Talk:Betelgeuse: Difference between revisions

Supersonic?

Does it really make any sense to describe an object moving through a vacuum as "supersonic"? Kendall-K1 (talk) 18:29, 26 November 2012 (UTC)

Had exactly the same thought. The statement is made twice in the article. Even if the star is travelling through a medium not originating from its own emissions and generating a bow shock, the density of that surrounding gas is so low that a person on a bicycle could probably be considered to be traveling at supersonic speeds in such an environment! One would have thought such meaningless concepts alluding to the speed of sound in space (where no one can hear you scream:-) would have been weeded out before this article was passed for FA status (particularly since it appears on WP's main page - making it a bit of an embarrassment). I suggest we replace the words 'supersonic speed' with 'high velocity'. Anyone any thoughts that can justify retention of 'supersonic'?1812ahill (talk) 20:40, 26 November 2012 (UTC)

I had remarked that in the featured article review: you can find the response there and note here whether you find it sufficient. Hekerui (talk) 20:57, 26 November 2012 (UTC)

This entire sentence: "Having been ejected from its birthplace in the Orion OB1 Association—which includes the stars in Orion's Belt—this crimson runaway has been observed moving through the interstellar medium at a supersonic speed of 30 km/sec, creating a bow shock over 4 light-years wide" is non-ecyclopedic langauge. " It's more suitable to a comic book. Starting a sentence with 'Having been ejected' is poor diction besides. I think it needs to be rewritten, and perhaps the following sentence as well, but I have not done it because this controvery exists. 72.179.53.2 (talk) 17:47, 28 November 2012 (UTC) Eric

The speed of sound in space is not meaningless. The sound speed is the speed at which changes in pressure can propagate in a medium, and while interstellar space is not dense enough that a human scream could be heard, neither is it a perfect vacuum in which a pressure wave could not propagate. Given the current state of articles addressing these concepts, perhaps it would be more useful if the supersonic re-directed to speed of sound? In the long term, coverage in supersonic should probably be made a bit more general to explain this. James McBride (talk) 23:21, 26 November 2012 (UTC)

Featured article review discussion: http://en.wikipedia.org/wiki/Wikipedia:Featured_article_candidates/Betelgeuse/archive1 The term supersonic is used correctly here and there is not a suitable alternate term. I would suggest linking the word "supersonic", but unfortunately the relevant Wikipedia article is basically just a few pretty pictures of jet planes with very little physics. "Transonic" has a paragraph about some supersonic flows in astrophysics, but hardly worth linking to. Lithopsian (talk) 21:25, 26 November 2012 (UTC)

Well ok, I guess, since it's already been discussed. It still seems odd to me. We don't talk about boats travelling through water at supersonic speeds, even though that's what they do by this standard. And I think the average reader is going to think "700 mph" (or metric equivalent) when told something is supersonic. Kendall-K1 (talk) 21:41, 26 November 2012 (UTC)

After dredging through the FA review I found:

I have reintroduced the word "supersonic" as it is used throughout the primary literature. See: Mohamed 2012, Introduction to Stellar Winds and Bow shock. I'm no astrophysicist, so I will defer to others on this issue. But it appears that this is an important distinction when describing stellar winds.--Sadalsuud (talk) 14:54, 15 October 2012 (UTC)

I appreciate the points being made, but although the term 'supersonic' is used in academic circles when discussing interstellar bow shock, I question whether it is appropriate for an encyclopedic article, particularly when the linked term refers users to pages about Earth's atmosphere as that could lead to confusion for non-experts. Also I note retention of 'supersonic' was done on the basis of only a singly user's decision (correct me if I am wrong). I also suspect that the word 'supersonic' is used in interstellar bow shock literature to ditinguish it from the more familiar phenomenon associated with boats as pointed out by the previous poster.1812ahill (talk) 21:52, 26 November 2012 (UTC)

Betelgeuse is moving at over 30km per *second*. What part of supersonic are you having problems with? You think there is no "sonic" in a vacuum? You thought interstellar space was a vacuum? I'm not a big fan of removing correct and useful statements just because someone doesn't understand them and can't be bothered chasing the references. If a large proportion of readers are not likely to understand the statement, don't just remove it. Explain it better. Lithopsian (talk) 22:12, 26 November 2012 (UTC)

Incidentally there is also a Wikipedia article on Bow Shock specific to stars, but that may just confuse things further. The fact that a star is moving supersonically and the fact that it has a bow shock are two separate things, although both can occur together. However a bow shock may simply be caused by a supersonic stellar wind. Lithopsian (talk) 22:12, 26 November 2012 (UTC)

Well thank you for that rather rude response. What part of 'there is no sonic in a medium as tenuous as that in interstellar space' do you not understand. The lede statement 'moving through the interstellar medium at a supersonic speed of 30 km/sec' is in itself relatively meaningless because the phrase 'the interstellar medium' implies a kind of ether disproved by the Michelson-Morley experiment. Don't know how good your Physics is (you dont have a user page), but such statements referring to 'x km/s' are usefully backed up with 'relative to [something or other better than the interstellar medium]'. The word 'sonic' when used even in our own atmosphere is pretty meaningless. Ask any supersonically flying pilot - it depends on pressure and temperature. I'm bowing out of this conversation now as it seems to have taken a turn for the worse. As I stated before the word is no doubt being used to distinguish it from ship's bow shock.1812ahill (talk) 23:08, 26 November 2012 (UTC)

The interstellar medium has nothing to do with the Michelson-Morley experiment. It instead refers to the gas and dust that exist in the space in between stars. It has a well defined pressure and density in the same way that the Earth's atmosphere does, and that speed and pressure likewise describe a speed of sound. As you point out, the pressure and density are location dependent, but that does not make the concepts of sonic or supersonic speed meaningless. It merely means that treating the speed of sound as fixed is an approximation, rather than being exactly true. James McBride (talk) 23:30, 26 November 2012 (UTC)

Sorry can't help myself. True, as you say, the local 'interstellar medium' rotates around the galactic core along with the rest of the galaxy. I just question the point of using a word involving sonic when the particles creating the phonons in that medium probably have a mean free path on the order of AUs.1812ahill (talk) 23:47, 26 November 2012 (UTC)

Whether or not you question the value of it, the sound speed in the interstellar medium is an incredibly useful and important concept. For one particularly important example, see the article on the Jeans instability, which has an intuitive explanation that relies upon the sound speed in a cloud in the interstellar medium. In short, if the timescale on which gravitational contraction occurs is faster than the timescale on which sound waves cross the cloud, then the cloud will collapse. This entire discussion suggests that there needs to be some work done on making the articles addressing sound speed and supersonic more general. I strongly disagree that the solution is removing the term supersonic from the article on Betelgeuse. James McBride (talk) 00:02, 27 November 2012 (UTC)

Having looked at your user page, I must concede. My (uncompleted) PhD was in nuclear physics. You clearly know more about this subject than I do. Caveat: the link to supersonic speed is inappropriate. Or, as you suggest that article needs extra work. Best wishes.1812ahill (talk) 00:29, 27 November 2012 (UTC)

I could, of course, be making that all up. :) I agree that the link given now is not ideal; I actually suggested above that for the time being, directing users who click on supersonic to the article on the speed of sound might be more useful. James McBride (talk) 00:39, 27 November 2012 (UTC)

And I assume as one of WPs directives, good faith (or whatever it's called), and you seem to know what you're talking about:) Just as a matter of interest, since you're online, what kind of velocities are we talking about in one's average interstellar medium - if the concept is actually meaningful in our Earth's atmosphere centric view. (Sorry if I'm forgrtting this not a forum)1812ahill (talk) 00:49, 27 November 2012 (UTC)

There is not any one number for the interstellar medium (ISM), as it includes gas in a few different phases. Each phase is in rough pressure equilibrium. This includes important, probably dominant, contributions from non-thermal sources of pressure, which actually means that a quantity like the Alfven wave speed (which describes the speed at which magnetic pressure propagates, rather than thermal pressure) is often more important than the speed of sound. Anyway, each phase is in rough pressure equilibrium, but they have very different temperatures and density, so a blob of gas could move through the cold component of the ISM at supersonic speeds, but be subsonic in a warmer ISM component (in much the same way, I suppose, as a jet could be supersonic at one altitude and subsonic at another). At order of magnitude level, in the cold ISM, the sound speed is of order 1 km/s, while for the warm ISM it is closer to 10 km/s. James McBride (talk) 01:16, 27 November 2012 (UTC)

Huthoff & Kaper (2002) give the speed of sound in neutral interstellar gas as 1 km/s at 100K, 10 km/s at 10,000K, and 100 km/s at 1,000,000K. A million kelvin is an extreme condition that only occurs in a few places such as in bubbles blown by supernovae or OB supergiants. Thus most runaways are moving fast enough to be supersonic. In addition, young hot stars have solar winds that are comfortably supersonic wrt the interstellar medium. Even the sun is thought to produce a bow shock where its wind drops to subsonic speeds, but this is very difficult to observe for a normal star and from the inside. Lithopsian (talk) 17:21, 27 November 2012 (UTC)

Thanks for the enlightenment. I'll have to read up on this more - a sound speed of the order of km/s implies very fast moving particles to me in such a tenuous environment. The idiosyncracies of 'sound' in the ISM as you describe do further re-enforce the almost otherworldy sense of 'sound' in the ISM as compared to the understanding the lay reader (probably) has of our familiar Earthly pressure wave, where magnetic or thermal pressure effects (except maybe in storms or fires) are of minimal importance, because our atmosphere is more like a fluid (in the sense that most people understand that word). Thanks for taking the time to explain. Well past my bed time now, so I'll call it a night. Again apologies for turning this into a forum.1812ahill (talk) 02:19, 27 November 2012 (UTC)

I wasn't even thinking of "sound" in a "vacuum" when I questioned "supersonic" for 30 km/sec (Mach 100). The reason why _I_ laughed out loud when I saw it was that it reminded me of the Far Side cartoon, "Young Carl Sagan on his first date": "Just look at all those stars, Gloria, there must be _hundreds_ of them!" Kotabatubara (talk) 02:11, 28 November 2012 (UTC)

Somebody seems to have reinserted that ill-constructed (and unencyclopedic) sentence, just the way it was. 83.254.151.33 (talk) 04:02, 12 December 2013 (UTC)

Mass loss

Copied from User_talk:Modest_Genius#Episodic_mass_loss

Hi Modest Genius:

After working closely with Casliber for 2.5 years on Betelgeuse we were finally able to cross the FA goal line recently. There is, however, one concern that I have and which could really use your expertise. It concerns the nature of stellar mass loss.

Where we ended up in a pickle, was we needed to reference the luminosity figure for Betelgeuse. Fortunately, we found a recent article from Mohamed 2012 which provides a little infobox on page 2 showing a mean luminosity of ~120,000L_☉. But in the infobox is a mass loss rate of ~0.03M_☉ every 10,000 years which sources a 2001 article from Graham Harper. In 2009, however, ESO published a Betelgeuse article referencing Ohnaka2009 which quotes ~1M_☉ per 10,000 years. It seems that the discrepancy has to do with episodic mass loss (huge gas plume) vs that which is emitted by the star's stellar wind.

So my question is simply this: Is the first paragraph of Circumstellar dynamics a fair representation of the facts?

Thanks for your help on this.--Sadalsuud (talk) 04:14, 19 October 2012 (UTC)

I don't have time to look into this right now, but might find some over the weekend. Modest Genius ^talk 15:44, 19 October 2012 (UTC)

No rush. There's a major conference on mass loss coming up in November. We're thinking of having the article featured on the main page then. I just want to make sure that the way we've summarized the information makes sense to an astrophysicist. Thanks again for any insights.--Sadalsuud (talk) 19:33, 19 October 2012 (UTC)

Well it took a month, but here are my thoughts:

• It's not bad, though a bit heavy going to read. You seem to have described the relevant observations quite well, though it would be nice to have a concluding paragraph to the section which sums up what all these mean, in layman's terms.
• 1e-4 M_sol/yr is NOT the same as 1 M_sol every 10,000 years. The former is an instantaneous rate, whilst the latter is the average over a long time period. If the mass loss is episodic, the latter can be much higher than the former implies, because the star will go through periods of low- and high-mass loss rate. Don't re-write the measurements in this way. Compare 'the car was travelling at 10mph' and 'the car travelled 88,000 miles in a year' - they mean different things, despite being numerically equivalent.
• Harper et al quote a mass loss rate of 3e-6 based on a combination of their model and earlier observations (section 8.1). I cannot see any estimate of Betelgeuse's mass loss rate in the Ohnaka paper, just a general statement about red supergiants (given as 'up to 1e-4'). Harper also references a value of 4e-6 found by Glassgold & Huggins (1986), and two other measurements of 1e-6. In the absence of any other information, I'd go with the Harper number and cite both Harper and Glassgold & Huggins. Delete the 1e-4 figure as misleading and not referring to this star.
• I don't like the second paragraph of that section, which reads like a rehashed ESO press release. Really, what does it matter which instrument was used, or who did the study? That's technical information which is of interest to researchers in the field, but not general encyclopaedia readers.
• There's no reason to talk up the mass loss from this particular star as somehow being massively important to the entire history of the universe. That's overblown, and belongs in the Stellar mass loss article, not Betelgeuse.

I hope that's helpful. Feel free to copy these comments to somewhere more useful (e.g. the article talk page). Modest Genius ^talk 17:31, 18 November 2012 (UTC)

GAIA observation limit

I used a source that gives the named brightness limit directly and I used "may" because the GAIA team has apparently weighed options to read out the CCD differently to get to bright stars and obtain better estimates than Hipparcos, presumably. Hekerui (talk) 13:55, 1 March 2013 (UTC)

Damn bright stars.....Casliber (talk · contribs) 20:29, 1 March 2013 (UTC)

Betelgeuse is in the Milky Way

Right? Wondering.--Tomwsulcer (talk) 18:34, 13 March 2013 (UTC)

Well, strictly speaking all stars we see are in the milky way....but if you're asking is it in the galactic plane, I am not sure - don't think so but need to check a star map. Casliber (talk · contribs) 19:57, 13 March 2013 (UTC)

Thanks Casliber!--Tomwsulcer (talk) 21:14, 13 March 2013 (UTC)

The galactic equator passes through Orion, so Betelgeuse is close to the galactic plane. It's coordinates in the galactic sphere show it 9 degrees south of the galactic plane. Tarl.Neustaedter (talk) 21:54, 13 March 2013 (UTC)

Cool factoid. I'm writing a sci-fi novel about a high schooler who builds a spaceship and flies to Betelgeuse -- a coming-of-age story -- and this stuff is helpful. To get to Betelgeuse, he travels faster than the speed of light (which isn't possible in reality, right?) but are there any ideas of how such warp speeds could be possible?--Tomwsulcer (talk) 22:13, 13 March 2013 (UTC)

See: warp drive; hyperspace; Alcubierre drive. Or Faster than light. And I'd recommend reading Milky Way. None of which have to do with Betelgeuse :-). Tarl.Neustaedter (talk) 22:28, 13 March 2013 (UTC)

Yes thanks. I figured you knew more than most about this subject. And this stuff may possibly be related to Betelgeuse if (1) I finish my sci-fi novel in which a hero travels to Betelgeuse at warp speed (2) it gets published (3) readers want to know if such blistering speeds are possible to Betelgeuse. That is, my question above is probably almost certainly and most unequivocally irrelevant. :)--Tomwsulcer (talk) 23:22, 13 March 2013 (UTC)

Eighth- or ninth-brightest?

The article Orion (constellation) says Betelgeuse is the eighth-brightest star in the night sky; this article says it's ninth. Which is correct? 108.246.205.134 (talk) 20:46, 18 September 2013 (UTC)

It's long-term variable, so its brightness is not always the same. If you look at List of brightest stars, you'll see how close the other stars are relative to the variation in Betelgeuse's brightness. Tarl.Neustaedter (talk) 02:36, 19 September 2013 (UTC)

Distance from sun

Is it not terribly misleading to quote its distance to so many decimal places in the lead, (641.8 light-years) when its estimated distance, according to the side bar is only known by ± 146 ly ? Dave3457 (talk) 20:46, 21 November 2013 (UTC)

yes - someone added that a few months ago unfortunately - corrected back now to rounder figure. Cas Liber (talk · contribs) 00:59, 22 November 2013 (UTC)

Jawza means Black Sheep

"The last part of the name, "-elgeuse", comes from the Arabic الجوزاء al-Jauzā', a historical Arabic name of the constellation Orion, a feminine name in old Arabian legend, and of uncertain meaning. Because جوز j-w-z, the root of jauzā', means "middle", al-Jauzā' roughly means "the Central One"."

That is basically true.. the Arabic word al-Jauza means any black sheep that had a white line "in the middle" of it. This gave the name elgeuse to Orion because of the white line of the belt! --Mando Salama (talk) 17:20, 1 August 2014 (UTC)