confusing points of reference? --170.28.224.114 19:58, 12 October 2006 (UTC)

This has to be taken a look at. What are the references? --FK65 14:25, 13 October 2006 (UTC)

Ok it has been corrected on the other articles. 130km/sec is the correct value. The 300km/sec is including the sun's angular speed of orbit around the Milky Way --FK65 14:30, 13 October 2006 (UTC)

Mixing measurements

Can we please avoid mixing lights years and (mega)parsecs? Surely one unit of measurement should suffice for an article.

Anyone care to edit? I'm not qualified. Ajkgordon 22:11, 2 November 2006 (UTC)

I suggest switching to light-years, with a mention of the unit of megaparsecs I am not qualified to make the conversions. Ratherhaveaheart 18:33, 30 November 2006 (UTC)

Another possibility is to write both, as in "326 Mly (100 Mpc)". Dr. Submillimeter 20:24, 30 November 2006 (UTC)

LINERs and starbursts

LINERs are a confusing topic. Some are AGN, and some are starbursts. Some are both AGN and starburst. Whether or not they should be listed in "active galaxies" is questionable.

Okay, I added in a few weasel words about their possible relationship.

A second point is that starburst galaxies should be mentioned here somewhere. Dr. Submillimeter 19:48, 6 December 2006 (UTC)

I was going to add it into a future section on "galaxy mergers, tidal interactions and changes in morphology." Would that be okay? Anyway it's in the to-do list now. — RJH (talk) 18:43, 9 December 2006 (UTC)

Although most starbursts are triggered by mergers/interactions, some may not. Among other things, bars could trigger starburst activity. (Bars can at least trigger nuclear star formation.) Maybe starbursts can be in a separate section? Dr. Submillimeter 14:45, 10 December 2006 (UTC)

Okay. — RJH (talk) 18:08, 10 December 2006 (UTC)

Biology

The following section was removed from the main article because unsourced speculation of this nature seems too much like crystal ball-ism/original thought, which violates WP:NOT. It would be very helpful if there were solid references on this topic (in regard to galaxies) so it could be re-inserted. Anybody?

Main article: Extraterrestrial life

Biology as we know it is currently assumed to exist only around single, third-generation G-type stars (like the sun) in the middle regions of the spiral arms of spiral galaxies. Elliptical galaxies, produced as a result of many galactic collisions, quickly lose their clouds of interstellar hydrogen gas, and cannot make new generations of stars. Irregular galaxies have few elderly stars and thus seem to have low concentrations of the heavier elements on which Earth-like biology depends. Even within spiral galaxies biology as we know it would appear to be limited to the middle reaches of the spiral arm, as in the galactic halo or outer spiral arms heavier elements are in short supply, whilst in the gas clouds around the galactic centre heavier elements are in concentrations too high, and interstellar interactions are too frequent to allow earth-sized planets to form in stable circular orbits around their stars.

Thank you. — RJH (talk) 23:21, 11 December 2006 (UTC)

Oldest is not highest redshift

The article makes reference to the "oldest galaxy found" but it is the "most distant" or "highest redshift" or "observed at the earliest cosmic time". Indeed, the galaxy found is not old at all -- it is young, but observed a long time ago. Since we don't know what happened to it since the time it released the photons we currently see, we can't say anything about its present-day age. This terminology ought to be changed. -- David W. Hogg 00:47, 4 January 2007 (UTC)

Correct. I updated the text. Thanks. — RJH (talk) 17:26, 4 January 2007 (UTC)

Field galaxies?

Misplaced Pages and the David Darling site define a Field Galaxy as being separate from a cluster. But the papers and scientific sources I checked say that a Field galaxy is a galaxy that is selected without regard to its environment. That is, it has nothing to do with whether it is isolated or not. (Thus I get 228 ghits on "isolated field galaxies", for example.) So now I suspect that the current wikipedia definition is wrong and should be changed to "isolated galaxies". Any suggestions? — RJH (talk) 19:24, 13 January 2007 (UTC)

I have seen "field galaxy" used to designate a few things. In some cases, the term is used to describe galaxies that are outside clusters but possibly inside smaller groups like the Local Group. (For nearby galaxies, people usually use divide things as to whether they lie inside or outside the Virgo Cluster. Those objects outside the Virgo Cluster are called "field galaxies" in these situations.) In other cases, however, the term is used to describe galaxies which are also outside of groups or crowded environment. In this version of the use of "field galaxy", people are using the term to indicate that the galaxies do not interact with other galaxies and thus evolved in isolation. I hope this discussion helps. Dr. Submillimeter 21:52, 13 January 2007 (UTC)

Thank you. I should probably qualify the text as such. — RJH (talk) 16:40, 15 January 2007 (UTC)

Good Article status

I think the article is very nearly there. It's very broad, largely well-written, has good images and is comprehensively referenced. My only concern is that in a few places the article uses unnecessarily scientific language without explanation, or where the prose is not quite clear enough. I have highlighted some from the intro, where the prose is particularly important, but there are a fe (only a few) in the rest of the article.

• I find the reference to 'unknown type of dark matter' in the first sentence confusing. Perhaps 'a quantity of dark matter'?
• Second paragraph: Galaxies organised by 'visual morphology': could one simply say 'shape'?
• Second paragraph: The two sentences starting with 'Peculiar galaxies' do not flow together well and hence confuse the reader.
• Final paragraph of into: 'The Milky Way appears to harbor just such an object within the core region': Does not specify the core region of what.

If these can be addressed then I would have no hestitation in passing it as a Good Article - congrats to all who have worked on it. The Land 21:24, 4 February 2007 (UTC)

Regarding dark matter: No one has identified what dark matter is made of. Most proposed particles and objects have been eliminated as possibilities. It is also standard to say that galaxies are comprised of stars, interstellar gas, and dark matter. Most standard dynamical galaxy models include at least stars and dark matter. Dr. Submillimeter 23:24, 4 February 2007 (UTC)

This has been addressed so I've passed the article. The Land 15:55, 5 February 2007 (UTC)

In answer to the first, there was a complaint during the PR regarding the use of the phrase "an unknown dark matter" so I added in "type". The use of "unknown quantity" is somewhat ambiguous because astronomers do have an idea of the mass, but not the composition. I'm still not completely happy with either phrasing, so it would seem to make sense to use Dr. Submillimeter's suggestion. — RJH (talk) 15:35, 5 February 2007 (UTC)

"Most common form" of galaxies

The introduction refers to elliptical galaxies as "the most common form" of galaxies. Is that including both dwarf and normal-sized elliptical galaxies? Also, do elliptical galaxies truly outnumber irregular galaxies? Should that assertion be made in this article? Dr. Submillimeter 23:30, 4 February 2007 (UTC)

The following news article appears to suggest that the combination of dwarf and giant ellipticals are the most common:

• Hoover, Aaron (June 16, 2003). "UF Astronomers: Universe Slightly Simpler Than Expected". Hubble News Desk. Retrieved 2007-02-05.

Is that inaccurate? Thanks. — RJH (talk) 15:58, 5 February 2007 (UTC)

• The article says that giant and dwarf ellipticals are "two of the most common types of galaxies in the universe". This is not the same as "the most common type of galaxy in the universe". Someone needs to go find some luminosity functions for galaxies sorted according to Hubble type. (It would also be preferable to find these people's journal papers rather than use a press release.) Dr. Submillimeter 16:38, 5 February 2007 (UTC)

"But together, dwarf and giant elliptical galaxies are the most common." I took that to mean that, taken as a whole, the elliptical is the most common shape. — RJH (talk) 17:40, 5 February 2007 (UTC)

I missed that part. It seems OK, although a citation to a journal article would be preferable. Also, it is worth checking to ensure that the professional astronomy community generally accepts these results (i.e. multiple papers have not been published recently with a contrary viewpoint or with a viewpoint critical of this paper). Dr. Submillimeter 18:10, 5 February 2007 (UTC)

Okay I'll keep looking for some more references. It should probably say "the most common form of nearby galaxies" as well, to exclude high redshift, peculiar types. Thanks. ...

...I found this reference that suggests E-type galaxies are only 10% of the total. So now I'm confused. — RJH (talk) 18:53, 5 February 2007 (UTC)

In one of my I quotedrelative number densities for ellipticals and for all galaxies (it's at the end of section 7) and ellipticals are far from the most common. If you agree, please change this. Incidently, shouldn't we include the terms 'early-type' and 'late-type' somewhere, as these are more common in the literature. Chrislintott 19:42, 5 February 2007 (UTC)

Recent edit RJHall helps - I wonder if it's now just confusing? Can I suggest that we just rewrite this to say 'The largest nearby galaxies are ellipticals...' and go on from there? Chrislintott 19:55, 5 February 2007 (UTC)

I believe that would be incorrect as there are also dwarf ellipticals. I added in a brief mention of the early-type/late-type galaxies, which are also discussed on the "Galaxy morphological classification" page. Thanks. — RJH (talk) 20:03, 5 February 2007 (UTC)

The largest nearby galaxies are elliptical galaxies; M87 is a good example. However, this does not mean that all elliptical galaxies are large, which I think is the point that RJHall does not like. Maybe the comment should be removed (if that has not been done already). Except for brief mentioning the terminology, we should avoid using the phrases "early-type" and "late-type"; it is well-accepted in the scientific literature, but it can be confusing or even misleading to the average reader. (I know some astronomers who get confused by the terms, too.) Dr. Submillimeter 20:55, 5 February 2007 (UTC)

Sorry for the mix-up on the ellipticals. It's mentioned down in the Ellipticals section that the largest are ellipticals; hopefully that's sufficient? As a compromise on the early-late-type phrase, I moved the comment to the notes section.— RJH (talk) 21:37, 5 February 2007 (UTC)

Excellent - I think that this problem is sorted now. What else needs fixing? Chrislintott 21:53, 5 February 2007 (UTC)

I've nominated the article for FAC, which should hopefully catch any remaining issues; including grammar. (Although I dread having an article like this go on the front page—it just becomes vandal bait.) Thanks. — RJH (talk) 22:55, 5 February 2007 (UTC)

Front page

Regretably it now appears that this article may appear on the wikipedia front page. Inevitably that will result a lot of vandalism and sloppy edits. But there's not much that can be done to prevent it; I did ask if it could be avoided, but there's no means to stop it. So the article will likely require a lot of cleaning up in the aftermath. — RJH (talk) 16:03, 1 March 2007 (UTC)

Just revert as you go along - its much easier than re-writing the article MelicansMatkin 00:19, 3 March 2007 (UTC)

Hey guys, haven't you noticed all that vandalism coming every single minute!?, wouldn't it be easier to protect the article rather that reverting edit per edit? --– Emperor Walter Humala · 00:53, 3 March 2007 (UTC)

Someone asks this every day. See WP:NOPRO for why we don't protect it. 153.104.209.212 01:54, 3 March 2007 (UTC)

Okay I think the page is mostly cleaned up now. — RJH (talk) 16:43, 5 March 2007 (UTC)

photo of Edwin Hubble

I think the copyright information listed for the photo of Edwin Hubble is incorrect. It's not a product of NASA, so it's not public domain. Just because it appears on a NASA web page, that doesn't mean NASA took the photo. Wikimedia Commons apparently got it from http://antwrp.gsfc.nasa.gov/apod/ap980621.html , where the credit line is "Mt. Wilson Archive, Carnegie Institution of Washington."--76.81.164.27 03:54, 3 March 2007 (UTC)

Thank you for catching that. I've removed the image and I'll try to find something else to add in there. — RJH (talk) 18:02, 4 March 2007 (UTC)

Dark matter

hey this is just a question, because i don't know, but i thought Dark Matter was only a theory? —The preceding unsigned comment was added by 82.26.111.66 (talk) 17:34, 3 March 2007 (UTC).

The presence of dark matter has been demonstrated observationally. According to theory, it should not exist. Dr. Submillimeter 19:25, 3 March 2007 (UTC)

The article keeps getting modified over this issue by an anonymous editor who apparently has an axe to grind. The current definition in this article reads:

A galaxy (from the Greek root galakt-, meaning "milk", a reference to our own Milky Way) is a massive, gravitationally bound system consisting of stars, an interstellar medium of gas and dust, and dark matter.

Would a definition such as the following alleviate concerns about the existence of dark matter?

A galaxy (from the Greek root galakt-, meaning "milk", a reference to our own Milky Way) is a massive, gravitationally bound system of stars and an interstellar medium of gas and dust, which, evidence suggests, is collectively held together by dark matter.

Alternatively a good reference could be used to anchor the text. Thanks. — RJH (talk) 18:50, 17 April 2007 (UTC)

The top one is better. The bottom one does not seem as graceful. I will attempt to find a reference. (Maybe Shu?) If an anonymous editor is causing problems, then maybe a dialogue needs to be started with the anonymous editor (if the editor can be identified). Dr. Submillimeter 20:37, 17 April 2007 (UTC)

Actually, it seems that more than one account has been used to revise the comments on dark matter. User:Holofect made changes to the article that were very similar to the changes by 204.246.226.132. Dr. Submillimeter 20:46, 17 April 2007 (UTC)

I left a note on the talk page for the anonymous user, and tagged the definition with a NASA reference. Maybe that'll help. Thank you. — RJH (talk) 21:44, 17 April 2007 (UTC)

I suggest using multiple references. I added a reference to Sparke & Gallagher. (I can't believe that their book was left out of this article.) Dr. Submillimeter 17:03, 20 April 2007 (UTC)

Well there's always room for improvement. I agree. Thank you. — RJH (talk) 20:13, 20 April 2007 (UTC)

Spirals

"Like the stars, the spiral arms also rotate around the center, but they do so with constant angular velocity. That means that stars pass in and out of spiral arms."

This surprising observation leaves one hanging out for an answer to the question of how the stars then move. The second sentence does not follow obviously from the first, unless the explanation is too well known to require spelling out. For example, it is, fairly, not stated anywhere explicitly that the sun is a star. But since stellar motion is referenced, how stars move needs to be stated.

Otherwise readers are left to speculate. Could it be in accordance with Kepler's laws? But the article states:

"As stars move into an arm, they slow down, thus creating a higher density; this is akin to a "wave" of slowdowns moving along a highway full of moving cars."

How does a star slow down? How does this square with Newton's first law? Perhaps interstellar gas provides the drag. But then, how does this gas become to be more concentrated in the spiral arms?

My point is not to question the factual correctness but to say that something is missing in the explanation.

Bluemin 18:31, 3 March 2007 (UTC)

I agree that more could be said on the topic. However, this is a general article on galaxies, and the explaination of spiral density waves, shear effects, bar effects, and random oscillations (among other things) would take up too much space. Dr. Submillimeter 19:23, 3 March 2007 (UTC)

I added in a brief comment about the gravitational attraction from the arm's higher density causing the velocity change of the stars. This matches the explanation on the spiral galaxy page. But, now that I think about it, the explanation seems contradictory. If a star spends less time passing through the density wave than through the remainder of the galaxy (due to a change in velocity), that would seem to reduce the net density of the arm. — RJH (talk) 16:58, 5 March 2007 (UTC)

Here is the way to think about it in simple terms:

The shapes of the spiral arms should not change as they move through the disk. Therefore, the tips of the spiral arms move at the same angular velocity as the parts of the spiral arms near the center. When the angular velocities are multiplied by radii, this demonstrates that the tips of the spiral arms are moving at much higher tangential velocities than the stars near the center.

However, the angular velocities of the stars are mostly independent of the angular velocities of the spiral arms. They therefore move into or out of the spiral arms (except at co-rotation radii, where funny things happen). When the stars hit the spiral arms, they are briefly slowed down by the gravitational overdensity, but then they speed up again once they pass through the overdensity. The effect is similar to rolling a ball over a small bump. At small radii, the stars actually move faster than the spiral arms, so they hit the trailing side of the arms. At large radii, however, the spiral arms move faster than the stars; the spiral arms overtake the stars.

Also, keep in mind that the stars are "collisionless", which means that they do not actually physically touch anything. All of the interactions are gravitational. On the other hand, the gas clouds in spiral galaxies are collisional. When they hit the spiral arms, the clouds physically strike other gas clouds. This causes the gas clouds to slow down a lot. Moreover, the gas clouds will collapse after these collisions, which leads to star formation.

Is this a more satisfactory description? Dr. Submillimeter 17:30, 5 March 2007 (UTC)

Yes I think so, although it doesn't really explain how the overdensity retards the stars. Would it temporarily slow the stars nearer the core and speed up the ones on the exterior? Also I've seen some editor-types object to terms like "funny things happen", but that should be easy to address. Thanks. — RJH (talk) 20:44, 5 March 2007 (UTC)

Yes, the stars just slow down when they are inside the waves and speed up again when they are outside (although I fear that a dynamicist would explain that it's more complicated than that). Dr. Submillimeter 22:19, 5 March 2007 (UTC)

Excellent writing! Can we just change the last sentence to something like 'Moreover, these gas clouds will be disturbed by the collisions, leading to episodes of star formation' which gets round the fact that the whole clouds don't necessarily collapse to form stars? Chrislintott 20:50, 5 March 2007 (UTC)

(I see that I have encountered someone who may know more about the dynamics of star formation regions than I do.) I generally think of star formation as triggered by cloud collapse, but that is because I study galactic structure on kpc scales. This is, of course, ignoring the fact that 10 M_ʘ clouds will fragment into about 10 stars each. Is this incorrect? Are the dynamics of star formation significantly different from this description? Or do you just want to state that not 100% of the clouds are turned into stars? Dr. Submillimeter 22:19, 5 March 2007 (UTC)

I was trying to get at the fact that the efficiency of star formation is low; the exact number is a matter of some dispute, but it seems unlikey to be more than 50% at the absolute maximum. I would say that the (large, pc scale) are disrupted, causing the collapse of smaller structures within them (usually called 'clumps' or 'cores'), but that's too detailed for this article. Also, it sounded slightly like you were implying one cloud forms one star - I know you didn't mean that, but that's what it seemed like to me. Does any of that make sense? It's been a long day.... Chrislintott 22:53, 5 March 2007 (UTC)

Yes, this makes sense. I had not written the above text for inclusion in an article, but feel free to edit it as you see fit. Dr. Submillimeter 10:12, 6 March 2007 (UTC)

A discussion about the effect of the spiral arms on the star formation rate might be an interesting addition to the spiral galaxy article. Thank you. — RJH (talk) 22:01, 7 March 2007 (UTC)

External link replacement

An anonymous editor (using several different IP addresses) seems determined to add in an external link to a web page that has the very irksome habit of resizing the browser to occupy the full screen. I tracked down the original news release contained within that web site and used that as a reference within the article. The external link is now redundant and I plan to keep reverting it whenever it appears. Sorry if this offends anybody. — RJH (talk) 15:30, 20 April 2007 (UTC)

I am deeply unoffended. Dr. Submillimeter 16:55, 20 April 2007 (UTC)

This anonymous user has attempted to add the link back twice in the past day. I think it may be possible to get the website blocked. I will ask for help at WP:AN. Dr. Submillimeter 08:58, 22 April 2007 (UTC)

Thank you. — RJH (talk) 22:31, 23 April 2007 (UTC)

A question

Hi, I have a question.

When we photograph a galaxy, isn't the side away from us actually showing a galaxy many years in the past compared to the side nearer us, because the light from the far side will have to cover the additional distance of the galaxy's diameter?

Thanks in advance. Arnavion 21:02, 1 June 2007 (UTC)

For inclined disk galaxies and for elliptical galaxies, this is indeed the case. However, galaxies are only a few tens of thousands of light years across, and the stars do not move or change much on time scales as short as tens of thousands of years, so we virtually see galaxies as they appear at a single moment in time (although some phenomena such as supernovae are exceptions).

(Note that these talk pages are for discussions about writing the articles themselves and not for asking questions about the topics. I don't mind answering questions, but other people might get grumpy.) Dr. Submillimeter 21:45, 1 June 2007 (UTC)

Thanks Dr. Submillimeter. And thanks for telling me about the rule. Arnavion 17:22, 2 June 2007 (UTC)

Although I personally encourage good questions as they sometimes show a gap that needs filling in the article. Sophia 17:34, 2 June 2007 (UTC)

Etymology

Can someone explain to me what this "Bob" nonsense is. Hera bob her son Zeus bobbed his bob? Huh? Is this for real? —Preceding unsigned comment added by 64.86.141.133 (talk) 11:59, August 30, 2007 (UTC)

It's just silly vandalism. The changes were reverted. — RJH (talk) 15:29, 30 August 2007 (UTC)

• Misplaced Pages featured articles
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