Talk:Hydrogen line/Archive 1

Merge from 21cm line
I've merged the text that was previously in 21cm line into this article. You may view it's history at Hydrogen line/history1. --Gmaxwell 05:36, 3 October 2005 (UTC)
 * Note that someone else merged the article histories, so it looks like there was a huge edit war that didn't actually happen. I'm sorry about that, I pointed this out to the person who did it, and he doesn't do it anymore. --Gmaxwell 08:10, 29 October 2005 (UTC)

Comments from Talk:21 centimeter radiation
yeah, add this to the hydrogen line thingy as a subsection and redirect this article to the proper place... id do it myself, but im a little busy now so ill look later S3raph1m 17:03, 25 January 2006 (UTC)

Sorry, I didn't notice the merge tag until now (I guess I was away when it was being edited). Can someone provide a rationale for this? The subject of this page is a nascent, but major observational effort in cosmology to measure the 21 centimeter line of neutral hydrogen in the early universe. This has nothing to do with the galactic measurements that are being discussed on the hydrogen line page; it would make just as much sense to merge it with cosmic microwave background. The title of this page is the name that it is given in cosmology, but it may not be ideal. Perhaps 21 centimeter radiation (Cosmology)? I will remove the merge tag for now. –Joke 17:37, 25 January 2006 (UTC)

well the 21 centimetre radiation is what is referred to as the hydrogen line (this is what i remember from a faq on seti@home anyway) its a particular type of radiation that is used as a baseline to see into the deep cosmos as its very easy to correct the redshift on it so distancing is accurate... i think, id have to research it but thats what i remember. the hydrogen line page is discussing using the 21 centimetre radiation... its up to you which one you put it in, as it would work in both...

one idea would be to add it to cosmic radiation and have the link from the hydrogen line page link directly to the sub category of the cosmic radiation page what do you think S3raph1m 23:52, 25 January 2006 (UTC)

Yes, the 21 cm radiation and hydrogen line are the same thing. My point is that it is used in two distinct ways: While the way in which the 21 cm line is used to derive redshifts is similar, the physical phenomena being measured are completely different. –Joke 03:53, 26 January 2006 (UTC)
 * as a diagnostic in galactic astronomy. This is discussed in hydrogen line.
 * as a tracer for the matter power spectrum in the early universe and a tool for mapping reionization. These measurements, covered on this page, are of a time before galaxies, or even stars, existed. That is discussed on this page.

then this page needs to be added to cosmic radiation, as it is just that and perhaps the hydrogen line page needs a little information about 21 centimetre radiation in it... have what is added to the cosmic radiation page, simply describe the phenomenom S3raph1m 18:51, 27 January 2006 (UTC)

Merge from 21 centimeter radiation
I'm proposing that 21 centimeter radiation be merged into this article. I know that this has been proposed before and opposed, however these two articles are talking about different applications of the same phenomenon. As such, I think that the content on 21 centimeter radiation, once the duplicated content has been removed, should become a section in this article entitled "Uses in cosmology". Mike Peel 21:35, 23 November 2006 (UTC)

H I
The accompanying article is lk'd from the Dab HI (which may move to Hi), as if H I were about the line, not the H I region that H I Rdr's to. This raises several questions: I'm tentatively cleaning up the Dab by making guesses about most of these, and probably some of them should be asked at talk:H I region, but i will be paying attention to any input available here. TIA. --Jerzy•t 05:09, 3 February 2009 (UTC)
 * 1) Is the region rather than the line indeed the primary use of "H I"?
 * 2) Is "H I" ever a reasonable way to refer to the line?
 * 3) Is the Dab entry correct in implying that "HI", used in whichever of those 2 senses, is an error? I.e., will careful writers always write it with the space between? (This is not a question abt removing the entry from HI, but abt whether to treat it as a "full member" of the league of senses of HI, or as a "See other", in this case bcz it is an easy and perhaps common misreading.
 * 4) Since it is a little hard to distinguish HI and H I when they are not together to compare, is it asking for trouble to write "H  I" (marked up "H&amp;nbsp;&amp;nbsp;I")? (Hmmm: "H  I" -- marked up "H < /nowiki> I" -- doesn't work.)
 * 5) Am i right in assuming that H I and H II are properly read "aitch one" and "aitch two" --  at least in cases where H1 and H2 are not explicitly under discussion?
 * 6) Is there an oral convention for avoiding confusion, where the context involves discussion of not only one or both lines or regions, but also of one or both of the chemical species of hydrogen? (Deuterium, i think, is 2H (if not D or 2D), and read "two aitch", right?)

"Mis"references
For the two Nature references, they both link to the same place/article. I don't understand the minutae of academic journals, but it appears that under the same article, "Observation of a Line...", there were multiple authors that began on different pages. I thought that each of the new page/author combos would have a unique link, but that's not the case. Any ideas how to specify the individual pages, or if it needs to be? (They all have the same DOI.) Uhjoebilly (talk) 03:43, 5 November 2010 (UTC)

Origin?
Where did the constant 21.10611405413 come from? I didn't find it in any of the references and quick googling didn't help either (besides google doesn't do partial matches on numbers) --Zom-B (talk) 15:18, 7 January 2013 (UTC)

I realize that this is a bit late, but in case you are still interested in how that number was derived, it comes from converting the frequency to a length. This calculator I find particularly useful in such situations. You will have to correct the value they supply for c, since they round it to 300Mm/s (How silly!), otherwise you will get 21.1207... cm (incorrect digits bolded for emphasis) Mocha2007 (talk) 00:49, 27 January 2016 (UTC)