Talk:Galaxy

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)

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. [1] SkyWatcher2025 (talk) 16:35, 24 January 2024 (UTC)

Corrected. Thanks. Praemonitus (talk) 18:42, 25 January 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'easter (talk) 16:04, 9 March 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:

• Burstein, D., Haynes, M. & Faber, S. Dependence of galaxy properties on viewing angle. Nature 353, 515–521 (1991). https://doi.org/10.1038/353515a0

Abstract:

• 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.

Johnjbarton (talk) 19:12, 6 March 2024 (UTC)

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)

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)

GLASS-z12 is now the oldest galaxy discovered

This newly discovered galaxy formed before EGS-zs8-1: GLASS-z12 Jcgam (talk) 19:53, 15 April 2024 (UTC)

It would be the earliest observed, not necessarily the oldest. 02:02, 16 April 2024 (UTC)

I've removed the previous entries for EGS-zs8-1. Frankly, I don't think this article should be the target of such statements; it's meant to cover galaxies in general terms, not specific record holders. Praemonitus (talk) 16:21, 16 April 2024 (UTC)

Thanks for the cleanup: that whole "early universe" section could use a lot of trimming. - Parejkoj (talk) 17:21, 16 April 2024 (UTC)