Talk:Milky Way/Archive 1

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Important News

I'm no expert on the matter, but I just read this article, which seems very important to me. Seems like our galaxy isn't the standard spiral at all. Here's the link: http://www.newscientist.com/article.ns?id=dn7854 80.140.218.64 12:41, 17 August 2005 (UTC)

Sag A*

A logged-out user posted "The centre of the Milky is a region called Stagitarius A*." - is this correct, besides the spelling? The interesting choice of spelling leads me to wonder if it was a troll attempt. --Pakaran 05:49, 4 Dec 2003 (UTC)

http://en.wiki.x.io/wiki/Sagittarius Lirath Q. Pynnor

Arms?

Seven arms in the galaxy? Is that correct? That seems like an extraordinarily large number. -- Decumanus 23:02, 8 Feb 2004 (UTC)

IIRC, the Milky Way is more specifically a barred-spiral galaxy, rather than a spiral one. This would imply two arms. See the beginning of the galaxy article for a reference. -- Anonymous

Correct, the Milky Way is a barred spiral but a barred spiral has more than two arms, since the bar splits up into several arms. The diagram in the article is very misleading. A more accurate, scientific diagram can be found here: http://www.ras.ucalgary.ca/CGPS/press/aas00/pr/pr_14012000/iconmw_plan.gif Svanimpe 16:20, 17 jul 2004 (CET)

How can any of you be sure there are any arms to it,it is impossible to be certain form earth's POV. Dudtz 11/1/05 4:10 PM EST

We don't look at the sky all night anymore

Most pages on the Milky Way, like this one, fail to confirm my suspicion that the river of stars rotates as the night progresses. I think that is what I am told in Chinese poetry, the translation of which is my main business, but I always like to check facts like that. We are so caught up in the views that modern technology gives us that we forget to remember what the ancients saw and wrote about.

Yes, too true. Because the earth rotates, the stars of the Milky Way (and just about everything else in the sky) appear to move from east to west (rotating counterclockwise around Polaris, and rotating clockwise around the Southern Cross). You cancel this rotation by slowly turning your head or the telescope in the opposite direction. Many telescopes have a equatorial mount, designed to make this easier to do. Then you can see that the stars of the Milky Way (except for the sun) and the stars of all other galaxies appear to stay perfectly motionless. Because this view has been almost exactly the same over the last few millennia, astronomers get tremendously excited when there is even the tiniest amount of change. So they spend lots of time talking about these changes (the Zodiac is the sun's yearly motion relative to the stars; the motion of the planets and the asteroids; supernova; rotating binary stars; satellites; etc.). --DavidCary 22:03, 12 Aug 2004 (UTC)

I'm confused, why should the galaxy itself be visible from earth? even if we're looking at the disk 'the long way' it's still several orders of magnitude larger than we are.. no matter what the distance...

Go outside at night and look up. thx1138 12:29, 10 February 2006 (UTC)

This doesn't sound right

...the Earth's axis of rotation is highly inclined to the normal to the galactic plane...

This needs to be changed. "To the normal to the.." doesn't make sense. --Viriditas 09:20, 25 Sep 2004 (UTC)

Well, it does sound right, but is a bit awkward. I'm trying to think of a better way to express it. ✏ Sverdrup 09:50, 25 Sep 2004 (UTC)

Are you trying to say that that the inclination of the galactic equator to Earth's equator is 62.9°? --Viriditas 11:29, 25 Sep 2004 (UTC)

Umm.. no. I realize now that it should really be "plane of orbit" not "axis of rotation". I don't think "normal to the galactic plane" is wrong, just awkward. I'm editing the article, please change what you don't agree with.✏ Sverdrup 12:05, 25 Sep 2004 (UTC)

Thanks, but I didn't say I didn't agree. I just said that the term, "normal to the galactic plane" is not commonly used in English, TTBOMK. Can you replace the word "normal" in that context? That's all. --Viriditas 12:18, 25 Sep 2004 (UTC)

See the disambig page normal; in geometry/maths, a normal is a vector/line perpendicular to another line or a plane. (I think) I solved it by not using the word normal in the article. ✏ Sverdrup 14:09, 25 Sep 2004 (UTC)

What do you think of this:

This reflects the fact that the highly inclined ecliptic plane of the Earth is normal to the galactic plane. That reads well and preserves the meaning you intended. Or not? --Viriditas 00:27, 26 Sep 2004 (UTC)

No, it's not necessarily true. A normal is exactly perpendicular to the galactic plane, while the accuracy in the original is just "highly inclined". ✏ Sverdrup 00:32, 26 Sep 2004 (UTC)

That makes sense. What about linking to ecliptic: This reflects the fact that the Earth's plane of orbit is highly inclined to the galactic plane. --Viriditas 02:53, 26 Sep 2004 (UTC)

Yes, absolutely. ✏ Sverdrup 13:03, 26 Sep 2004 (UTC)

Having started the confusion in the first place by using the jargon normal without a definition (I wanted to avoid any need for a formal definition of the angle between two planes), I have now tidied this up. Note that the Earth's equatorial plane and the plane of the ecliptic are inclined by 23.5 degrees (occasionally rising to 24+ due to nutation IIRC). I think the local standard orientation for the solar system is the plane of Jupiter's orbit (the ecliptic, Earth's plane of orbit, is pretty close), as I recall reading that its orbital angular momentum is 60% of that of the solar system (which should not be that hard to check on the back of an envelope). Maybe the Uranians, with their 98 degree axial tilt, are the ones closest to alignment with the galaxy and the rest of us are out of step :-) -- Alan Peakall 18:14, 13 Dec 2004 (UTC)

Milky Way: Galaxy vs silvery river in the sky

I'm wondering whether this article should be split in two, one being the milky river bit, with the attendant history, and another purely about our Galaxy. They would be Milky Way and Milky Way Galaxy, so that Milky Way can be used by regular people wanting to know about the glowing river in the sky, and its nebulous characteristics, but not about astrophysics or whatnot. The other can be about the characteristics of our galaxy. 132.205.15.43 01:56, 15 Apr 2005 (UTC)

That'd be silly. It's like having one article on the big yellow really bright thingy that's up in the sky during the day, and anther article on the star at the center of our solar system --Ctachme 05:36, 17 Apr 2005 (UTC)

It's separated in the Japanese Wikipedia. It would be akin to the separation as Sun versus Solar System. Or Alpha Centauri versus the various components (A,B,C (Proxima)), or Eta Carinae versus Carina Nebula; besides, the Milky Way is a much more non-astronomical way of thinking of the sky versus the Milky Way Galaxy. The great swath of light above our heads is a significant feature that is not equivalent to the Galaxy which it is part of. 132.205.15.43 01:44, 19 Apr 2005 (UTC)

I feel splitting the two would be appropriate. Astronomers and astronomy textbooks do distinguish between them. Splitting them is equivalent to how there are separate pages for the Sun and solar deity (or list of solar deities). As with the Sun example, the Milky Way Galaxy is the modern scientific understanding, while the Milky Way is the protoscience used by various cultures prior to modern science. --zandperl 02:18, 19 August 2005 (UTC)

As there hasn't been any objection, I am doing so. Milky Way Galaxy will be the astronomical, while Milky Way will talk about the mythology. --zandperl 15:41, 27 August 2005 (UTC)

Need of Image

Can't there be a photo of the Milky Way on this page that isn't X-ray or a diagram or something? Scorpionman 7 July 2005 11:32 (UTC)

As a general user of Wikipedia, the first thing I thought of when reading this article was that it needs an actual image of the Milky Way

Worth contacting Axel Mellinger [1]? He doesn't seem to specify any terms of use, he may be willing to license a version of his fantastic panorama under the GFDL. I could consider releasing this image under GFDL if it would be useful, although it's not terribly representative of the whole milky way. Worldtraveller 12:16, 4 August 2005 (UTC)

Tall order considering that we can't really leave our galaxy to take a picture of it.  :) I suggest one of the following:

• 2MASS IR Galactic Center
• 2MASS Infrared Sky

Two reasons for my recommendation: IR penetrates much of the dust/gas so we get a better idea of what's behind those clouds we can't see through, and the images are public domain. --zandperl 02:57, 19 August 2005 (UTC)

This article

A while ago it was suggested to me that this article is one that has rich potential for becoming featured, but currently wouldn't make the grade. I'd love to see this get to featured status, but feel it might need substantial restructing and expanding. I have been thinking about a possible structure for a revised article and thought I would float it here and see what people think. I could see the article looking like this:

1. Intro section
2. History of understanding

   Certainly a summary of a main article but outlining the progress of thought from inexplicable milky way to disc-shaped object containing the entire universe to just one of many galaxies to our present knowledge.

3. Formation

   Current understanding of the formation of this galaxy.

4. Structure

   Good content here already, details of possible bar, spiral arms, bulge, disk and halo

5. Age
6. Contents

   Outline of the main constituents, ie stars, open clusters, globular clusters, GMCs, etc, with notes on how each is distributed

7. Rotation

   Rotation curve, evidence for dark matter

8. Location of the Solar System

   How we determine where abouts the solar system is

9. Satellite galaxies

   Brief outline of the coeval dwarfs

What else should we have? Worldtraveller 12:16, 4 August 2005 (UTC)

ReqImage

Requested images:

1. Milky Way ... the swath of light in the night sky, preferably framing Orion
2. http://www.news.wisc.edu/news/images/Milky_Way_galaxy_sun05.jpg something like this ( related to: http://www.news.wisc.edu/11405.html )
3. Milky Way subgroup schematical layout

• 132.205.44.43 19:01, 17 August 2005 (UTC)

Orbital speeds

The orbital speed of stars in the Milky Way does not depend much on the distance to the center: it is always between 20 and 25 km/s for the Sun's neighbours [3].

The problem with this statement is that the Sun's neighbors are all roughly the same distance from the center of the Milky Way, so regardless of whether it's a function of distance or not they'll all have the same orbital speed. --zandperl 02:41, 19 August 2005 (UTC)

Sun's orbit shape

The sun's orbit around the galaxy is expected to be roughly elliptical with the addition of perturbations due to the galactic spiral arms and non-uniform mass distributions.

Is there a reference for this statement? As I recall, an elliptical orbit is only expected for a spherically symmetric mass distribution, including the special case of only a central mass (Kepler's Laws). An essentially disk-like distribution of mass, as is our galaxy, would result in a "merry-go-round" orbit: an ellipse with vertical pogo-stick motion superimposed. --zandperl 12:50, 19 August 2005 (UTC)

Oh probably there is, but I'm working from my memories of the last time I had to calculate this. Most of the mass we orbit is well inside the space of our orbit and hence well-approximated as being pointlike. The pertubations due to the spiral arms are roughly 5% in the velocity, and introduces little bumps in the x-y motion. Yes, I neglected the vertical motion, which may be worth mentioning, but from the point of view of our orbit around the galaxy is even less important. The amplitude of our vertical motion is ~0.3 kpc, or ~4% of the 8.5 kpc distance to the center. In other words, in terms of figuring out how we move about the galactic center it is not a big deal. Dragons flight 14:43, August 19, 2005 (UTC)

Oops, the vertical is 0.1-0.2 kpc. Shows what I get for working from memory, but again a small effect compared to other motions. Dragons flight 20:43, August 19, 2005 (UTC)

I believe we can assume the mass inside an orbit is point-like iff the mass inside the orbit has spherical symmetry. Since we are significantly outside the nucleus of the galaxy, we cannot assume this. --zandperl 15:34, 27 August 2005 (UTC)

Diameter and Circumference

The Galaxy is about 100,000 light years in diameter .. and about 250,000 light years in circumference.

Unless I'm making an incorrect assumption about how circular the galaxy is, wouldn't the circumference of the galaxy be closer to 300,000 ly? I'm basing this off the assumption that $C=\pi d$. If it were perfectly circular, wouldn't the Milky Way have a circumference of approximately 314,159 ly, which can be comfortably rounded down to "about 300,000 light years"? --mpeg4codec 16:20, 11 September 2005 (UTC)

Obviously, we all just decided that π = 2.5, of course. I've updated this to read 80-100 thousand in diameter and 250-300 thousand in circumference which is consistent with the different ways people define the size of the galaxy. Dragons flight 16:55, September 11, 2005 (UTC)

Diameter

The diameter of the Milky Way is listed as 80,000 ly, but I believe it's closer to 120,000 lightyears since the discovery of an extra, outlying spiral arm (not containing visible stars, but with large amounts of neutral hydrogen) and a ring of stars wrapped around the galaxy. I am searching for sources, but are the ring of stars and the starless spiral arm known to you? DaMatriX 20:27, 24 January 2006 (UTC)

More details on outer arm?

It would be nice to expand the outer arm stuff referred to here (I couldn't find anything on the Internet about it): WilliamKF 22:31, 7 February 2006 (UTC)

Milky Way extending further is a clear possibility and is supported by evidence of the newly discovered Outer Arm extension of the Cygnus Arm.

Chinese, Japanese, and Korean translation removed

I removed the translation of "Milky Way" in those three Asian languages. They are not written in the same characters and in Korean's case, there is a different meaning.

• Chinese: 银河系 yín hé xì (galaxy system or silver river system)
• Japanese: 銀河系 zingakei (galaxy system or silver river system)
• Korean: 우리 은하 uri unha (our galaxy)

I'm not sure how to reconcile all this. However they all have "silver river" in their translations (yín hé, zinga, unha). But saying "silver river galaxy" is redundant since "silver river" is galaxy, apparently. I am thinking that these translations would be more appopriate in galaxy, but I am not sure about Chinese treatment of the word, because the Chinese article is named 河外星系 hé wài xing xì --Chris S. 05:37, 13 February 2006 (UTC)

Ironic

Isn't it ironic that we named our galaxy the Milky Way Galaxy. Then F. Mars created the Milky Way candy bar. And now our galaxy is now known to be a barred spiral galaxy with a bar at the center. This is what I think of when my power goes out! Check this out if you don't get what i'm talking about! — Hurricane Devon ( Talk ) 12:16, 16 February 2006 (UTC)

Question about bar

I have a question. What are the main arms to come of the ends of the bars? — Hurricane Devon ( Talk ) 01:44, 17 February 2006 (UTC)

I found out. This sais the main arms to come off the bars of the Milky Way are the Norma-Cygnus Arm and the Sagittarius Arm.
— Hurricane Devon ( Talk ) 14:01, 18 February 2006 (UTC)

Perseus Arm

I removed the statement that the distance between the local arm and the Perseus arm is 2000 lyrs. It is not supported by the given reference, http://www.ras.ucalgary.ca/CGPS/press/aas00/pr/pr_14012000/pr_14012000map1.html and in fact, given that the galaxy's diameter is 10^5 lyrs, the map given there suggests that the distance is much larger. AxelBoldt 01:40 Jan 11, 2003 (UTC)

Local Bubble/Fluff

The recent change from Local Bubble to Local Fluff, although technically correct, is not necessarily an "either-or" The Sun is still well within the Local Bubble. The Local Fluff is also known as (perhaps more within the field) as the Local Interstellar Cloud (LIC). Recent evidence suggests the LIC is not alone (surprise) and the acronym CLIC has also started to be used for "Complex/Cluster of Local Interstellar Clouds". --mh 21:28, Feb 8, 2005 (UTC)

orbital velocity vs distance from galactic centre

These 2 sentences appear to contradict each other:

"The orbital speed of stars in the Milky Way does not depend much on the distance to the center: it is always between 200 and 250 km/s for the Sun's neighbours [1] (http://zebu.uoregon.edu/~imamura/123/lecture-2/mass.html). Hence the orbital period is approximately proportional to the distance from the star to the Galaxy's center (without the power 1.5 which applies in the case of a central mass)."

I think you might be missing the change in term from orbital speed to period. If speed is constant, since the orbital path length increases linearly with R, the period is also linearly proportional to R.--mh 04:37, Apr 17, 2005 (UTC)

Age

So if I get it right, the Milky Way Galaxy is about the same age as the universe (some 13 billion years) as a whole? That doesn't make sense. It can't be created at the same time.

Universe: 13,700 +/- 200 million; Milky Way: 13,600 +/- 800 million

This is consistent with the Milky Way being a first generation galaxy whose original stars are only a few hundred million years younger than the universe itself. Obviously it won't actually be older than or the same age as the big bang. Dragons flight 13:16, August 1, 2005 (UTC)

Right. It does make sense. I just didn't know our Galaxy came to be that soon after the Big Bang. Thanks!

Also, globular clusters within our galaxy are only a slight bit younger than the galaxy and universe--for a while astronomers thought the GC's were _older_ than the universe! Pretty much EVERYTHING formed right after the Big Bang. Check out the timeline. --zandperl 02:29, 19 August 2005 (UTC)

Since there was just an edit changing age to 13600 bya, here are sources for 13600 mya:

• [2]
• [3]

A thought -- confusion could have resulted from the variation in use of "," vs "." for a separator. Lomn | Talk 21:10:47, 2005-08-17 (UTC)

Is it also possible it's due to the different use of "billion" throughout the world? --zandperl 02:29, 19 August 2005 (UTC)

The universe has no age.Dudtz 11/1/05 4:11 PM EST

I've edited the paragraph in question to reflect the fact that we are discussing the age of the oldest stars in the galaxy. I'm not sure it has any meaning at all to talk about the age of the Galaxy as a whole. I'd suggest moving this section to a less prominent place in the article. Thoughts? Chrislintott 10:56, 30 January 2007 (UTC)

so yeah it is really cool dont u think haha