Talk:Galaxy/Archive 3

Deletion discussing of interest related to measuring the distance of galaxies
Posting Articles_for_deletion/Roxy's_Ruler here to get more knowledgeable people to review, in case there's some scientific validity to the information posted there. TimTempleton (talk) (cont)  22:18, 10 March 2019 (UTC)

Outlines of galaxies
This is an entry in the "see also" section, but just a redirect to this very page, so probably not very useful. --131.169.89.168 (talk) 13:32, 11 April 2019 (UTC)
 * I changed the redirect to Galaxy morphological classification, which should be more useful. ~Anachronist (talk) 15:54, 11 April 2019 (UTC)

The mystery of galaxy Q 6188 in Cetus
I don't know if I'm at the right place to ask questions about the mysterious galaxy Q 6188 in Cetus (??). It is the only galaxy in Uranometria 2000.0 (1987 edition) which is catalogued as "Q" (see charts 261 / 262). Could this be an error? That's the conclusion of Wolfgang Steinicke of the book -Galaxies and How to Observe Them-. According to Steinicke, this galaxy is also catalogued as Mrk 960 and PGC 2845. The coordinates of this galaxy are (J2000.0) R.A. 00:48.6 / Decl. -12°44'. Danny Caes, Ghent-Belgium. 2A02:1812:151F:5500:6DB0:E10C:2CAE:9FF2 (talk) 10:29, 22 April 2019 (UTC)

Possibly All Galaxies Have A Supermassive Black Hole At Their Heart
The current hypothesis is that every galaxy has a supermassive black hole at its heart and that a galaxy can't form without its SBH. 73.85.203.93 (talk) 15:15, 16 January 2020 (UTC)


 * Do you have a specific edit you want to make to the article? Because this topic is already discussed here. - Parejkoj (talk) 18:20, 16 January 2020 (UTC)


 * Yes... The current hypothesis by the majority of cosmologists is that every galaxy has a supermassive black hole at its heart and that a galaxy can't form without its SBH. 73.85.203.93 (talk) 19:47, 16 January 2020 (UTC)


 * This isn't accurate. We have discovered galaxies without supermassive black holes in their centers. See, for example, A2261-BCG. –Prototime (talk · contribs) 18:31, 11 July 2020 (UTC)
 * It is correct but perhaps needs to be more clear. As it has to do with formation...not about the fact they can survive without one after formation. To quote the NASA source at A2261-BCG.."Astronomers have proposed two possibilities for the puffy core. One scenario is that a pair of merging black holes gravitationally stirred up and scattered the stars. Another idea is that the merging black holes were ejected from the core. Left without an anchor, the stars began spreading out even more, creating the puffy-looking core.".-- Moxy 🍁 03:08, 12 July 2020 (UTC)
 * That isn't necessarily correct either. M33 doesn't have a supermassive black hole in its center, and some scientists conclude that it likely never did, as has pointed out elsewhere. –Prototime (talk · contribs) 07:41, 12 July 2020 (UTC)

Estimate of number of galaxies revised?
"the unseen galaxies are less plentiful than some theoretical studies suggested, numbering only in the hundreds of billions rather than the previously reported two trillion galaxies." https://hubblesite.org/contents/news-releases/2021/news-2021-01 ChaTo (talk) 12:54, 14 January 2021 (UTC)

Featured article concerns
There is a decent amount of uncited text in this article, and most of the sources are from before 2010, suggesting that newer discoveries or theories may not be well-represented. I have concerns that this article meets the featured article criteria, and it may require a featured article review. Hog Farm Talk 19:21, 22 February 2021 (UTC)

Include the Circumgalactic Medium
The CGM has been found to play a substantial role in a Galaxy's life (star formation) since it contains the same if not more baryonic matter than the host galaxy and thus will govern the galaxy's accretion of new matter. (https://arxiv.org/abs/1903.05644) For this reason, adding a note about the CGM should be pertinent to this article. For example, in the "Spirals" section it is stated that "Though the stars and other visible material contained in such a galaxy lie mostly on a plane, the majority of mass in spiral galaxies exists in a roughly spherical halo of dark matter which extends beyond the visible component, as demonstrated by the universal rotation curve concept." and a simple change could be "Though the stars and other visible material contained in such a galaxy lie mostly on a plane, the majority of mass in spiral galaxies exists in a roughly spherical halo of dark matter which extends beyond the visible component, this reservoir of baryonic matter is known as the Circumgalactic Medium, as demonstrated by the universal rotation curve concept." What are other people's thoughts on this?


 * The CGM plays a vital role in both galaxy and stellar evolution. While a section should be added to the galaxy page, it is such a vast and diverse subject (for example, see https://arxiv.org/abs/1709.09180) that it really should get its own page. At this time, Googling the circumgalactic medium returns mostly scientific articles. Having a page to introduce the main ideas to newcomers within the field would be beneficial.

BradAstroClass (talk) 00:18, 7 May 2021 (UTC)

Milky Way's Diameter
The article states that the Milky Way has a diameter of 'at least 30,000 PS or 100,000 LY, when the most recent research (shown on the Milky Way page as 185,000 LY +/- 15,000 LY) lists a diameter of roughly 60,000 PS or 190,000 LY. I would have edited that, however the article is locked. — Preceding unsigned comment added by 65.28.183.104 (talk) 08:24, 9 August 2021 (UTC)


 * Nope, the new diameter listed here is the correct one as far as galaxy sizes go.


 * The "most recent research" about the Milky Way's size has conflated their standard of how they define it. They probed the disk stars and used the outer fringes of the galactic disc as their definition of the diameter, leading to overly large diameters upwards of 150 kly. But this is simply not how we define the sizes of galaxies, and is a very crude way of doing so.


 * Galaxy sizes are measured through isophotes, variations of the half-light radius, Petrosian, surface brightness, and scale lengths. The most established one is the isophote: the D25 standard or the 25.0 mag/arcsec2; which is used by Uppsala, ESO, the RC2 and RC3 catalogues, and an infrared variation is used by 2MASS and an r-band variation by SDSS. It is also used by the NASA/IPAC Database and HyperLEDA. There are decades worth of studies going back to the 1950s that focuses on this way of defining galaxy sizes, so it is simply very hard to object against this method.


 * If you look at the Milky Way article, this new, smaller size is the one which uses D25. Unfortunately, this is the most recent one that we could find. That being said, unless you can provide a new source that uses a widely accepted standard for defining galaxy sizes, this smaller size for the Milky Way will not change. SkyFlubbler (talk) 17:31, 23 August 2022 (UTC)

Section "Properties" should be expanded
I am quite surprised that this section only lists the magnetic fields. There are so many topics out there about the properties of galaxies, like their morphology, internal activity, spectrum, diameter, environment and interaction, components (arms, disks, haloes, dark matter, and their central black holes) and their evolution - their properties as they change over time. This should easily be the largest section of this article. I hope we can expand this and cover these topics, even if just a brief summary of them. SkyFlubbler (talk) 17:41, 23 August 2022 (UTC)


 * + There are also several unsourced statements and lack of updates. Listing it at WP:URFA/2020. 2001:4455:690:C100:E970:55AE:B71E:53A1 (talk) 23:44, 13 July 2023 (UTC)

Semi-protected edit request on 1 August 2023
GNz11 is now not the farthest galaxy AirbusA330772673 (talk) 12:29, 1 August 2023 (UTC)
 * Red question icon with gradient background.svg Not done: it's not clear what changes you want to be made. Please mention the specific changes in a "change X to Y" format and provide a reliable source if appropriate. WikiVirusC (talk) 14:49, 1 August 2023 (UTC)

milky way diameter
The milky way diameter is wrong. NASA has it at 100k LY. https://imagine.gsfc.nasa.gov/features/cosmic/milkyway_info.html https://imagine.gsfc.nasa.gov/science/featured_science/milkyway/ GhostOfKepler (talk) 18:07, 31 July 2023 (UTC)
 * Those pages give very approximate values; they are not primary references for a size measurement. - Parejkoj (talk) 16:48, 1 August 2023 (UTC)

galaxy
https://commons.wikimedia.org/wiki/File:NGC_4414_(NASA-med).jpg#/media/File:NGC_4414_(NASA-med).jpg 207.204.103.47 (talk) 16:10, 26 October 2023 (UTC)

Distinction from other nebulae
Hubble's year of classification of galaxies should be corrected from 1936 to 1926.

Hubble's classification paper, published in 1926. SkyWatcher2025 (talk) 16:35, 24 January 2024 (UTC)


 * Corrected. Thanks. Praemonitus (talk) 18:42, 25 January 2024 (UTC)

Dependence of galaxy properties on viewing angle
I cannot access this article but as a review in an major journal is seems like a good source to be discussed in the intro to "Properties".

Review: Abstract: Johnjbarton (talk) 19:12, 6 March 2024 (UTC)
 * Burstein, D., Haynes, M. & Faber, S. Dependence of galaxy properties on viewing angle. Nature 353, 515–521 (1991). https://doi.org/10.1038/353515a0
 * Galaxies are three-dimensional objects projected onto the sky at random angles of inclination. Deduction of their true structure from their appearance requires an understanding of the variation of their apparent diameter, luminosity and surface brightness with viewing angle. But these variations in turn depend on galactic structure. It has taken astronomers several decades to realize that only with a direct knowledge of galactic distances can these two effects be disentangled.

Isophotal diameter
The section on "Isophotal diameter" had some citation needed tags, but the more I read to look for references, the less I liked the content in the section. Some of it was misleading and I deleted it.

I will delete two more paragraphs. One is on Redman's 1937 paper (should have cited his 1938 correction) but only cited the primary source and incorrectly (AFAICT) credited Redman with defining a standard. Instead the paper compares two methods of measure diameters of things-not-yet-called-galaxies, one of which is the isophotal comparison method. This is a notable historic paper for isophotal techniques, but needs a secondary reference and not a whole paragraph in Galaxy.

Similarly there is paragraph on Holmberg that is almost certainly not correct in that his method differs from modern technique. But the reason to delete here is not incorrectness but rather too much detail for an article on Galaxy. We don't need a partial history of isophotal techniques here. Johnjbarton (talk) 16:45, 8 March 2024 (UTC)

Galaxy morphological classification
The section here on types and morphology is almost four pages; it links Galaxy morphological classification as the main page but it is two-ish.

It seems to me that the Galaxy morphological classification is about the classification systems rather than their results? Johnjbarton (talk) 17:45, 6 March 2024 (UTC)
 * What do you mean by "their results"? - Parejkoj (talk) 17:54, 6 March 2024 (UTC)
 * I mean that the Galaxy article has 4 pages to describe different kinds of galaxies, while the "main" it points to is about systems. So the section "Types and morphology" is not a summary of Galaxy morphological classification as implied by "main" tag.  Is that clearer? Johnjbarton (talk) 18:41, 6 March 2024 (UTC)
 * Tweaked accordingly. XOR&#39;easter (talk) 16:04, 9 March 2024 (UTC)

average number of stars
This article states that the number of stars in a typical galaxy is believed to be around 100 million. But the Milky Way contains upwards of 100 billion stars, which is 1000 times the supposed average. Now, that may be correct - I'm no expert - but it does seem improbable that our galaxy is so atypical. It seems more probable that someone somewhere has written 'million' instead of 'billion'. 220.235.71.22 (talk) 05:45, 7 December 2023 (UTC)
 * Good catch, thanks. Even the linked reference (which I'm not keen on, but it'll do) says 1e9 or more, so I fixed that in the text. - Parejkoj (talk) 18:27, 7 December 2023 (UTC)
 * I believe that the origin of the difference between the average number of stars per galaxy and the number in the Milky Way is simple: the Milky Way is in fact "atypical". The Milky Way is a typical spiral galaxy:
 * Goodwin, S. P., John Gribbin, and M. A. Hendry. "The relative size of the Milky Way." The Observatory 118 (1998): 201-208.
 * But spiral galaxies are large, but most galaxies are smaller Dwarf galaxies:
 * "Dwarf galaxies represent the dominant population, by number, of the present day Universe"
 * Mateo, Mario (1998). "Dwarf Galaxies of the Local Group". Annual Review of Astronomy and Astrophysics. 36 (1): 435–506. arXiv:astro-ph/9810070.
 * The referenced cited says:
 * "In an email with Live Science, lead author Christopher Conselice, a professor of astrophysics at the University of Nottingham in the United Kingdom, said there were about 100 million stars in the average galaxy."
 * and later
 * "Some estimates peg the Milky Way's star mass as having 100 billion "solar masses," or 100 billion times the mass of the sun. Averaging out the types of stars within our galaxy, this would produce an answer of about 100 billion stars in the galaxy. This is subject to change, however, depending on how many stars are bigger and smaller than our own sun. Also, other estimates say the Milky Way could have 200 billion stars or more."
 * So the article should cite 100 million for the average. I will look for a better reference. Johnjbarton (talk) 17:06, 9 March 2024 (UTC)

Intro not a summary.
The intro has a lot of interesting material about the numbers of stars and galaxies that does not appear in the article. Conversely parts of the article do not appear in the intro. Johnjbarton (talk) 18:16, 9 March 2024 (UTC)
 * I agree; the lead needs a rewrite. It doesn't even cover the Observation history, Variants, or the Formation and Evolution sections. There should probably be up to a paragraph for each of the major sections. Praemonitus (talk) 18:56, 9 March 2024 (UTC)